Can spot urine protein/creatinine ratio replace 24 h urine protein in usual clinical nephrology?

BACKGROUND: This prospective study was designed to assess whether a single urine specimen, collected at the time of clinical assessment, could be used to estimate 24 h urinary protein excretion.

METHODS: The spot urinary protein/creatinine ratio (P/Cr) and 24 h urine protein and creatinine excretion were determined from 103 patients attending a nephrology clinic. Using the spot urine P/Cr, a regression equation was derived, predicting 24 h protein excretion. The performance of the regression equation was tested on a new cohort of 85 patients. Within this new cohort, agreement between the predicted and the actual 24 h urinary protein excretion was assessed using the Bland-Altman technique. The ability of spot urine P/Cr and dipstick urinalysis to predict different threshold levels of protein excretion was assessed using a series of receiver operator curves.

RESULTS: A logarithmic relationship exists between the spot P/Cr and 24 h protein excretion, with a correlation of 0.92, P < 0.0001. Agreement between the actual and predicted 24 h urine protein and between spot and 24 h urine P/Cr ratios was suboptimal at higher levels of protein excretion. Discriminant spot urine P/Cr values reliably predicted protein excretion thresholds of 0.3, 0.5, 1.0 and 3.0 g/day. Urinalysis was a poor test for quantitating protein excretion.

CONCLUSION: Random spot urinary P/Cr predicts actual 24 h protein excretion with reasonable accuracy in patients with lower levels of protein excretion but is unreliable in patients with high protein excretion and should not be used in the clinical setting unless 24 h urine collection is unavailable.