Project Summary/Abstract In the funded parent grant (R01-AA014988-13), we are examining the effects of an 8-week contingency management procedure using transdermal alcohol monitoring to reduce drinking among 440 individuals arrested for driving while intoxicated (DWI). The current application is a timely and cost-effective competitive revision proposing to add the measurement of an alcohol-use biomarker, phosphatidylethanol (PEth). The goals of this project are to extend our previous work by determining the extent to which PEth can be used to: (a) confirm abstinence, (b) characterize alcohol consumption levels, and (c) enhance the parent grant’s ability to objectively characterize drinking levels during post-intervention follow-up. There is a need for alcohol biomarkers that can objectively identify alcohol use (or confirm abstinence) and characterize individuals’ patterns of drinking. PEth, measured in whole blood, has unique characteristics that make it a sensitive biomarker for alcohol use. Our recent pharmacokinetic study showed that PEth 16:0/18:1 (a homologue of PEth) can be used to detect as little as 1 to 2 standard alcohol drinks, supporting the notion that it may be useful in confirming alcohol abstinence (Javors et al., 2016). Furthermore, Ulwelling and Smith (2018) proposed that cut-off values of PEth 16:0/18:1 may be clinically useful in identifying alcohol-use drinking levels, including: "none/light" (< 20 ng/ml), "moderate" (20–199 ng/ml), and "heavy" (> 200 ng/ml). However, because these values were derived from a literature review, they require further validation. Our recent work also demonstrated that PEth 16:0/18:1 has a relatively long window of detection (half-life = 7.8 days) and can be detected up to 28 days after the last drink. These data provide indirect support for the idea that PEth 16:0/18:1 could be used as a marker for characterization of long-term drinking levels. In contrast, we have shown that PEth 16:0/20:4 has a much shorter half-life of 2.1 days, which may be more advantageous for characterizing recent drinking. The parent grant is monitoring alcohol consumption via transdermal alcohol monitoring during a contingency management intervention among individuals arrested for DWI. It includes post-intervention visits that rely on self-reported drinking. We propose to now include measurement of PEth 16:0/18:1 and 16:0/20:4 to address the gaps discussed above and characterize levels of drinking during the post-intervention period. We will collect blood samples weekly during the 8-week intervention phase of the study and during the post-intervention visits (3, 6, 9, and 12 months). Our revison primary aims are: to determine the accuracy of PEth 16:0/18:1 for detecting and identifying patterns of alcohol consumption (verified by transdermal alcohol monitoring); to develop and validate an algorithm for PEth 16:0/20:4 (shorter half-life) to detect and identify patterns of consumption (verified by transdermal alcohol mon...