Personal non-commercial use only. The Journal of Rheumatology. Copyright © 2004. All rights reserved Christopher-Stine, et al: Protein:creatine ratio in lupus 1557 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. Quantitation of proteinuria by 24-hour urine collection is a cornerstone of monitoring disease activity in patients with lupus nephritis 1-3 . Such collections, however, are often inac- curate due to collection errors 4-6 . According to the National Kidney Foundation KDOQI Guidelines, proteinuria can be accurately assessed by the use of the urine protein-to-creati- nine ratio (U pr:cr) 7 . The ratio is determined by dividing the urine protein (mg/dl) by the urine creatinine (mg/dl). The numerical outcome of the ratio is roughly equal to the 24-h protein excretion in g/day per 1.73 m 2 body surface area 8 . The validity and reliability of this method has been vali- dated in diabetic 9 and nondiabetic 10 nephropathy. An addi- tional prospective cross-sectional study was performed in patients with various glomerular diseases to determine the accuracy of predicting 24-h proteinuria from the Upr:cr ratio. A good correlation and precision of agreement were found between the 2 methods across a wide range of urinary protein, regardless of the level of renal function 11 . There is, however, a paucity of data regarding the utility of Upr:cr ratio for a 24-h urine collection in monitoring proteinuria in lupus nephritis 12 . In this preliminary study, our objective was to evaluate the use of the Upr:cr ratio compared to 24- h urine protein excretion within the same 24-h urine collec- tion as a measure of proteinuria in a cohort of patients with lupus nephritis undergoing intravenous cyclophosphamide therapy. MATERIALS AND METHODS Proteinuria in 8 patients with biopsy-proven lupus nephritis treated with cyclophosphamide was monitored by total protein excretion and Upr:cr ratio in 24-h urine collection. There was a broad 24-h protein range from 112 mg/day to 8456 mg/day. A median of 16 (range: 9-22) measurements per patient was collected over a median of 47 months (range: 18-90). The adequacy of the 24-h collection was assessed by comparing the total crea- tinine in the sample to the predicted creatinine [(22–(age/9)*kg in women and 28–(age/6)*kg in men] 8 . 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% was defined as an under-collection [(predicted–measured)/predicted × 100 > 20%]. Likewise, collections in 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. Objective. To evaluate the 24-hour urine protein-to-creatinine (U pr:cr) ratio compared to 24-h urine total protein excretion as a measure of proteinuria in patients with lupus nephritis. Methods. Proteinuria in 8 patients with lupus nephritis treated with cyclophosphamide was moni- tored by total protein excretion and U pr:cr ratio in 24-h urine collections. A median of 16 measure- ments 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 (R 2 = 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-collec- tions 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 LUPUS NEPHRITIS PROTEINURIA PROTEIN-TO-CREATININE RATIO Personal, non-commercial use only. The Journal of Rheumatology. Copyright © 2004. All rights reserved.