can be an indicator of hydration status, other biochemical values must be considered in order to assess hydration status versus kidney function.
An elevated BUN:creatinine ratio (greater than 25) was seen in 2 of 37 elderly, long-term care patients who experienced no febrile episodes and no documentation of impaired oral intake (Weinberg et al., 1994a). The BUN:creatinine ratio remained relatively constant over a 6-month period in stable male residents (Weinberg et al., 1994b). Still, although the BUN:creatinine ratio, like BUN itself, has been used to assess hydration status, lack of specificity hinders its use as a measure of hydration status.
Urine volume is often used as an indicator of hydration status. If healthy individuals have urine outputs of approximately 100 mL/ hour, they are probably well hydrated (see Figure 4-4). Higher urine outputs (300 to 600 mL/hour) are probably indicative of fluid excess (Freund et al., 1995; Lee, 1964). If urine output falls to less than 30 mL/hour for extended periods with an average diet, the person is probably dehydrated (see Figure 4-4).
The color of urine darkens or lightens with low or high output levels (because the solute load is either concentrated or diluted, respectively). Thus urine color has been used as an indicator of hydration status (Wakefield et al., 2002). However, no precise relationship between urine color and hydration level exists. Furthermore, diet, medications, and vitamin use can affect urine color. Nonetheless, urine color can provide a good educational tool for dehydration or overhydration (Casa et al., 2000). A urine color chart for athletes to teach them about proper hydration is available (Casa et al., 2000). Although not nearly as precise as biochemical measures, urine color can give a crude indication of hydration status.
Because urine becomes more concentrated with dehydration, both urine specific gravity and urine osmolality have been used as indicators of hydration status. Urine specific gravity and urine osmolality increase with dehydration and are strongly correlated (r = 0.82−0.97) with each other (Armstrong et al., 1994; Popowski et al., 2001). It should be noted that the validity of the urine specific gravity and