A reference range for endogenous gamma-hydroxybutyrate in urine by gas chromatography-mass spectrometry.

Gamma-hydroxybutyrate (GHB) has been implicated in drug-facilitated sexual assault (DFSA). The interpretation of GHB levels in biological samples collected for evidence is complicated by the natural presence of this compound in the body, and by its extremely rapid elimination after ingestion. There is a lack of agreement regarding a suitable cut-off concentration, which can reliably separate endogenous concentrations in urine from those reflecting ingestion. We have developed a method for the analysis of low levels of GHB in urine and have used it to establish a reference range for normal females. The method uses liquid-liquid extraction, silyl-derivatization, and gas chromatographic-mass spectrometric analysis. The limit of detection was 0.1 mg/L, and the method was linear from 0.1 to 5.0 mg/L. Our analysis of 50 urine samples donated by normal women indicates an upper limit of normal for urinary GHB of 1.46 mg/L or 323 microg GHB/mmol of creatinine. We propose that a 5 mg/L cut-off for urine GHB concentration, or 1000 microg GHB/mmol creatinine, will separate endogenous GHB concentrations from those reflecting GHB ingestion in antemortem samples with greater than 99% confidence, providing that a specific assay method comparable with that we describe is used. We demonstrate that urinary GHB concentrations fall with age and that this can be corrected for by measurement of the GHB/creatinine ratio.