Aim: A prospective cross-sectional study was performed on 170 patients with various glomerular diseases to study the accuracy of predicting 24-hour proteinuria from the spot urine protein-creatinine ratio (Up/Uc). A cost-benefit analysis was performed for the New Zealand health economic system to obtain the best cut-off values for proteinuria.
Subjects, methods and results: Two spot urine samples (Up/Uc1 and Up/Uc2) were collected on the same day as the collection of a 24-hour urine. A randomly chosen subsample of 50 patients provided a second set of urine samples. The correlation and precision of agreement between the two methods were examined. The predictive intervals were calculated for derived 24-hour proteinuria. The level of agreement was evaluated by the Bland-Altman method and concordance analysis. The limits of agreement were evaluated against the clinical limits of agreement. A cost-benefit analysis (CBA) was performed to obtain the optimum operating points on receiver operating characteristic (ROC) curves for the best decision threshold. Correlations of r = 0.97 and 0.99 were observed between Up/Uc1, Up/Uc2 and 24-hour proteinuria, respectively. The 95% predictive intervals were wide. A high concordance correlation coefficient was obtained. The most of the differences between the two methods fell within the clinical limits of agreement. The Up/Uc1 of 0.26 and 3.20 represent the best thresholds to detect normal and nephrotic proteinuria, respectively.
Conclusions: Despite wide confidence intervals, a good correlation and precision of agreement were demonstrated between the two methods across the whole range of proteinuria, regardless of the level of renal function. The difference between the two methods was less than the biological variability in the protein excretion and its measurement, enabling the methods to be used interchangeably. The optimum thresholds for abnormal and nephrotic range proteinuria were obtained.