VARIABILITY IN SNOW-URINE ASSAYS

Urea nitrogen : creatinine ratios in snow urine have become popular fo r assessing the extent of winter nutritional deprivation in ungulates. During winter 1992-1993, we collected 10-17 sequential snow-urine sam ples (402 total) from 27 individually identifiable free-ranging adult female elk. Within-animal variance accounted for 91% of the total vari ance (351.66 mg(2)/dL(2)) in creatinine, 86% of the total variance (63 7.03 mg(2)/dL(2)) in urea nitrogen, and 82% of the total variance (0.5 6) in urea nitrogen : creatinine ratios. This substantial within-anima l variability was unexpected and led to experiments that examined whet her the variability was due to sample collection and measurement techn ique or actually reflected biological variability. Factors examined in cluded dilution effects, measurement (assay) repeatability, and short- term (<24 h) within-animal constancy in metabolite excretion. No dilut ion effects were detected when the initial concentrations of snow-urin e samples were diluted less than or equal to 75% with water. Measureme nt variability accounted for 0.78, 0.37, and 27.7% of the total varian ce in creatinine, urea nitrogen, and urea nitrogen : creatinine ratios , respectively. Within-animal metabolite excretion was reasonably cons tant within 24 h, suggesting that creatinine provides a valid index fo r comparing urinary metabolites. We conclude that variability in urea nitrogen : creatinine ratios due to dilution, measurement variability, and short-term temporal variability in metabolite excretion was small compared with the total within-animal variance. Urea nitrogen : creat inine ratios should provide an accurate estimate of the true ratios of these metabolites in an elk's bladder urine. However, the interpretat ion of urea nitrogen : creatinine ratios is often complex, since they reflect the immediate dynamics between fat depletion, protein cataboli sm, and dietary intake. Differences in ratios between collections may be partially due to variations in recent dietary intake or restriction , in addition to true differences in long-term nutritional status. The best method for statistically analyzing snow-urine data remains unres olved.