Use of urine specific gravity to improve screening for albuminuria

The albumin to creatinine ratio (ACR) can be used to measure urine albumin excretion rates, but is inconvenient and expensive. More rapid and less expensive screening methods estimate only albumin concentration and are subject to errors caused by variation in urine volume. We examined whether urine specific gravity could be used in place of urine creatinine to correct albumin concentration for differences in urine volume in 50 patients. Urine specific gravity accurately estimated urine creatinine concentration (r = 0.79, P < 0.001). The albumin estimated-creatinine ratio (ACestR) in random spot urine sample correlated with urine albumin excretion measured in a 24-hour urine collection (r = 0.98, P < 0.001), as did the ACR (r = 0.95, P < 0.001). For determining microalbuminuria, the sensitivity (0.88) and specificity (0.93) of the ACestR were similar to those of ACR (0.89 and 0.93, respectively). Unfortunately, the sensitivity (0.63) of the Micral-Test was relatively poor, and was only slightly improved by correcting for urine specific gravity (0.69) in this small sample of patients. Nevertheless, these results suggest that as rapid methods for measuring urine albumin concentration improve, combining them with urine specific gravity might produce a less expensive and more convenient alternative to the ACR.