Correlation of 24-hour urinary protein quantification with spot urine protein:creatinine ratio in lupus nephritis

Twenty-four hour urine collection has been the foundation for monitoring patients with lupus nephritis. However, the use of protein to creatinine ratios in spot urine samples is now widely used. We aimed to evaluate the validity of this method cross-sectionally and longitudinally.

A cross-sectional retrospective study was conducted. Records of 486 lupus nephritis patients were searched for paired results of 24-h quantification of urinary protein and a random spot urine protein to creatinine ratio and were examined over a three-year period.

Ninety-five lupus nephritis patients had paired results and were included in the final analysis, male/female 14/81, mean age 36.5 years. Over a three-year period there were a total of 137 samples from 95 patients. For the entire dataset, there was a significant correlation between protein:creatinine ratio and 24-h urine collection protein (mg), Spearman Rho correlation coefficient was 0.869, p < 0.0001 with (R (2 )= 0.504). There was also a strong correlation for longitudinal data, n = 14 at two-years Rho 0.910, p < 0.0001 with (R (2 )= 0.878), n = 8 at three-years Rho 0.909, p < 0.0001 and (R (2 )= 0.73). We have shown for the first time in a UK population of lupus nephritis patients, well trained in producing 24-h collection, that the spot protein:creatinine ratio correlates well with 24-h urinary total protein excretion. Having a simple, reliable, reproducible and cost-effective test such as the spot urine protein:creatinine ratio is therefore a valuable tool with which to monitor disease progression.