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 m², 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.

clearance; d-asparagine; d-serine; fractional excretion; glomerular filtration rate; kidney transplantation.