D-Asparagine is an Ideal Endogenous Molecule for Measuring the Glomerular Filtration Rate

INTRODUCTION: An ideal endogenous molecule for measuring glomerular filtration rate (GFR) is still unknown. However, a rare enantiomer of serine, d-serine, is useful in GFR measurement. This study explored the potential of other d-amino acids for kidney function assessment. METHODS: This was a cross-sectional observational study of 207 living kidney transplant donors and recipients, for whom GFR was measured using clearance of inulin (C-in). Associations between levels of d-amino acids and GFR were analyzed using multivariate factor analysis. Fractional excretion (FE), a ratio of the clearance of a substance to C-in as a standard molecule, was calculated to monitor the excretion ratio after glomerular filtration. Dissociation from an ideal FE of 100% was assessed as a bias. Proportional bias against C-in was calculated using Deming regression. RESULTS: Multivariate analysis identified the blood level of d-asparagine to reflect GFR. Means of blood d-asparagine and clearance of d-asparagine (C-d-Asn) were 0.21 μM and 65.0 ml/min per 1.73 m(2), respectively. Inulin-based FE (FE(in)) of d-asparagine was 98.67% (95% confidence interval [CI]: 96.43–100.90%) and less biased than those of known GFR markers, such as FE(in) of creatinine (147.93 [145.39–150.46]; P < 0.001) and d-serine (84.84 [83.22–86.46]; P < 0.001). A proportional bias of C-d-Asn to C-in was −7.8% (95% CI, −14.5 to −0.6%), which was minor compared to those of clearance of creatinine (−34.5% [−37.9 to −31.0%]) and d-serine (21.2% [13.9–28.9]). CONCLUSION: D-Asparagine acts similar to inulin in the kidney. Therefore, d-asparagine is an ideal endogenous molecule that can be used for GFR measurement.