Urine protein-to-creatinine ratio in an untimed urine collection is a reliable measure of proteinuria in lupus nephritis

Abstract

Objective . To evaluate the accuracy of urine protein-to-creatinine (P/C) ratio in an untimed urine specimen as compared with 24 h total protein excretion for measurement of proteinuria in patients with lupus nephritis. Methods . Proteinuria in patients with lupus nephritis was assessed by 24 h total protein excretion and spot urine P/C ratio. Correlation and limits of agreement between the two methods were evaluated. The discriminant cutoff values for spot urine P/C ratio in predicting 24 h protein ‘threshold’ excretion of ≥0.3, ≥0.5, ≥1.0 and ≥3.5 g/day were determined using receiver operating characteristic curves. Results . A total of 165 samples were available for assessment with 21.8% excluded due to inadequate collection. A strong correlation ( r = 0.91, P < 0.0001) was found between spot urine P/C ratio and 24 h urine protein excretion. Bland–Altman plot showed the two tests had acceptable limits of agreement in low level of protein excretion (−0.86 to +0.92 g/day when protein excretion was Conclusion . This study supports the recommendation of using spot urine P/C ratio in screening and monitoring proteinuria in patients with lupus nephritis. However, in assessing the exact amount of proteinuria, the urine P/C ratio may have unacceptably wide limits of agreement in high protein excretion range.