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Urine Protein-to-Creatinine Ratio Is a Reliable Measure of Proteinuria in Lupus Nephritis
LISA CHRISTOPHER-STINE, MICHELLE PETRI, BRAD C. ASTOR, and DEREK FINE
ABSTRACT. Methods. Proteinuria in 8 patients with lupus nephritis treated with cyclophosphamide was monitored by total protein excretion and U pr:cr ratio in 24-h urine collections. A median of 16 measurements per patient were collected over a median of 47 months. Adequacy of the 24-h collection was assessed by comparing total urine creatinine to the predicted creatinine. Collections in which the difference between the predicted 24-h urine creatinine and the measured 24-h urine creatinine was greater than or equal to 20% were defined as inadequate collections. Results. Timed 24-h urine collections were frequently inadequate (30.2% of total collections were under-collections, while 14.3% were over-collections). We found 87.5% of patients had at least one under-collection whereas 75% had at least one over-collection. Correlations between total protein and U pr:cr ratio for individual patients ranged from 0.87 to 0.99 (mean 0.95). For the entire sample, the correlation (R2 = 0.89) of total urine protein to Upr:cr ratio was excellent. Excluding the 38 under-collections led to improvement in the overall correlation (0.94). Excluding the 18 over-collections led to a correlation of 0.89. Excluding both under-collections and over-collections led to a correlation of 0.94. Conclusion. In patients with lupus nephritis, the 24-h U pr:cr ratio is highly correlated with the 24-h urine protein excretion when the collections are adequate. The error of the estimate is higher when collections are poor. (J Rheumatol 2004;31:1557-9) Key Indexing Terms:
SYSTEMIC LUPUS ERYTHEMATOSUS
From the Department of Medicine, Divisions of Rheumatology and Nephrology, and the Department of Epidemiology, The Johns Hopkins Bloomberg School of Public Health; and the Welch Center for Prevention, Epidemiology and Clinical Research, The Johns Hopkins University, Baltimore, Maryland, USA. The Hopkins Lupus Cohort is supported by RO1 AR043727 and the General Clinical Research Center M01RR00052. L. Christopher-Stine, MD; M. Petri, MD, MPH, Department of Medicine, Division of Rheumatology; B.C. Astor, PhD, MPH, Department of Epidemiology, The Johns Hopkins Bloomberg School of Public Health, and the Welch Center for Prevention, Epidemiology and Clinical Research; D. Fine, MD, Department of Medicine, Division of Nephrology. Address reprint requests to Dr. L. Christopher-Stine, 1830 East Monument Street, Suite 7500, Baltimore, MD 21205. E-mail: LChrist4@jhmi.edu Submitted October 16, 2003; revision accepted February 27, 2004. |