Is morning urinary protein/creatinine ratio a reliable estimator of 24-hour proteinuria in patients with glomerulonephritis and different levels of renal function?

Background. This cross-sectional study was conducted to determine whether a spot urine protein/creatinine ratio (UPr/UCr) provides accurate quantitation of 24-hr urinary protein excretion (24-hr Prot) in out-patients with primary glomerulonephritis (GN) and different renal function levels. Methods: Patients were classified into three groups according to creatinine (Cr) clearance (ml/min) and into five categories according to morning U-Pr/U-Cr. Correlation between 24-hr Prot and U-Pr/U-Cr was calculated according to the three renal function levels. The Bland and Altman method was used to assess agreement between 24-hr Prot and U-Pr/U-Cr. Agreement Emits were obtained calculating the mean difference between 24-hr Prot and morning U-Pr/U-Cr +/- 2SD. Sensitivity and specificity were determined for different renal function levels and U-Pr/U-Cr cut-off values. Results: High correlation coefficients (r=0.91, 0.95 and 0.98) were observed in patients with normal, reduced and severely reduced renal function. Differences and variability between 24-hr Prot and U-Pr/U-Cr tended to increase with higher proteinuria levels, and this trend was observed for the three renal function levels. The best U-Pr/U-Cr cut-off values to detect abnormal or nephrotic proteinuria. were, respectively, 0.3 and 2.6. Conclusions: Correlation and agreement between U-Pr/U-Cr and 24-hr Prot was good for all renal function levels, but demonstrated more marked differences as urinary protein excretion increased. Morning UPr/UCr had good sensitivity and specificity for the diagnosis of 24-hr Prot, even in patients with reduced renal function.