Background/Purpose: The Protein/Creatinine ratio in a spot urine sample (PCR) has been accepted as a substitute for the protein content in a 24 hours urine collection (24H-P), but there is not enough evidence on its usefulness for the prospective follow up of lupus patients.  The purpose of this study was to determine if PCR can accurately measure the change in proteinuria level over time when compared to the “gold standard” test 24H-P.

Methods: Analysis on data from a single lupus centre between May 2008 and December 2014 was conducted. Results of concurrent urine samples for PCR and 24H-P were identified. Patients with diabetes mellitus, end stage kidney disease and kidney transplantation were excluded.

The gold standard test in this study is 24H-P. At baseline visit, urine samples with abnormal 24H-P (³0.5 g/day) and normal 24H-P (<0.5 g/day) were identified. In the group with abnormal 24H-P, the first follow up visit for each patient with 50 and 100% improvement, based on 24H-P, was identified and compared to the PCR results at this visit. In the group with normal 24H-P, the first follow up visit for each patient with worsening (³0.5 g/day) based on 24H-P was identified and compared to the PCR results at this visit. Abnormal PCR is >0.05 g/mmol.

Standardized Response Mean (SRM) for 24H-P and PCR for 50 and 100% improvement and for worsening were calculated. We hypothesized that both tests (24H-P and PCR) will change in the same direction but the effect captured by 24H-P is larger than PCR.

Results: 

1188 paired samples from 230 patients.

• 24H-P 100% improvement: 60 patients started with abnormal 24H-P (³0.5 g/day) and had 100% improvement on follow up. Mean duration for improvement: 17.5 months. Among this group, only 53.3% had 100% improvement based on PCR. SRM for 24H-P: -1.00 (95% CI: -1.31 to -0.69) (large effect). SMR for PCR: -0.69 (95%CI: -0.97 to -0.41) (moderate effect).
• 24H-P 50% improvement: 64 patients started with abnormal 24H-P (³0.5 g/day) and had 50% improvement on follow up. Mean duration for improvement: 13.6 months. Among this group, only 56% of patients had 50% improvement based on the PCR. SRM for 24H-P: -1.27 (95% CI: -1.61 to -0.95) (large effect). SMR for PCR is -0.63 (95% CI: -0.90 to -0.36) (moderate effect).
• 24H-P worsening: 43 patients started with normal 24H-P (<0.5 g/day) and became abnormal (³0.5 g/day) on follow up. Mean duration for worsening of proteinuria: 10.5 months. At first visit all patients had normal 24H-P but 34.9% had abnormal PCR (≥0.5 mg/g). At the first visit with abnormal 24H-P, only 79.1% of these patients had abnormal results by PCR. SRM for 24H-P: 0.81 (95% CI: 0.46 to 1.15) (large effect). SMR for PCR: 0.53 (95%CI: 0.21 to 0.85) (moderate effect).

Magnitude of change for improvement (50 and 100%) and worsening: SRM_24H-P > SRM_PCR

Conclusion: PCR is not as accurate as 24H-P in determining clinically important change in proteinuria levels (50, 100% improvement and 100% worsening) in lupus nephritis. Thus 24 H-P should be used to monitor improvement or deterioration in LN in clinical practice.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/how-accurate-is-spot-urine-proteincreatinine-ratio-in-measuring-the-change-over-time-in-proteinuria-level-compared-to-the-24-hour-proteinuria-test/