Background The ethanol metabolites, ethyl glucuronide (EtG) and ethyl sulfate (EtS) are biomarkers of recent alcohol consumption that provide objective measures of abstinence. Our goals are to better understand the impact of cutoff concentration on test interpretation, the need for measuring both metabolites, and how best to integrate test results with self-reports in clinical trials. Methods Subjects (n=18) were administered, one week apart, 3 alcohol doses calibrated to achieve blood concentrations of 20, 80 and 120 mg/dL respectively. Urinary EtG/EtS were measured at timed intervals during a 24 hour hospitalization and twice daily thereafter. In addition, participants from 2 clinical trials provided samples for EtG/EtS and drinking histories. Cutoffs for EtG/EtS of 100/50, 200/100 and 500/250 ng/mL were evaluated. Results Twelve hours following each challenge, EtG was always positive at the 100 and 200 cutoffs, but at 24 hours sensitivity was poor at all cutoffs following the low dose, and poor after 48 hours regardless of dose or cutoff. Similarly, in the clinical trials EtG sensitivity was good for detecting any drinking during the last 24 hours at the two lowest cutoffs, but under 40% during the last 24-48 hours. Sensitivity was reduced at the 500 ng/mL cutoff. Discrepancies between EtG and EtS were few. Comparison of self- reports of abstinence and EtG confirmed abstinence indicated under-reporting of drinking. Conclusions Any drinking the night before should be detectable the following morning with EtG cutoffs of 100 or 200 ng/mL. Twenty-four hours after drinking, sensitivity is poor for light drinking, but good for heavier consumption. At 48 hours, sensitivity is low following 6 drinks or less. Increasing the cutoff to 500 ng/mL leads to substantially reduced sensitivity. Monitoring both EtG and EtS should usually be unnecessary. We recommend EtG confirmed self-reports of abstinence for evaluation of outcomes in clinical trials.