Spot urine osmolality/creatinine ratio in healthy humans.

BACKGROUND Spot urine albumin/creatinine ratio is a reliable estimate of 24-hour urine albumin excretion. In a pilot study, we observed that the spot urine osmolality/creatinine ratio (U(osm)/U(cr)) in healthy adults is reproducible. Therefore, we postulated that U(osm)/U(cr) of a spot urine sample may give an overall estimate of urinary excretion of solutes, renal concentrating ability and body hydration status. METHOD Early morning spot urine samples were collected from healthy humans, frozen and analyzed in batches to establish spot U(osm)/U(cr) and its variation in relation to sex, age, body weight and height. RESULTS Two hundred and twenty-nine healthy volunteers participated. They were stratified into seven age groups: (a) 1.5-5, (b) >5-10, (c) >10-20, (d) >20-30, (e) >30-45, (f) >45-60, and (g) >60 years. Fifteen males and 15 females were allocated to each age category. A spot urine sample was collected from all subjects in the morning after the first void and was analyzed for osmolality and creatinine. The influence of age, sex, body weight and height on spot U(osm)/U(cr) was investigated using multiple linear regression. Only height showed a significant correlation (R(2) = 0.02). Further analysis after excluding the 1.5-5 years age group revealed no significant correlation between age, sex, body weight and height and the U(osm)/U(cr) ratio. CONCLUSION Spot U(osm)/ U(cr) of healthy humans is a consistent index in steady state and needs no correction for sex, age and body weight above the age of 5 years.