Spot Urine Protein To Creatinine Ratio Corrected With Estimated Urine Creatinine Generalized From Cystatin C Is Invaluable For Estimating Daily Urinary Protein In Patients With Decreased Muscle Mass.

Abstract Background: Spot urine protein to creatinine ratio (sUP/Cr) is used to evaluate proteinuria; however, conventional equations do not reflect 24-hour (24H) urine creatinine (24HUCr), especially in patients with decreased muscle mass. This study aimed to establish a methodology for estimating 24HUCr in patients with decreased muscle mass and to assess whether multiplying the sUP/Cr with estimated 24HUCr would improve the accuracy of 24HUP estimation.Methods: Nine patients with decreased muscle mass and 14 patients with normal muscle mass were included. Serum samples were obtained during 24H urine collection. Serum creatinine (sCr) and cystatin C (sCysC) as well as urine creatinine and protein concentrations were measured. The correlation coefficient (R) between the estimated glomerular filtration rate (eGFR) calculated with sCysC (eGFRCysC) and 24H creatinine clearance was obtained. We defined the formula as eGFR = R × [(UCr × 24H urine volume × 1.73)] / [(sCr × 1440 × body surface (m2)] and then e24HUCr (UCr × 24H urine volume) = [eGFR × sCr × 1440 × body surface (m2)] / (1.73 × 100 × R).Results: The new formula, i.e., e24HUCr = eGFRCysC × sCr × 1440 × BS/1.73 × 100 × 0.97, was used to estimate 24HUCr. The sUP/Cr corrected with e24HUCr from the new formula was closer to the actual 24HUP compared to that corrected with e24HUCr derived from existing formulas.Conclusion: The sUP/Cr corrected with the estimated urine creatinine generalized from sCysC is useful for estimating 24HUP in patients with normal and decreased muscle mass.