Objectives: The quantification of 24 hrs urinary protein excretion is valuable for diagnosing and monitoring renal disease. However, because of its practical difficulties, the spot urinary protein/creatinine (P/C) ratio has been utilized. We aimed to evaluate the analytical performance of P/C ratio by comparing with the qualitative urinary protein values and the microscopic urine sediment analysis.
Methods: We obtained 5,538 urinary samples from the outpatients of Juntendo University Hospital. Testing for urinary P/C ratio was performed by Atlas Pro12 (cut-off 150 mg/g x Cr), urinary protein (proteinuria) was detected quantitatively by full-automated system ATLAS XL (cut-off 30 mg/dL). Microscopic exams were conducted following to the JCCLS reference method.
Results: The P/C ratio demonstrated higher sensitivity but lower specificity for urinary abnormal casts detected by microscopic exams compared to proteinuria (sensitivity; P/C 87%, proteinuria 77%. specificity; P/C 74%, proteinuria 93%). From the comparative study with microscopic exams, both P/C and proteinuria performed high positive rate (> 80%) for the granular cast type and mixture cast type. For the cellular cast type, however, the positive rate of P/C was 56% and that of proteinuria was only 36%. The overall abnormal casts by microscopic exams showed better correlation with the positive P/C ratio than proteinuria.
Conclusion: This study emphasizes that a spot urine P/C ratio is useful in screening for the further microscopic exams. P/C ratio can be a convincing index of urinary protein excretion when attenuation urine is doubted.