Objectives: The accurate assessment of proteinuria is critical for the management of lupus nephritis. Measuring the protein to creatinine (P/C) ratio in random spot urine (RSU) samples has been introduced as an alternative to the 24-hour (24h) urine collection method. However, it remains unclear as to whether the RSU P/C ratio is reliable for assessing lupus nephritis (LN) in routine clinical practice.
Methods: In total, 275 pairs of 24h urine and RSU samples from 102 patients with biopsy-proven LN were analysed. The correlation and concordance between the P/C ratios in the two sample types were assessed by Pearson or Spearman correlation and intra-class correlation coefficient (ICC) using mixed models for repeated measurements, respectively.
Results: The mean 24h urine P/C ratio was 3.2 ± 4.9. Overall, RSU P/C ratio correlated strongly with the 24h urine P/C ratio (r=0.944, p<0.001) with an excellent agreement (ICC=0.949, 95% confidence interval [CI]: 0.69-1.00). Subgroup analyses revealed that the correlation remained high in class II, III, IV, and V LN (rho=0.868, p<0.001; rho=0.649, p=0.007; r=0.945, p<0.001; and rho=0.900, p=0.001, respectively). The correlation between the 24h urine and RSU P/C ratio in the range of 0.5 to 3 was good (r=0.720, p<0.001) with ICC of 0.659 (95%CI 0.554-0.812). RSU P/C ratio ≥0.5 could predict 24h PCR ≥0.5 with 91.7% sensitivity and 70.2% specificity, whereas RSU P/C ratio ≥1.0 increased specificity up to 94.7%.
Conclusions: The RSU P/C ratio is an excellent alternative to the 24 hour P/C ratio for assessing the presence of clinically significant proteinuria in LN. RSU P/C ratio >1.0 may prompt directly to a renal biopsy, whereas RSU P/C ratio between 0.5-1.0 should be followed by a confirmatory 24h urine collection.