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z Mechanisms and Methods: Looking At the Impact of Health Insurance on Health The relationships between health insurance and access to health care, and health insurance and care received, have been the subject of hundreds of studies over the past several decades. More recently, the relationship between health insurance and health outcomes has also been examined. This chapter describes the Committee's analytic approach to its critical review of this research to inform the understanding of the relationships between health insurance, health care, and health outcomes for adults. The chapter is organized in three sections. First, it outlines the mechanisms by which the Committee postulates that health insurance affects health-related out- comes. Whether one has health insurance, a regular source of care and, if one is uninsured, the length of time that one is without coverage all influence access to care and affect health-related outcomes. The second section discusses issues related to the measurement of health insurance effects and considerations of research design that affect the inferences that can be drawn. It explores analytic strategies to distinguish the effects of health insurance status from those of personal attributes that are correlated with health insurance, including health status, race and ethnicity, and socioeconomic status, which may confound) the results of studies that relate health insurance to health outcomes. The section gives particular attention to the two-way causal relation- ship between health status and insurance status.2 It also describes the major popu- iSee Appendix C for definitions of technical terms such as "confound." 2Perhaps most problematic for determining the effects of health insurance on personal health is that an individual's health condition may be directly related to enrollment in particular kinds of insurance 25
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26 CARE WITHOUT COVERAGE: TOO LITTLE, TOO LATE ration surveys and databases that provide information about Americans' use of health care and epidemiological information about health status and disease preva- lence. These sources provide the data for many of the most informative studies reviewed. The final section of this chapter presents the Committee's methods for sys- tematic review and synthesis of the research on health insurance status effects. This section describes how studies were identified for inclusion in the review, the criteria for evaluating the methodological quality of studies and study results, and how the Committee's findings are presented in Chapter 3. MECHANISMS AND MEASURES OF ACCESS TO HEALTH CARE Health insurance facilitates access to health care by removing or diminishing financial barriers to obtaining care. Among people who have insurance, the extent of cost sharing also influences the use of health care (Newhouse et al., 1993; Zweifel and Manning, 2000~. An extensive body of research consistently finds a strong and positive relationship between health insurance and access to care, even as the definitions and measures of access have been strengthened. Population- based surveys conducted over the past three decades have evaluated access to primary care in relation to health insurance status with measures such as any physician visit within a year, the number of physician visits per year, having a regular source of care, and the ability to obtain care when needed (Freeman and Corey, 1993; HaLner-Eaton, 1993; Newacheck et al., 1998; Nelson et al., 1999; Zuvokas and Weinick, 1999; Haley and Zuckerman, 2000; Kasper et al., 2000; Shi, 2000; Weinick et al., 2000; Hoffman et al., 2001~. Public policy and health care industry interests in high-quality and efficient health care have developed in tandem with the progress of clinical effectiveness research over the past decade. The standards of evidence for the efficacy of health insurance in promoting better health outcomes have evolved from enumerating physician visits to measurable improvements in effective processes of care. The notion of "access" itself has shifted from a simple measure of utilization to mea- sures that incorporate the quality of care and health outcomes. In 1993, the Institute of Medicine (IOM) Committee on Monitoring Access to Personal Health Care Services reconceptualized access as "the timely use of personal health services to achieve the best possible health outcomes" and recommended a set of health outcome measures that could serve to monitor populations over time for access to basic health services (Millman, 1993~. programs (e.g., as a medically needy or disabled Medicaid beneficiary and other Medicaid enrollees who join the program only when they incur a hospitalization or need other expensive care; in the case of nongroup private health insurance, only persons in good health may be accepted for coverage). See Box 2-1 for further discussion.
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MECHANISMS AND METHODS 27 In studies of access to care and health outcomes, several factors mediate the relationship between health insurance and health-related outcomes. These include being able to see a provider when one believes care is needed, having a regular source of health care, having continuity of coverage, and the duration of periods without health insurance. Measures for each of these factors provide some infor- mation about an individual's or population's access to health services that supple- ments the measurement of health insurance status at a given point in time. These measures are discussed below. Getting Care When Needed The ability to see a physician or other health care provider when one believes medical attention is needed is a fundamental and intuitive measure of access to health care. Most Americans mistakenly believe that people without health insur- ance have this level of access (IOM, 2001a). Although the lack of health insurance is not the only reason someone might not be able to see a health care practitioner when needed, it is a major one.3 Adults without health insurance are far more likely to go without health care that they believe they need than are adults with health insurance of any kind (Lurie et al., 1984, 1986; Berk and Schur, 1998; Burstin et al., 1998; Baker et al., 2000; Kasper et al., 2000; Schoen and DesRoches, 2000; Davidoff et al., 2001; Holahan and Spillman, 2002~. While the overall percentage of adults who reported that cost prevented them from seeing a doctor in the previous 12 months increased only slightly from 10 percent to 11 percent between 1991 and 1996, the proportion of uninsured adults who reported this barrier to care increased from 28 to 35 percent, and the fraction of insured adults reporting this barrier decreased slightly from 8 to 7 percent (Nelson et al., 1999~.4 In 1998, nearly 70 percent of uninsured adults in poor health could not see a doctor at some time during the year because of cost (Ayanian et al., 2000~. A study that polled 1,100 patients four months after their initial visit to an emergency department found that patients who lost their health insurance were more than twice as likely as those who maintained their coverage to have delayed seeking care in the four-month interval (Burstin et al., 1998~. Evaluations based on professional judgment confirm findings based on a sub- jectively determined need for care. In one study with a national probability sample of almost 3,500 adult respondents, a physician panel identified 15 serious condi- tions for which they deemed medical attention necessary (Baker et al., 2000~. In 3Other reasons include not being able to find a provider within traveling distance and not being able to find a practitioner that participates in one s health plan (particularly a problem for Medicaid enrollees in states with low provider payment levels), language or cultural barriers, and understanding of symptoms and health conditions as requiring professional attention. see the IOM report, Access to Health Care in America (Millman, 1993) for a discussion of barriers to access. 4All of these differences are statistically significant.
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28 CARE WITHOUT COVERAGE: TOO LITTLE, TOO LATE an analysis that adjusted for demographic and economic characteristics and also for health status and having a regular source of care, the authors found that an uninsured adult was much less likely than an insured adult to get care for a reported symptom (odds ratio ~OR] = 0.43~. Examining only those symptoms for which the respondent thought care was needed, those without insurance were even less likely to have received care (OR = 0.28~. Among those who did not receive needed care, the uninsured were far more likely than those with insurance to report that they did not get care because of cost (95 percent and 23 percent, respectively) (Baker et al., 2000~.5 A lack of health insurance acts not only as an initial barrier to care but may continue to impede the receipt of appropriate, effective care. Even if uninsured patients receive primary care, referrals to specialists, ancillary diagnostic and treat- ment services and medications are more difficult to obtain. Primary care providers who treat uninsured and other low-income patients report greater difficulty in arranging for referrals and services that they cannot directly provide for their uninsured patients than for those who are insured (Fairbrother et al., 2002~. Persons who never present themselves to a health care provider are not accounted for in health services research that documents and measures utilization and outcomes with hospital administrative records, patient chart reviews, and clinic encounter forms. This is a "blind spot" and source of bias in studies of health insurance effects because overall, persons without health insurance are estimated to use roughly two-thirds of the services that those who do have insurance use (Marquis and Long, 1995~. Because those without health insurance are less likely to see a provider than are others with insurance and thus are less likely to be included in research documentation, studies that rely on health care records to compare groups who received some care may overstate utilization by uninsured populations. Having ~ Regular Source of Care In addition to supplying the financial resources that enable one to obtain health care when needed, insurance coverage also improves receipt of appropriate care by facilitating the use of a regular source of care or primary care provider.6 5Previously reported in IOM (2001 a). 6Measurement of a usual or regular source of care is frequently based on survey participant re- sponses to a single question such as "Is there a place or health care practitioner that you routinely go to for medical care?" However, some studies use more restrictive definitions of a regular source of care (e.g., they exclude hospital emergency departments as a positive response to the question.) The studies included in this review that document a regular source of care may use different operational definitions. One problem with evaluating utilization outcomes in terms of having a regular source of care is that frequent users of services are more likely to report having a regular source; the direction of causality is not clear. As the concept of primary care has been further conceptualized, the attributes associated with a regular source of care have become more specific (Starfield, 1992, 1998; Shi, 2000).
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MECHANISMS AND METHODS 29 Both health insurance and having a regular source of care contribute indepen- dently to the utilization of health services (Solis et al., 1990; Mosen et al, 1998; Mandelblatt et al., 1999; Zuvokas and Weinick, 1999; Cummings et al., 2000; Breen et al. 2001~. Having a regular source of care enhances the appropriate use of ambulatory care as measured by receipt of preventive services, management of chronic conditions, and population rates of avoidable hospitalizations (Bindman et al., 1995; Starfield, 1995; Pappas et al., 1997; Kozak et al., 2001 ~ . The independent contribution that having a regular source of care makes to the receipt of appropriate care reinforces rather than diminishes the importance of health insurance, because health insurance is an important determinant of obtain- ing and maintaining an ongoing relationship with a health care provider. Adults with health insurance are much more likely than those who are uninsured to have a regular source of care, a consistent finding across states with very different health care resources and provider configurations (IOM, 2001a; Holahan and Spillman, 2002~. An analysis based on the 1997 National Health Interview Survey found that among adults eligible for Medicaid, 42 percent of those not enrolled in the program did not have a regular source of care, whereas only 12 percent of those with Medicaid coverage lacked one (Davidoff et al., 2001~. Even those uninsured adults who have chronic conditions are substantially more likely to lack a regular source of care than are chronically ill adults with health insurance. Among unin- sured adults, 19 percent with heart disease, 14 percent with hypertension, and 26 percent with arthritis do not have a regular source of care, compared with 8, 4, and 7 percent, respectively, of their insured counterparts (Fish-Parcham, 2001~. Someone without health insurance who can identify a regular source of care may still face difficulties in obtaining recommended and effective health care services that are outside the scope of practice of their regular provider, such as referrals to specialists, ancillary services, and hospital-based care. Continuity of Care and Coverage Not only is continuity of care, as measured by having a regular source of care, important, continuity of insurance coverage is also critical to the receipt of appro- priate care. Breaks in coverage can disrupt care relationships to the detriment of quality health care. Being uninsured for longer periods of time can be expected to have larger effects on utilization of services (and consequently on health) than being uninsured for shorter periods. Although only a few studies have examined health outcomes by length of time uninsured (Lurie et al., 1984, 1986; Ayanian et al., 2000; Kasper et al., 2000; Baker et al., 2001), a number of studies have looked at intermediate measures of access to care and volume of services (e.g., any physician visit or number of visits within a year) and find strong relationships between breaks in coverage and length of time uninsured, on the one hand, and reduced access to and utilization of services, on the other (Burstin et al, 1998;
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30 CARE WITHOUT COVERAGE: TOO LITTLE, TOO LATE Ayanian et al., 2000; Kasper et al., 2000; Schoen and DesRoches, 2000; Hoffman et al., 2001~.7 One study examined clinical outcomes in a small cohort of patients with high blood pressure who were followed after some of them lost Medi-Cal coverage (California's Medicaid program). Blood pressure control for those who lost public insurance was worse at six months than at one year following the loss of coverage. The authors hypothesize that the deterioration in blood pressure control was reversed to some extent after new care arrangements were established following the termination of benefits (Lurie et al., 1986~. In another study that compared transitions among health care plans to the complete loss of coverage, researchers found evidence of a temporary weakening of access to care among those who changed plans and a more pronounced loss of access among those who lost health insurance (Burstin et al., 1998~. The experience of intermittently insured adults, in terms of both access and outcome measures, falls between that of continuously insured adults and continuously uninsured adults but is more similar to the latter than the former (Schoen and DesRoches, 2000; Baker et al., 2001~. These results suggest that continuity and stability in health insurance coverage contribute to reliable access and effective health care. Finally, in a large-scale survey comparing persons uninsured for shorter or longer time periods, measures of access, appropriate care, and health status were worse for those uninsured for longer periods (Ayanian et al., 2000~. These findings are particularly relevant to the interpretation of results for Medicaid enrollees, whose coverage tends to be of limited duration and who may have been uninsured before they were enrolled in Medicaid (IOM, 2001a; Perkins et al., 2001~. In fact, more than half of single women with Medicaid at the beginning of a year have lost their coverage before the end of the year, and between one-third and one-half of them are uninsured a year after they lose Medicaid coverage (Short and Freedman, 1998; Garrett and Holahan, 2000~. METHODOLOGY AND MEASUREMENT OF HEALTH INSU12ANCE EFFECTS Chapter 1 poses the problem of how to determine whether a "dose" of health insurance is effective in improving an individual's health. Experimental studies provide a stronger research design than observational studies for concluding that insurance itself affects health. However, most of the research examining the im- pact of health insurance on health-related outcomes is based on observational 7Although access to health care services measured at this most general level (i.e., in terms of the number of visits to physicians and self-reported delays in seeking care) is not the focus of the Committee's review, it can be postulated that longer times without coverage that result in more greatly reduced utilization and delayed care would similarly affect the specific health-related outcomes examined in this report.
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MECHANISMS AND METHODS 3 (nonexperimental) and, with several important exceptions, cross-sectional (point- in-time, nonlongitudinal) studies. Indeed, earlier reviews ofthe research on health insurance status and health outcomes have highlighted the need for longitudinal studies and close examination of natural experiments, for example, when public coverage programs are expanded to new population groups or cut back or among state-level programs with differing eligibility standards (Weissman and Epstein, 1994; Brown et al., 1998~. The one major randomized trial of health insurance, the RAND Health Insurance Experiment, did not have an uninsured group, although it did include a group with 95 percent cost sharing and a very high deductible, which approximated no coverage for routine care (Newhouse et al., 993~.8 Because of the great potential for biases in observational studies, the associa- tions observed between health insurance status and health may reflect the effects of factors other than health insurance, such as income, social status, or education. Much of the observational research, especially that conducted in the past decade, has included extensive statistical adjustment for some of these potentially con- founding factors. Such adjustment strengthens the analytic design of observational studies and increases the likelihood that the observed associations between health insurance and health outcomes represent a causal relationship. However, addi- tional, less readily measured personal characteristics covary with health insurance status and may also affect health outcomes. This section examines the sources of potential bias and the analytic strategies used to address them in observational studies of health insurance effects. It also reviews the personal characteristics that are most likely to covary with health insurance status namely, health status, race and ethnicity, and socioeconomic status (SES) and considers how they may confound findings of health insurance effects in observational studies. 9 Limits of Observational Studies Sources of Bias In relying on observational studies to determine whether and how health insurance affects health-related outcomes, the first limitation is the wide variability 8See footnote 8 in Chapter 1. 9See the Committee's earlier report, Coverage Matters, for an analysis of personal characteristics related to health insurance (IOM, 2001a). In this report "race-ethnicity" is used to refer to the classification of population groups according to U.S. Census categories unless otherwise indicated. "Socioeconomic status" encompasses a variety of indicators, including income, educational attain- ment, type of employment or job classification, and residential area. Health insurance status itself has often been used as a measure of SES. Most studies of health insurance status effects include at most two or three SES indicators in addition to race-ethnicity as control variables.
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32 CARE WITHOUT COVERAGE: TOO LITTLE, TOO LATE in how the term "health insurance" is defined and measured. In cross-sectional studies, a participant's health insurance status is measured at one point in time, when other personal information is collected or when health care services are used and utilization is documented in administrative or clinical records. Even longitu- dinal studies that follow a study participant across time often measure health insurance status only at the beginning of the study period. Because health insur- ance status may change over time (with the important exception of those who qualify, on any basis, for Medicare), the classification of study participants as "insured" includes not only those who always had insurance, but also those who may have recently been uninsured and now have coverage. Likewise, those clas- sified as uninsured include not only those who have always been without health insurance, but also those who may have had coverage until recently. The result of the point-in-time measurement of health insurance status is to diminish the ability to measure any effect of health insurance on health outcomes because the groups being compared have overlapping membership. Therefore, the actual effect of health insurance, if it were to be measured in terms of its duration, may be greater than found in observational studies that look at insurance status at a single point in time. Most studies of health insurance effects do not fully account for a second kind of bias in measuring health insurance coverage, namely, the wide range of benefit packages and of cost-sharing and provider participation arrangements subsumed under the category of general health insurance. Health insurance plans that cover a wide array of benefits or that require no or limited cost sharing (deductibles and coinsurance) from enrollees can be expected to affect patient and provider behav- ior differently from plans that cover fewer benefits and require more cost sharing (Zweifel and Manning, 2000~. Finally, health insurance plans differ in terms of provider payment and participation rates and arrangements, affecting enrollees' ease of access to care and patterns of utilization. Another potential source of bias in observational studies is the nonrandom distribution or selection of study participants among health insurance status cat- egories. Health status is itself a determinant of health insurance coverage. It is closely related to the likelihood that someone has insurance and to the kind of coverage he or she has (IOM, 2001a). Working-age adults in better health are more likely to work full time and at higher paying jobs, thus increasing their chances of having employment-based health insurance. At the same time, those with access to employment-based coverage who anticipate needing health care are somewhat more likely to take up the offer of coverage than similar persons in good health with no expectation of significant health care use and expense. Adults in poor health without access to employment-based coverage are more likely to seek individually purchased coverage but may find that they cannot obtain health insurance because of preexisting conditions. Those in the poorest health who are unable to work (i.e., who are recognized as permanently and totally disabled) or who have very low incomes or high medical expenses may qualify for Medicare or Medicaid. Box 2-1 reviews the eligibility requirements for Medicaid and Medi-
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MECHANISMS AND METHODS 33 care that contribute to the distinctive health status profile of working-age adults with public health insurance. Thus, health status differs systematically among persons grouped by health insurance categories because health status is one criterion by which people qualify for coverage, and analytic adjustments for underlying health status tend to be incomplete at best. Health-related behaviors such as smoking, exercise, and diet also affect individual health and are sometimes but not usually included as covariates in analyses of health insurance effects.l° These health behaviors are strongly re- lated to educational attainment, which itself is correlated with health insurance status (IOM, 2001a). Finally, additional personal characteristics, such as the will- ingness to live with risk and the value placed on good health or health care, may bias the selection of enrollees in private health insurance plans, including those who take up the offer of employment-based coverage and those who seek to purchase individual coverage. Because most of those offered employment-based coverage do accept it and relatively few purchase individual coverage, this source of selection bias is not likely to substantially affect the overall comparisons be- tween insured and uninsured adults. Major Covariates Personal characteristics that vary with health insurance status may confound analyses of the effects of health insurance on health-related outcomes because they are independently associated with these outcomes. In addition to health status, which has just been discussed, the most important among these characteristics are race and ethnicity and SES. Race and ethnicity are frequently included in analyses that examine the effects of health insurance status on health outcomes. SES, however, is more difficult to separate completely from health insurance status, even when it is represented in multivariable statistical analyses by employment, educational attainment, or income. In addition, distinctive ethnic and cultural population groups with different economic and behavioral characteristics are sub- sumed under the broad racial and ethnic categories most commonly used in research. For example, Mexican and Cuban Americans and Puerto Ricans are all categorized as Hispanic; likewise, African and Caribbean immigrants may be categorized with African Americans as Black. Racial and ethnic minority groups and persons with lower SES often face barriers to obtaining health care, such as lack of transportation, concerns with personal safety, and provider shortages, that extend beyond those related to being uninsured. Even with insurance and health care, disadvantaged groups often have worse health on average than do socially privileged groups. 10See the report Promoting Health: Intervention Strategies from Social and Behavioral Research, (IOM, 2000b) for a review and analysis of behavioral factors in health.
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34 CARE WITHOUT COVERAGE: TOO LITTLE, TOO LATE Rates of health insurance coverage differ for various racial and ethnic groups: African Americans, Asian Americans and Pacific Islanders, and Hispanics are much more likely to be uninsured than are non-Hispanic whites. Eighteen and a half (18.5) percent of African Americans, 18 percent of Asian Americans and Pacific Islanders, and 32 percent of Hispanics are uninsured, compared with 10 percent of whites (Mills, 2001~. Ethnic differences in insurance status partially reflect differ- ences in the rates of employment-sponsored insurance coverage. Public insurance programs (e.g., Medicaid) only partially make up for these disparities in employ- ment-sponsored coverage (Gabel, 1999; Monheit and Vistnes, 2000; IOM, 2001a). Insurance is also correlated with socioeconomic status. Forty (40) percent of adults who live in lower-income families (defined as having incomes less than 200 percent of the federal poverty level and 40 percent of adults without a high school diploma are uninsured, compared with 10 percent of adults with at least a college degree (Hoffman and Pohl, 2000; Monheit and Vistnes, 2000~. In addition, the factors of race ethnicity and SES are intertwined. In the United States, ethnic minorities face more limited educational and occupational opportunities. Many ethnic minorities live in highly segregated communities (Massey and Denton, 1989; Cutler et al., 1999~. These neighborhoods may be
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MECHANISMS AND METHODS 35 more crowded, offer fewer economic opportunities, be more geographically iso- lated from health care providers, and have more environmental health hazards. Each of these factors may adversely affect health. Boxes 2.2 and 2.3 illustrate how health insurance is just one of several sources of health disparities among racial and ethnic groups and persons of different SES. Context of Care Every study that examines the care and outcomes for people without health insurance does so within a particular economic and institutional context of health care for the dominant population, namely, those with health insurance. Nationally, one out of every six or seven adults receiving health care is uninsured. Among states, the ratio of uninsured to insured ranges from one in ten (Minnesota and Rhode Island) to one in four (Texas, Arizona, New Mexico) (IOM, 2001a). Two points follow from this observation. First, national studies that examine the health care and outcomes of uninsured adults mask a wide range of health care financing contexts within which care is rendered. Second, any evaluation of the care of uninsured adults reflects the financing and resource environment that
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36 CARE WITHOUT COVERAGE: TOO LITTLE, TOO LATE prevails at that time and, if the balance of uninsured to insured populations changes, the general processes and outcomes of care may also change, for both those who have and those who lack health insurance. This phenomenon will be addressed in a subsequent Committee report on community impacts of uninsured populations.
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MECHANISMS AND METHODS 37 National Surveys and Databases Many of the studies reviewed draw on a number of publicly sponsored surveys with national or state-level probability samples, epidemiological databases, and disease registries. Box 2.4 identifies these surveys and data sources. The size of these sample surveys, the comprehensiveness of reporting systems, and the collec- tion of comparable data periodically over several years and even decades contrib- ute to the quality of the information available from these sources.
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38 CARE WITHOUT COVERAGE: TOO LITTLE, TOO LATE
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MECHANISMS AND METHODS 39
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go ~~ ~~oQ~ comer Too Of~ Coo ## METHODS Of THE SYSTEMIC OTE~U~E RENEW Ago SYNTHESIS The Committcc~ l~cr~turc rcvic~ updates and broadens the scope of number of extensive rcvic~s of rcsc~rch mc~suhng the coca of acute insurance Fetus on bc~th-rcktcd outcome. Not~lc prior conthhubons include Dan ~6 ~~ ~~c ~ D~, ~ background Parr prepared by the Congrc~ion~1
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MECHANISMS AND METHODS 4 Office of Technology Assessment (OTA, 1992~; Falling Through the Safety Net: Insurance Status and Access to Health Care (Weissman and Epstein, 1994~; "Monitor- ing the Consequences of Uninsurance: A Review of the Methodologies" (Brown et al., 1998~; and No Health Insurance? It's Enough to Make You Sick (American College of Physicians-American Society of Internal Medicine, 1999~. To these earlier reviews, the Committee contributes the development and application of explicit criteria used to identify and select studies for inclusion and to assess their methodological strength. This section describes the selection crite- ria, the evaluation of research quality, and how the individual study findings are presented in Chapter 3. Identification of Studies and Inclusion Criteria The systematic literature review includes clinical and health services research and population surveys that are structured to examine the independent effect of health insurance on some health-related outcome. It includes studies with the following dependent variables or outcomes: · General health status (self-reported or medically evaluated) · Disease-specific clinical indicators (e.g., blood pressure) and stage of dis- ease at diagnosis or treatment · Mortality (e.g., in-hospital; longer-term survival rates) · Functional status, limitations, disability · Use of services for specific conditions that are associated with improved health outcomes (e.g., periodic dilated eye exams for diabetics) · Screening and other secondary preventive services · Use of appropriate procedures (e.g., diagnostic and treatment services after acute myocardial infarction) · Adverse events due to medical mismanagement · Hospital admissions for preventable conditions The Committee excluded those studies that measured only basic access to care (e.g., number of physician visits per year, presence of a regular source of care, difficulty reported in obtaining care when needed) because the relationship be- tween health insurance and access is well established. This literature is discussed earlier in this chapter and in the Committee's first report, Coverage Matters (IOM, 2001 a). All studies selected for systematic review include uninsured subjects. Some excluded studies, for example, were limited to comparisons of health-related outcomes among types of insurance coverage (e.g., fee-for-service or indemnity coverage versus health maintenance organization or managed care) and included no information on uninsured patients. The following three categories are the most common classification of insurance status groups for the purposes of the Committee's analysis:
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42 CARE WITHOUT COVERAGE: TOO LITTLE, TOO LATE 1. privately insured (employment-based or individually purchased coverage); 2. publicly insured (Medicaid, State Children's Health Insurance Program, or Medicare for disabled or end-stage renal disease beneficiaries); and 3. uninsured. Studies that combine publicly insured and uninsured persons within a single category are included in the literature review because they may offer some insight into the factors that affect health-related outcomes, even though they do not yield results specific to uninsured adults and thus are of limited value in measuring the effects of uninsurance as such. Likewise, studies of health services utilization that report combined results for publicly and privately insured adults are included. Reporting findings for a single category of publicly and privately insured adults also presents a problem for interpretation because these insured groups differ in their health status, with publicly insured adults tending to have worse-than- average health status and privately insured adults better-than-average health status. This difference is rarely adequately controlled for analytically. The literature review focuses on outcomes for U.S. adults between 18 and 65. This review excludes perinatal and pediatric studies (as noted earlier, these will be reviewed in the next report of the Committee) and studies that are limited to the population over age 65, virtually all of whom have at least medical and hospital insurance, primarily through the Medicare program. To the extent that the re- search is discussed by disease category (e.g., diabetes care and outcomes), some studies may include children. These studies may be considered in the next Com- mittee report as well. Almost all of the studies reviewed share a conventional and imprecise defini- tion of health insurance as meaning general medical and hospital coverage. A few of the studies that examine preventive or mental health services assess health- related outcomes as a function of a specific insurance benefit package. These exceptions are noted in the discussion in Chapter 3. Studies that address dental services have been excluded from the review because coverage for dental services is minimal or missing from most basic health insurance plans for adults (KPMG, 1998~. Furthermore, studies of dental care and outcomes tend to identify coverage for dental services specifically. Likewise, studies that exclusively consider institu- tional, long-term, or custodial care as a function of insurance coverage were excluded because these services are not usually included in health insurance ben- efit packages. Studies of rehabilitation services were included. The Committee applied its selection criteria to studies identified in PubMed searches conducted between March and June 2001 and updated monthly thereaf- ter through November 2001.11 Studies cited in the published literature surveys 1lThe electronic search included publication dates back to 1965; however, most citations were for studies published after 1985. The search terms included "insurance status," "insurance, longitudinal," "insurance, cohort," "uninsured, longitudinal," "uninsured, cohort," "payer status," "payer source," "medically indigent," and "uncompensated care."
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MECHANISMS AND METHODS 43 noted earlier in this chapter were also included if they met the criteria set out above. In addition, unpublished studies of which the Committee became aware through experts were included if they met the criteria listed above. A total of 131 primary research studies were reviewed and rated. i2 This primary research bibliog- raphy is included in Appendix B. Evaluation of Research Quality The goal of the Committee's literature review is to evaluate the nature and quality of the evidence in the aggregate for particular kinds of health-related out- comes, rather than to judge the results of specific research studies. Thus, individual studies were evaluated in light ofthe information that they could contribute to the body of evidence on health insurance effects. To carry out that evaluation, two reviewers rated each study in the research bibliography (see Appendix B). Box 2- 5 presents the methodological review criteria. One reviewer was a member of the Subcommittee on Health Outcomes for the Uninsured and the other was an Institute of Medicine staffmember or consultant with training complementary to that of the first reviewer. Studies that were judged to be of poor quality by both reviewers were not used in formulating the Committee's findings. If studies were judged to be of fair or good methodological quality by one reviewer and poor by the other, they were submitted to a third reviewer (a Subcommittee or Commit- tee member). Quantitative results are presented in Chapters 3 and 4 only for studies that received a fair or good evaluation from two reviewers. Presentation of Committee Findings Chapter 3 presents the Committee's findings based on the literature review, with the evaluation of studies organized into categories that reflect either specific diseases or type of service such as preventive and screening services or hospital care. The categories reflect practical considerations, including ease of presentation and summary of results. Findings regarding specific health conditions are more easily understood within the context of similar clinical research and can be related to larger populations at risk. Service-based categories permit synthesis of findings across studies with consistent outcome measures. This may be especially useful because health insurance coverage rules are often structured by service categories (e.g., preventive and screening services may be excluded or covered without any cost sharing to promote their use) but may also specify exclusions based on condition (e.g., mental illness). i2Separate articles using the same sample and analytic design, as noted in Appendix B. are counted as a single study here.
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44 CARE WITHOUT COVERAGE: TOO LITTLE, TOO LATE In developing its summary findings, the Committee considered the volume of evidence within a given category in terms of the number and representativeness of studies and the degree of consistency among studies. The Committee gave greater weight to longitudinal than to cross-sectional studies and, among cross-sectional studies, placed more importance on those that measured and appropriately consid- ered key covariates. Regardless of the Committee's evaluation of methodological quality, however, all studies with results inconsistent with the Committee's sum- mary findings are discussed in Chapter 3 and included in Appendix B.
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Representative terms from entire chapter: