Background: Urine-ethyl glucuronide (EtG) concentrations are considered as a specific marker of recent alcohol consumption. We describe false-positive EtG screening results by the DRI (R) ethyl glucuronide enzyme immunoassay caused by chloral hydrate intake. Methods: Urine-EtG-screening: DRI (R) EtG enzyme immunoassay (Thermo Fisher Scientific Microgenics) on a Hitachi 912 analyzer. EtG- and ethyl sulfate (EtS) confirmatory analysis: LC-MS/MS with an ESI source in the negative ionization, selective reaction monitoring mode. Patient: ethanol-abstaining women under buprenorphine-treatment (medication with levetiracetam, gabapentin, clomethiazol and chloral hydrate). Proband: self-medication with 500 mg chloral hydrate after a 5-day ethanol abstinence. EtG analysis for both in subsequent urines. Check for cross reactions of the pharmaceuticals with the EtG immunoassay by addition of pure substance (2 g/L each) to EtG-free urine. Results: EtG concentrations up to 8.0 mg/L or 7.0 mg/g creatinine (cut-off 0.5 mg/L or mg/g) for the patient and up to 0.28 mg/L or 0.35 mg/g for the control subject (after 500 mg chloral hydrate) were obtained by the immunoassay. LC-MS/MS could not confirm these EtG results. In fact, EtG and/or EtS were not detectable in any of the urine samples by LC-MS/MS (lower limit of detection 0.01 mg/L). Cross reactions of the pharmaceuticals, incl. the chloral hydrate metabolites trichloroethanol and trichloroacetic acid, with the DRI (R) EtG immunoassay results were ruled out (by spiking experiments) as the underlying cause for the false-positive EtG immunoassay results. Conclusions: Trichloroethyl glucuronide as an important chloral hydrate metabolite remains the most probable cross reacting substance with the DRI (R) EtG immunoassay (unproven because of lack in pure standard). The chloral hydrate self-medication experiment clearly points to an association of the false-positive EtG immunoassay results and chloral hydrate intake. Chloral hydrate medication has to be considered as a cause for false-positive EtG screening results by the DRI (R) EtG immunoassay even in cases with regular chloral hydrate treatment (250-1000 mg) and the more in patients with chloral hydrate tolerance (taking g/day). It is recommended that positive EtG immunoassay results always be confirmed by a more specific technique such as LC-MS/MS, including ethyl sulfate as a second minor ethanol metabolite. (C) 2008 Published by Elsevier Ireland Ltd.
