We experienced a case of a 36-year-old female with rapidly progressive glomerulonephritis (RPGN) due to anti-neutrophil cytoplasmic antibody (ANCA)-associated nephritis and systemic lupus erythematosus (SLE) nephritis. Chiral amino acid metabolomics revealed a prominent profile of D-serine in this patient. At the fulminant period of RPGN, the level of plasma D-serine, a potential biomarker in CKD that reflects actual glomerular filtration ratio (GFR), was extremely high. On the other hand, urinary fractional excretion (FE) of D-serine, which was usually much higher than that of L-isoform, was 0% in this patient. These abnormal D-serine profiles normalized in response to the intensive treatment. Normalizations of blood D-serine levels were in parallel with those of blood creatinine levels and potentially reflect the recovery of GFR. FE of D-serine increased transiently before the normalization of D-serine profile, suggesting that kidney promotes urinary excretion of D-serine for the normalization of plasma D-serine level. These unexplored clinical features of D-serine well reflected the clinical course of this patient. Blood D-serine level can also serve as a biomarker in acute kidney injury (AKI) or RPGN, and, in combination with FE of D-serine, may render the clinical practitioners to judge the efficacy of intensive treatments.
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