The annotation of the LC-MS feature as 3-hydroxyhexanoic acid was confirmed by measuring an IRIS spectrum for a reference standard of this molecule (Figure ), which indeed gives high spectral similarity (Sspec = 969) with the spectrum of the LC-MS feature. The stereochemistry of the hydroxide remains ambiguous because it cannot be determined directly from an IR measurement. In general, 3hydroxycarboxylic acids are biomarkers for fatty acid oxidative disorders of both long-and short-chain 3-hydroxyacyl-CoA dehydrogenases . Specifically, 3-hydroxyhexanoic acid is increased in the serum of diabetic ketoacidotic patients and has been observed in body fluids of patients with 3-hydroxy-3methylglutaryl-CoA synthase deficiency and metastatic melanoma . The patient in this case was in ketosis as exemplified by the high body fluid concentrations of acetoacetate and 3-hydroxybutyric acid. After the identification of this m/z 131.0713 feature, we observed 3-hydroxyhexanoic acid in many plasma samples of ketotic patients. As such 3-hydroxyhexanoic acid may serve as ketosis biomarker.