The relatively poor correlation between random and 24-hour urine protein excretion in patients with biopsy-proven glomerular diseases

Random urine protein creatinine ratios are used to estimate 24-hour urine protein excretion, which is considered a diagnostic gold standard. However, few studies are available of the sensitivity and specificity of this estimation in patients with glomerular proteinuria. To clarify this, we measured the urine protein and creatinine centrally in random and 24-hour urine collections at biopsy and longitudinally every 6 months in individuals participating in the Nephrotic Syndrome Study Network (NEPTUNE) cohort with glomerular disease. In the initial developmental cohort, 302 patients had same day random and 24-hour samples with a total of 827 paired measurements across all visits. The protein excretion (g/day) was higher in adult than pediatric patients. The correlation between the random urine protein creatinine ratio and 24-hour urine protein excretion was moderate in both groups (r of 0.60 and 0.67, respectively). However, the log₁₀ transformation of values strengthened correlations in both groups (r of 0.85 and 0.82, respectively). Associations were moderately stronger among obese patients. Prediction equations were developed and validated in 232 unique cases from NEPTUNE (R² of 0.65). Thus, in patients with glomerular disease and proteinuria, the urine protein creatinine ratio correlates only moderately with 24-hour urine protein excretion. However an estimating equation was developed to derive 24-hour urine protein excretion from random urine protein creatinine ratio values with improved precision. The long-term prognostic value of log₁₀-transformed random protein creatinine ratios values requires future study.