Random Spot Urine Protein/Creatinine Ratio Is Unreliable for Estimating 24-Hour Proteinuria in Individual Systemic Lupus Erythematosus Nephritis Patients

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Abstract
Background: Recently the American Rheumatologic Association (ARA) recommended random spot urine protein/creatinine ratio (P/C) to monitor systemic lupus erythematosus (SLE) glomerulonephritis (GN). Shortly afterward, 2 works were published, designated Study 1 and Study 2, which are the only studies to test spot P/C in SLE GN. Here we evaluate Study 1 and Study 2, which came to different conclusions. Methods: Study 1 compared spot P/C to the P/C of intended 24-hour collections >50% complete, which reliably estimates 24-hour proteinuria. Study 2 compared spot P/C to the protein content of intended 24-hour collections >80% complete. To compare studies, Study 2 data were converted to P/C ratios. Results: Study 1 and Study 2 were found to be in agreement. Both showed that spot P/C and 24-hour P/C were highly correlated, but only when compared over the entire P/C range (0–8.0) (r = 0.842). Over the P/C range 0.5–3.0 (the most common P/C range encountered in SLE GN), correlation was present, but concordance was poor, rendering random P/C ratio unreliable. Conclusions: Random spot P/C ratio is unreliable for detecting moderate proteinuria change. For example, random spot P/C would not reliably diagnose British Isles Lupus Assessment Group (BILAG) Category A or B proteinuric flares.