Substantial changes in methods have occurred during the 50 years of vitamin B studies considered in this report. Appendix E summarizes information about methods used to determine laboratory values related to B vitamin and choline status. Methodological problems have been documented for folate (see Chapter 8) and other nutrients (see Appendix E).

Caution was used in the interpretation of study results. Some examples of points that were considered follow:

• The method of measurement of urinary excretion of the vitamin (e.g., fasting, random, or 24-hour specimens) introduces different types of errors. The use of creatinine corrections to allow for assay of random fasting urine samples rather than 24-hour collections may need to include considerations of differences in creatinine excretion by age.

• Depletion-repletion studies assess requirements by identifying intakes that return status indicators to the prestudy baseline values. Baseline values have been those of motivated healthy individuals on self-selected diets or on diets containing a recommended level. The assessed requirements based on this approach of returning values to baseline are invariably similar or higher than the baseline vitamin intake. This is addressed in more detail in Chapters 7 and 14.

• Some studies are too short to determine whether a tested level of nutrient intake will be sufficient to stabilize the laboratory test result at a lower but satisfactory value.

• Many laboratory values change during pregnancy (NRC, 1978) and the postpartum period (sometimes with differences between lactating and nonlactating women) (IOM, 1991). A decreased value does not necessarily mean that intake was inadequate.

The terms sensitivity and specificity each have different meanings when applied to laboratory tests as compared with public health applications. Analytic sensitivity is defined as the amount of a nutrient that results in a doubling of background blank in an assay. Analytic