Phosphatidylethanol (PEth) is a phospholipid that forms in red blood cell membranes only when alcohol is present, making it a direct and highly specific biomarker of consumption. A PEth level reflects drinking over roughly the previous two to four weeks, with published thresholds distinguishing abstinence, moderate intake, and heavy use — a much longer and more pattern-oriented view than urine EtG’s ~80-hour window. Collection is simple (often a fingerstick dried blood spot), and incidental exposure to sanitizers or mouthwash does not produce meaningful PEth, which removes the classic EtG dispute. PEth is increasingly used in fitness-for-duty evaluations, professional health monitoring, and safety-critical return-to-duty programs.