Protein-to-creatinine ratio in spot urine samples as a predictor of quantitation of proteinuria

Background: Normal individuals usually excrete very small amounts of protein in the urine. Persistently increased protein excretion is usually a marker of kidney damage. Quantifying protein in urine is commonly used in the diagnosis of kidney diseases, detection of treatment effects and evaluation of prognosis. We evaluated the use of the total protein-to-creatinine ratio (P/C) in spot urine specimens as a predictor of urine protein excretion in 24-h collections. Methods: The correlation between P/C in first morning and random urine specimens and urinary protein excretion in 24-h collections were analyzed. The cutoff value of P/C in first morning urine specimens for screening urinary protein excretion of I and 3 g in 24-h collections was determined by receiver operating characteristics (ROC) curve. Results: For patients with Ccr less than or equal to 10 ml/min, correlation between the urine protein excretion in 24-h collections and the P/C in first morning urine specimens was not significant. For patients with Ccr > 10 ml/min, the correlation was highly significant. Similar results were obtained for random urine specimens. By ROC curve analysis, the P/C of 0.94 and 2.84 g/gcr in first morning urine specimens represent the best threshold to detect urine protein excretion of 1 and 3 g in 24-h collections, respectively. There is a good correlation between P/C in first morning urine specimens and random urine specimens from inpatients and outpatients. But the P/C in random specimens is significantly higher than that in first morning specimens in outpatients. Conclusion: The P/C in spot urine samples could be used as an alternative to urine protein excretion in 24-h collections in patients with Ccr >10 ml/min. The P/C in first morning urine samples is better than that in random specimens, especially for outpatients. (C) 2004 Elsevier B.V. All rights reserved.