Determination of urine volume and glomerular filtration rate using d-serine and d-asparagine

Abstract Background and hypothesis Measurement of glomerular filtration rate (GFR) is subject to inaccurate urine collection. Clearances of d-serine and d-asparagine, rare enantiomers of amino acids, are the measures of GFR since they are almost free of tubular secretion and reabsorption. We hypothesize that d-serine and d-asparagine can accurately determine urine volume and GFR. Methods This cross-sectional observational study included 209 living kidney transplant donors and recipients for whom GFR was measured using the clearance of inulin. Assuming that urine excretions of d-serine and d-asparagine are constant and using urine levels of d-serine and d-asparagine from each urine collection, an equation for estimated urine volume (eUV) was established. Based on the eUV, the abnormal urine volume was replaced with an estimate with which the GFR was re-evaluated. Result Clearances of d-serine and d-asparagine were minor in proportional biases when compared with that of creatinine. Using 627 urine collection, the equation for eUV (mL/min) was established as 21.88/urine d-Ser (0.40 + 0.20 × log10(urine d-Asn)). Using eUV, we identified 20 instances where urine collection volumes varied significantly from the estimated values. After replacement with eUV, measured GFR (mGFR) was corrected to adjusted mGFR, which was within approximately 20 mL/min/1.73m2 of the mGFR. Conclusion d-Serine and d-asparagine are nearly completely excreted in urine after glomerular filtration, enabling the estimation of urine volume and correct mGFR. Besides reflecting GFR, d-serine and d-asparagine can be used to estimate urine volume. By applying the eUV method, mGFR determined using clearance methods becomes more accurate.