Background and aims Phosphatidylethanol 16:0/18:1 (PEth) is a biomarker for alcohol intake. It has a half‐life of 7.9 days. Chronic alcohol consumption causes high PEth values. It can take weeks before PEth values fall below the decision limit for ‘alcohol abstinence’. Our aim was to validate whether alcohol abstinence can be determined based on two consecutive PEth results above the decision limit. Design Observational study. Setting Belgium, February 2019. The study was linked to a social initiative in Belgium, ‘Tournée Minérale’. Participants Adults (aged > 18 years, n = 796) with varying drinking habits who self‐reportedly refrained from alcohol consumption during the study. Measurements A validated liquid chromatography–tandem mass spectrometry method was used to quantify PEth in participants’ dried blood samples, collected at three time‐points via remote fingerprick‐based self‐sampling. Findings A population‐based algorithm to evaluate abstinence based on 95% prediction limits was developed by fitting a linear mixed‐effect model to discern patterns in PEth elimination over time. It took intra‐ and inter‐individual variability into consideration. The algorithm was included in a two‐step decision tree, assessing whether (i) PEth values fell within the prediction interval and (ii) the slope between two PEth values was consistent with no alcohol consumption. Data for 74 participants reporting no alcohol intake during the study were used for validation. With a detection limit of ‘four units spread over 14 days’, the sensitivity and specificity of the decision tree was 89%. Conclusions Claims of alcohol abstinence can be verified using a two‐step decision tree for phosphatidylethanol 16:0/18:1 values, even when those values are above the limit for ‘alcohol abstinence’.

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