BACKGROUND AND AIMS: Alcohol is a well-established risk factor for numerous health conditions, making screening for hazardous alcohol use in healthcare a critical task. While self-reported data suggest that alcohol consumption varies across seasons, this seasonal fluctuation has not yet been confirmed using objective biological markers. This study aimed to measure whether phosphatidylethanol (PEth) captures variations in hazardous alcohol use across two temporal resolutions: month of the year and day of the week.

DESIGN: Observational cross-sectional study based on data from medical records.

SETTING: Healthcare services, Region Dalarna, Sweden, between 2017 and 2023.

PARTICIPANTS/CASES: Adult patients (n = 62 431, 50% females) screened for hazardous alcohol use with PEth within primary care.

MEASUREMENTS: This study utilizes test results from PEth, with results >0.30 μmol/l defined as hazardous alcohol use. We compared the prevalence of hazardous alcohol use across months and weekdays using logistic regression while adjusting for sex, age, smoking status, the Charlson Comorbidity Index and psychiatric diagnoses.

FINDINGS: The prevalence of hazardous alcohol use increased between May and August, ranging from 13.2% to 15.9%, compared with 10.7% in November. This corresponds to a 48% relative increase in the peak month of July [prevalence ratio (PR) = 1.48, 95% confidence interval (CI) = 1.33-1.64]. Hazardous alcohol use was also more prevalent among patients tested on Mondays (13.0%) compared with Thursdays, with the lowest prevalence (12.0%). The difference was particularly pronounced among female patients, with a 14.0% higher relative prevalence on Mondays (PR = 1.14, 95% CI = 1.02-1.27).

CONCLUSIONS: In Sweden, the prevalence of hazardous alcohol use appears to fluctuate seasonally and, to a lesser extent, across weekdays, as measured by blood tests for phosphatidylethanol, a biomarker for hazardous alcohol use. November showed the lowest prevalence and July the highest, consistent across age, sex and the year of the observational period. Hazardous alcohol use showed a slight elevation of prevalence during Mondays compared with Tuesday to Friday.