The urine protein to creatinine ratio as a predictor of 24-hour urine protein excretion in type 1 diabetic patients with nephropathy☆

Abstract The purpose of this study was to determine the usefulness of a random urine specimen protein to creatinine (P/C) ratio in predicting 24-hour urine protein excretion (24 UP) in type 1 diabetic patients with overt nephropathy. Two hundred twenty-nine outpatient diabetic subjects enrolled in the Collaborative Study Group's multicenter clinical trial of "Angiotensin-Converting Enzyme Inhibition in Type 1 Diabetic Nephropathy" provided specimens for study, which encompassed a wide range of proteinuria (0.05 to 13.3 g/d). Twenty-four hour urine collections for total protein and creatinine (g/d) were obtained in the outpatient setting. This was followed shortly thereafter by an untimed single urine specimen for protein and creatinine (mg/dL). For longitudinal analysis, 33 patients provided two 24-hour urine collections with concomitant random urine specimens, separated by at least a 3-month period. Across the range of proteinuria that we studied, the log random urine P/C ratio correlated to log 24 UP ( r = 0.90). The regression line was almost identical to the line of unity, which indicates that a patient's 24 U/P (in g/d) can be predicted directly from the random urine specimen P/C ratio (P/C = 24 UP in g/d). However, the standard deviations associated with these predictions were large, especially at the higher 24 UP values. Of the 33 patients who provided two time-separated specimens, the direction of change in P/C ratio was discordant with the direction of change in 24 UP in 14 of the 33 repeat specimens. We conclude that in patients with type 1 diabetes and nephropathy, the P/C ratio of a random urine specimen may be used to estimate a range of 24 UP excretion. The reliability of predictions is best when used in patients with lower levels of proteinuria, becoming less accurate in the nephrotic range and higher. We recommend caution in applying overconfidence to the values obtained with this technique and cannot recommend using this technique in following trends of proteinuria, or in following a patient's response to therapy.