Urinary excretion of 2-methyl-2,3-dihydroxybutyric acid and other metabolites, such as S-(2-carboxypropyl)-cysteine, S-(2-carboxypropyl)-cysteamine, and N-acetyl-S-(2-carboxypropyl)-cysteine, is useful to diagnose ECHS1 deficiency. Here, ECHS1 is linked to hyperinsulinemic hypoglycemia, familial, 4.