Stable isotope dilution analysis of 4‐hydroxybutyric acid: An accurate method for quantification in physiological fluids and the prenatal diagnosis of 4‐hydroxybutyric aciduria

A quantitative assay for 4‐hydroxybutyric acid was developed using D₆‐4‐hydroxybutyric acid as an internal standard. 4‐Hydroxybutyric acid was isolated by liquid chromatography and the amount quantified by selected ion monitoring, ammonia chemical ionization gas chromatography/mass spectrometry of the trimethylsilyl derivatives. The concentrations of 4‐hydroxybutyric in control physiological fluids were: 2.64 ± 3.46 mmol mol⁻¹ creatinine in urine, 1.09 ± 2.87 μmol l⁻¹ in plasma, 0.98 ± 1.17 μmol l⁻¹ in cerebrospinal fluid and 1.28 ± 0.47 μmol l⁻¹ in amniotic fluid. The concentration of 4‐hydroxybutyric acid in the amniotic fluid from a pregnancy at risk for 4‐hydroxybutyric aciduria was 2.30 μmol l⁻¹, indicating an unaffected fetus. The stable isotope dilution assay of 4‐hydroxybutyric acid in physiological fluid samples is a rapid, sensitive and accurate method for quantification, as well as a valuable technique for the prenatal diagnosis of 4‐hydroxybutyric aciduria.