Urinary betaine/creatinine ratio enhances diagnostic accuracy of PLA2R-Ab in idiopathic membranous nephropathy.

BACKGROUND: The sensitivity of serum anti-phospholipase A2 receptor antibody (PLA2R-Ab) in diagnosing idiopathic membranous nephropathy (IMN) remains suboptimal, underscoring the need for complementary biomarkers, particularly in seronegative cases. This study aimed to identify urine metabolic signatures associated with IMN to improve diagnostic insight. METHODS: Untargeted liquid chromatography-tandem mass spectrometry (LC-MS/MS) metabolomic profiling was performed on urine samples from IMN patients and healthy controls (HCs). Differential metabolites were identified using multivariate statistical analyses and further evaluated through metabolic network and pathway enrichment analyses. Key metabolites were subsequently validated using targeted metabolomics and ELISA in an extended cohort comprising 53 IMN patients (30 PLA2R-Ab positive and 23 PLA2R-Ab negative), 30 HCs, 23 patients with IgA nephropathy (IgAN), and 7 with minimal change nephropathy (MCN). Core metabolites were validated using 5-fold cross-validation. RESULTS: Untargeted metabolomics revealed 116 significantly altered metabolites in IMN patients. Enrichment analysis highlighted notable perturbations in pathways including Ferroptosis, Taurine and hypotaurine metabolism, and Glycine, serine, and threonine metabolism. Among eight quantitatively validated metabolites, urinary betaine was consistently elevated in IMN. The urinary betaine-to-creatinine ratio (Betaine/uCr) was significantly higher in the IMN group compared with HCs and IgAN patients (all p < 0.05), and was independent of common clinical parameters. Betaine/uCr alone discriminated IMN from HCs with an AUC of 0.83 and exhibited high specificity (96%) in distinguishing IMN from IgAN. Importantly, combining Betaine/uCr with PLA2R-Ab serology improved diagnostic performance (AUC = 0.91) compared to either marker alone. 5-fold cross-validation showed that the results were stable and reliable. CONCLUSIONS: Urinary metabolomics uncovered characteristic metabolic disturbances in IMN, involving pathways related to podocyte injury and osmotic regulation. The betaine/uCr showed preliminary potential as an auxiliary diagnostic marker when combined with PLA2R-Ab, though its specificity required further investigation. Therefore, broader studies are needed to establish its clinical utility.