Specific gravity and creatinine as corrections for variation in urine concentration in humans, gorillas, and woolly monkeys.

Hormones excreted in the urine are widely used to assess the physiological and psychological condition of unrestrained animals. In order to control for variation in the water concentration of urine samples, the hormone concentration is often indexed to the concentration of creatinine. Because there are several problems with using creatinine, we have investigated the efficacy of specific gravity as an alternative basis for adjusting the hormone concentration in humans, gorillas, and woolly monkeys. In an experimental manipulation of human urine hydration, ten volunteers drank a water load proportional to body weight, and provided complete urine collection and saliva samples for four consecutive 20 min intervals. From the urine, we measured cortisol (radioimmunoassay), creatinine (colorimetric assay), and specific gravity (refractometer). Only cortisol was assayed from saliva. During 80 min following water ingestion, cortisol, creatinine, and specific gravity declined as urine became diluted; however, total cortisol excretion remained constant. Only cortisol concentration indexed to specific gravity accurately reflected the consistent cortisol excretion. Specific gravity and creatinine-corrected cortisol values were highly correlated but were significantly different. Salivary cortisol provided evidence for the relative stability of serum cortisol. To determine the utility of these corrections in other primates, we compared specific gravity- and creatinine-corrected cortisol in urine samples from captive gorillas (N=16) and woolly monkeys (N=8). As with the human study, the two corrections were strongly correlated in each species, but the means were different. Specific gravity correction was superior in revealing the circadian variation in cortisol.