Screening for albuminuria is important for early detection of renal involvement in diabetes [1]. Because of considerable day-day variation in albumin excretion, the median result of three overnight or first morning collections is taken when accurate diagnosis is needed [2]. Measuring urinary albumin creatinine ratio (ACR) corrects for inaccuracies in timing of collections and dilutional effects [3]. If ACR measured on a single mixed sample of equal-sized aliquots taken from each of the three collections could provide results of similar accuracy to the median value of ACR from three separate collections, accurate measurement of ACR might be obtained at lower cost.
For 1 month all triplicate collections for ACR from our clinic were analysed by both methods. The samples were collected on alternate nights from 29 patients with ACR ranging from 0.3 to 44.8 g/mol, stored at room temperature and brought directly to the laboratory. Samples were analysed within 6 days of collection and confirmed negative for nitrites (Nephur 6 test strips). After mixing thoroughly, a 2.0 ml aliquot was taken from each collection and assayed separately. The median of the three measurements was calculated (ACR Med). A second 2-ml aliquot was taken from each sample and the three aliquots mixed together to form a 6.0 ml mixed sample (ACR Mix), which was then assayed for ACR. The three single samples and the mixed sample were analysed in a single run using immunoturbidimetry [4] (Roche Diagnostics, Basel, Switzerland). Intra-assay coefficients of variation were 4.8% for albumin and 2.9% for creatinine. Analyses were performed on log-transformed data. The geometric mean (interquartile range) for ACR Med was 6.2 (2.820.3) vs 5.6 g/mol (2.820.5) for ACR Mix (not significant on paired t-test). Correlation between the two measurements was highly significant (r = 0.93, P<0.001). Figure 1 shows the difference between the two measures of ACR against their mean [5]. The mean (±2 SD) of the means of the differences between the two measures of albuminuria was 0.14 g/mol (±2.9 g/mol) indicating a high degree of agreement, which appeared constant over the whole range of albuminuria.
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Conflict of interest statement. None declared.
Department of Diabetes, Endocrinology and Internal Medicine Guy's Campus Kings College London London UK Email: mike{at}krimholtz.com
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