Is morning urinary protein-to-creatinine ratio a reliable estimator of 24-hour proteinuria in patients with kidney diseases?

Abstract

Introduction: Proteinuria is the most frequent marker of kidney damage. Although 24-hour urinary proteinuria is the gold standard, the measurement of proteinuria from albumin urinary creatinine ratio is proposed as much useful metod. Objective: To evaluate the accuracy of urine protein-to-creatinine (P/Cr) ratio in morning urine specimens as compared with 24-hour total protein excretion for the measurement of proteinuria in patients with different kidney diseases and different renal function levels. Methods: Proteinuria in the studied patients was assessed by 24-hour protein excretion (24-hour PRT) and spot urine P/Cr ratio. The analysis of concordance between 24-hour PRT and P/ Cr was carried out using intraclass correlation coefficient (ICC), paired t-test and Bland-Altman plots. The discriminant cutoff values for spot urine P/Cr ratio in predicting 24-hour protein "threshold" excretion were determined using receiver operating characteristic curves (ROC), as well as sensitivity and specificity. Results: A total of 303 patients were included in the study. The concordance between 24-hour PRT and P/Cr ratio was excellent (ICC 0.931). Systematic overestimation of PRT by urinary P/ Cr ratio was disclosed (mean difference 0.138, p=0.011). The P/ Cr of 0.25 (sensitivity 0.90; specificity 0.96), 0.66 (1.00; 0.91) and 2.55 (1.0; 0.97) g/g reliably predicted 24-hour urine total protein equivalent "thresholds" at 0.2, 1.0 and 3.5 g/day. The chronic renal failure group independently positively influenced the difference between 24-hour PRT and P/Cr. It means the lower the kidney function the higher is the difference between the two proteinuria measurements. Conclusion: This study supports the recommendation of using spot urine P/Cr ratio in proteinuria screening in patients with different kidney diseases. The obtained results indicated better agreement between morning P/Cr and 24-hour PRT in patients with lower proteinuria and better kidney function.