Viewpoint On
Gutiérrez-Peredo Gabriel B., Montaño-Castellón Iris, Gutiérrez-Peredo Andrea Jimena, Aguilar Ticona Juan P., Montaño-Castellón Fabio, Batista Oliveira Filho José César, Pinto Almeida Antonio Raimundo: Comparison of urinary protein/creatinine ratio as an alternative to 24-hour proteinuria in Lupus Nephritis: TUNARI study. Nephron DOI 10.1159/000531333
The “gold standard” for evaluation of urinary total protein excretion is a timed (preferably 24-hour) collection (uPER). But compliance (inability to accurately collect the specimen) can limit its utility. The comparison (ratio) of the urinary total protein concentration and urinary creatinine concentration (uPCR) in an untimed, random “spot” urine collection is commonly used as an approach to overcome the limitations of uPER, but this adds a degree of variability due to fluctuation of protein and creatinine excretion during the day and night. This variability may limit the utility of uPCR, especially in lupus nephritis.
Gutierrez-Peredo and colleagues address this issue by comparing the performance of uPER (training provided but collections unsupervised) and random “spot” uPCR in 75 patients with lupus nephritis of varying severity under treatment with steroids, immunosuppressants, and RAS inhibitors.
As is well known, the two tests are well correlated (r2 = 0.59), but the scatter of results was high and tended to increase with higher grades of proteinuria. Interestingly, the median uPER was 1.2 g/24 h and the uPCR was 1.4 g/g. These values suggest systematic under-collection of timed urine or low urine creatinine excretion or both. The predicted and measured 24-hour urine creatinine excretion is not given. While the AUC of the uPCR for prediction of uPER is fairly high (0.92), this statistic is insufficient to judge the clinical utility of uPCR as an estimate of uPER. A test of misclassification errors would have been better. The spread of the values of uPCR relative to uPER tells a story of substantial variability. A compromise of using an aliquot of a presumed 24-hour collection determining the uPCR was not tested. I prefer this latter metric for assessing proteinuria by uPCR, as it avoids much of the variability of the uPCR collected randomly by “spot” urine specimens.
