An unresolved issue: The relationship between spot urine protein-to-creatinine ratio and 24-hour proteinuria

OBJECTIVE: To investigate the relationship between spot urine protein-to-creatinine (sP/Cr) ratio and 24-h protein excretion in patients with different diagnoses. METHODS: This retrospective study analysed data from the medical records of patients admitted for24-h proteinuria determination who also had sP/Cr ratio data for the same day. RESULTS: A total of 1222 urine samples obtained from 694 adult outpatients were analysed. The mean ± SD age of the patients was 53.6 ± 15.9 years. The mean ± SD 24-h proteinuria and sP/Cr were 1.7 ± 2.4 g/day and 1.8 ± 2.4, respectively. The correlation between the sP/Cr and 24-h protein excretion was high (R(2) = 0.89). The sP/Cr ratio accounted for 72% of the variability in 24-h proteinuria in the entire study population. Areas under the curve for 24-h proteinuria at 0.3 g/day, 1.0 g/day and 3.0 g/day were 0.940, 0.966, and 0.949, respectively. The mean + 2SD limits of agreement were between +2.99 and –2.73 g/day according to the Bland Altman analysis. CONCLUSION: This current study found a clinically unacceptable deviation between 24-h proteinuria and sP/Cr ratio. Therefore, the sP/Cr ratio cannot replace 24-h proteinuria. A new method using spot urine protein and creatinine values that is able to minimize under or over estimation is still warranted.