Background In 2006, the American College of Rheumatology (ACR) Ad Hoc Committee on Systemic Lupus Erythematosus (SLE) recommended the urinary protein level in a spot (untimed) urine specimen (determined as urinary protein-to-urinary creatinine ratio; spot P/C ratio) over a 24-hour urine protein excretion. In addition, the committee stated that renal function refers to the estimated glomerular filtration rate (GFR) and selected the MDRD study equation. However, several subsequent reports suggested that the spot P/C ratio may be inaccurate in the assessment of the degree of proteinuria in lupus nephritis (LN). In addition, there is no consensus as to which estimating equation is preferred for estimated GFR in LN.
Objectives We aimed to evaluate the validity of spot P/C ratio and estimated GFR by the modified MDRD study equation as measures of proteinuria and renal function in patients with LN.
Methods A total of 61 patients with active LN who were admitted to our hospital from 2010 through 2015 were included. LN was pathologically confirmed in 51 patients and renal biopsy was not performed in the other 10 patients. In addition, 22 SLE patients without active LN in whom spot P/C ratio, 24-hour urine protein excretion, estimated GFR, and 24-hour urine creatinine clearance were measured were also included. All the patients met the revised ACR classification criteria for SLE. Clinical and laboratory data were retrospectively collected from the electronic medical records and statistically analyzed.
Results The spot P/C ratio and the 24-hour urine protein excretion were moderately correlated (n=64, Pearson's r =0.62). However, the Bland-Altman plot demonstrated proportional bias (progressive deviation with values). Agreement of the spot P/C ratio ≥0.5 and the 24-hour urine protein excretion ≥0.5 g was moderate (Cohen's κ =0.56) whereas that of ≥0.2 was almost perfect (κ =0.83). Measured 24-hour urine creatinine excretion ranged from 220 mg to 2120 mg; the mean and median values were 860 mg and 840 mg, respectively. Among the 10 patients in whom serial data could be obtained in more than 3 times, the spot P/C ratio and the 24-hour urine protein excretion were highly correlated (r =0.87 to 0.99), except for 2 patients (r = -0.30 and 0.68, respectively). In these serial samples derived from the 8 patients, there was substantial agreement between the spot P/C ratio and the 24-hour urine protein excretion about whether increased or decreased compared to previous data (κ =0.65). The estimated GFR by the modified MDRD study equation and the body surface area (BSA) corrected 24-hour urine creatinine clearance were moderately correlated (n=81, r =0.62). However, the Bland-Altman plot demonstrated both fixed and proportional bias. Agreement of the estimated GFR and the BSA-corrected 24-hour urine creatinine clearance ≥60 (ml/min/1.73 m2) was moderate (κ =0.48).
Conclusions Our results supported the validity of spot P/C ratio and estimated GFR by the modified MDRD study equation as convenient screening and monitoring measures of proteinuria and renal function in patients with LN. However, agreement between these convenient measures and measures based on 24-hour urine collection is not always good. Spot P/C ratio and estimated GFR should be used with attention to their limitation.
Disclosure of Interest None declared