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SAT0404 Spot Urine Protein/Creatinine Ratio is Useful in Screening for Proteinuria but Should Not Substitute 24 hours Urine Collection Sample to Quantify Proteinuria in Lupus
  1. J.E. Medina,
  2. D.D. Gladman,
  3. M.B. Urowitz,
  4. J. Su,
  5. A. Sabapaty,
  6. Z. Touma
  1. Internal Medicine, Division Of Rheumatology, University of Toronto, Toronto, Canada

Abstract

Background The 24 hr urine (24H-P) collection sample is the gold standard test for proteinuria measurement in lupus but this test is cumbersome for the patients. Spot Urine Protein Creatinine Ratio (S-UPCR) in single voided urine sample has been accepted to measure proteinuria instead of 24H-P. There is no agreement amongst the existing studies on the utility of S-UPCR vs. 24H-P in screening/monitoring proteinuria. The variance between the studies is related to the use of inappropriate statistical analyses.

Objectives To determine the utility of S-UPCR as a screening and diagnostic test in the assessment of proteinuria in lupus.

Methods This is a retrospective analysis performed on the data from a single lupus cohort on patients seen between May 2008-December 2014. Inclusion criteria: Laboratory records with non-missing data for S-UPCR and 24H-P done at the same time. Exclusion criteria: Patients with diabetes mellitus, end stage kidney disease and kidney transplantation. The main variables were 24H-P, S-UPCR and the ratio of 24H-P and creatinine (24H P/C).

Laboratory records were divided into 4 groups according to 24H-P: Group 1: <0.5, group 2: 0.5-0.99, group 3:1-1.99, and group 4:≥2 g/day. Descriptive statistics were used for continuous/ categorical variables. Pearson correlation coefficient was measured for 24H-P and S-UPCR. Agreement was determined by Inter Class Correlation Coefficient (ICC), Concordance Correlation Coefficient (CCC) and Bland-Altman plot between 24H P/C and S-UPCR.

The best cut off for S-UPCR comparable for 24H-P of 0.5 g/day was determined with ROC curve and linear regression (controlling for age and sex).

Results 1730 laboratory records from 421 patients were retrieved; 85.3% women, 52.5% Caucasian and mean disease duration 11.61±9.56 years.

24H-P and U-SPCR correlation: For all records n=1730 r =0.73; group1 n=923 r=0.3; group 2 n=297 r=0.2; group 3 n=246 r=0.2; and group 4 n=264 r=0.5 (all p significant). Although the correlation of all records is strong, the results of the correlations among different groups showed weak to moderate correlation.

ICC(2, k) for 24H P/C and U-SPCR: All records ICC=0.85; group 1 ICC=0.41; group 2 ICC=0.67; group 3 ICC=0.62 and group 4 ICC=0.78. CCC for 24H P/C and U-SPCR: All records CCC=0.84; group 1 CCC=0.47; group 2 CCC=0.57; group 3 CCC=0.55 and group 4 CCC=0.81. ICC and CCC should have exceeded 0.9 to ensure reasonable validity.

Bland-Altman plot: For proteinuria ≥0.5-2g/day (groups 2, 3 and 4), the agreement between 24H P/C and U-SPCR is poor (Figure 1). U-SPCR overestimated the amount of proteinuria compared to 24H-P.

ROC curve: U-SPCR of 0.08 g/mmol (800 mg/mmol) has the best sensitivity (93%) and specificity (72%) for screening for proteinuria when compared to 24H-P (cut of 0.5 g/day). The linear regression analysis also identified 0.08 as the best cut off.

Conclusions The correlation/agreement (Pearson, ICC, CCC and Bland-Altman) analyses confirmed that U-SPCR has no reasonable validity in accurately measuring proteinuria compared to 24H-P. The results of the ROC showed that U-SPCR can be used as a screening test (qualitative test) and the best cut off for 24H-P of 0.5 g/day is 0.08. 24H-P should be the gold standard test to accurately measure proteinuria especially to follow a patient's response to therapy.

Disclosure of Interest None declared

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