OBJECTIVE To estimate the within-person variability (SD_i) for the overnight urinary albumin excretion rate (AER) in diabetic patients with persistent microalbuminuria.

RESEARCH DESIGN AND METHODS Thirteen normotensive diabetic patients in stable medical control, with normal renal function and without any associated cardiovascular or other clinical disorders, collected overnight urines at monthly intervals during one year. AER was determined by radioimmunoassay.

RESULTS Analysis of individual series of AER showed a significant trend (P ≤ 0.01) in three cases. The remaining patients (eight men, two women; seven with insulin-dependent diabetes mellitus) had a median mean AER 102 μg/min (range 30–238 μg/min). Because the individual mean AERs were significantly associated with their respective SD_iS, the data was log_e transformed. Log₂SD_i was estimated as 0.420, and its 90% probability range (0.353–0.490) was calculated using the bootstrap method.

CONCLUSIONS The high within-person variance for AER means that only people with an initial AER in the range of 53–76 μg/min have a high probability (P ≥ 0.95) of being classified as microalbuminuric (20–200 μg/min) on a subsequent specimen. However, subjects with an initial AER ≥53 or ≥80 μg/min have a 95 and 99% probability of persistent microalbuminuria. The large variability of AER limits its potential as a serial marker to detect any gradual deterioration of established renal dysfunction.