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Rheumatology Advance Access originally published online on October 25, 2006
Rheumatology 2007 46(4):649-652; doi:10.1093/rheumatology/kel360
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© The Author 2006. Published by Oxford University Press on behalf of the British Society for Rheumatology. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Urine protein-to-creatinine ratio in an untimed urine collection is a reliable measure of proteinuria in lupus nephritis

Y. Y. Leung, C. C. Szeto1, L. S. Tam1, C. W. K. Lam2, E. K. Li1, K. C. Wong1, S. W. Yu1 and E. W. Kun

Department of Medicine and Geriatrics, Tai Po Hosptial, 1Department of Medicine and Therapeutics, and 2Department of Chemical Pathology, Chinese University of Hong Kong, Hong Kong SAR.

Correspondence to: Y. Y. Leung, Department of Medicine and Geriatrics, Tai Po Hospital, 9 Chuen On Road, Taipo, N.T., Hong Kong SAR. E-mail: katyccc{at}hotmail.com


  Abstract

Objective. To evaluate the accuracy of urine protein-to-creatinine (P/C) ratio in an untimed urine specimen as compared with 24 h total protein excretion for measurement of proteinuria in patients with lupus nephritis.

Methods. Proteinuria in patients with lupus nephritis was assessed by 24 h total protein excretion and spot urine P/C ratio. Correlation and limits of agreement between the two methods were evaluated. The discriminant cutoff values for spot urine P/C ratio in predicting 24 h protein ‘threshold’ excretion of ≥0.3, ≥0.5, ≥1.0 and ≥3.5 g/day were determined using receiver operating characteristic curves.

Results. A total of 165 samples were available for assessment with 21.8% excluded due to inadequate collection. A strong correlation (r = 0.91, P < 0.0001) was found between spot urine P/C ratio and 24 h urine protein excretion. Bland–Altman plot showed the two tests had acceptable limits of agreement in low level of protein excretion (–0.86 to +0.92 g/day when protein excretion was <2.0 g/day). The limits became wider as the protein excretion increased. The spot urine P/C ratios of 0.45 (sensitivity 0.92; specificity 0.88), 0.7 (0.92; 0.89) and 1.84 (1.0; 0.86) mg/mg reliably predicted 24 h urine total protein equivalent ‘thresholds’ at ≥0.5, 1.0 and 3.5 g/day.

Conclusion. This study supports the recommendation of using spot urine P/C ratio in screening and monitoring proteinuria in patients with lupus nephritis. However, in assessing the exact amount of proteinuria, the urine P/C ratio may have unacceptably wide limits of agreement in high protein excretion range.

KEY WORDS: Proteinuria, Urine protein-to-creatinine ratio, Lupus nephritis

Submitted 23 August 2006; revised version accepted 19 September 2006.
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