A questionnaire, a blood test and a simple count identified three overlapping but different groups of drinkers

Design

Retrospective cohort using linked primary care records and national registries

Population

3,679 patients with biomarker plus AUDIT; 8,934 with biomarker plus reported weekly intake, Sweden

Primary outcome

Concordance between phosphatidylethanol and self-reported alcohol measures

Effect

Hazardous use in 26.5% by AUDIT, 14.6–18.3% by biomarker, 8.5% by weekly drinks; each identified patients the others missed

Phosphatidylethanol is a direct alcohol biomarker with a window of a few weeks, and it is increasingly offered as a way round the unreliability of asking. This cohort tested that assumption against Swedish primary care records linked to national registries, comparing the biomarker with the Alcohol Use Disorders Identification Test in 3,679 patients and with documented weekly intake in 8,934.

The three methods did not agree on who was drinking hazardously. The questionnaire classified 26.5% as hazardous, the biomarker 14.6% and 18.3% in the two samples, and self-reported weekly drinks only 8.5%. Each method identified patients the others did not. Correlation between biomarker level and both self-reports was moderate and, importantly, non-linear — so a biomarker value cannot be read back into a number of drinks.

The subgroup pattern is the part worth carrying into a consultation. At the same biomarker level, men and patients with higher education or income reported greater weekly consumption, while younger patients scored higher on the questionnaire. Under-reporting, in other words, is not uniform and not concentrated where clinicians usually expect it. The authors' conclusion is the right one: interpret the biomarker alongside what the patient tells you, not instead of it.

• A normal phosphatidylethanol does not exclude hazardous drinking captured by a questionnaire, and vice versa
• Do not convert a biomarker level into an estimated weekly intake — the relationship is non-linear
• A questionnaire asks about pattern and consequences; the biomarker asks only about recent quantity
• Reporting differs systematically by sex, age, education and income, so calibrate to the person rather than to a stereotype
• In Indian practice the biomarker is a send-out where available at all, which makes the structured questionnaire the workhorse

Why it matters

It undercuts the reason clinicians reach for the blood test, which is the hope that it settles a question the patient's answer left open.

Don't overread it

Retrospective and observational, and the two samples were selected by having had a biomarker test ordered — patients tested at all differ from those not tested, which will inflate prevalence in every arm.

The statistics, in plain English

The gap between 26.5% by questionnaire and 8.5% by weekly drinks is a fact about the definitions as much as the patients: a questionnaire score of 8 or above captures consequences and dependence features, while 10 or more drinks a week is a pure quantity threshold. They are measuring different things, which is why combining them finds more people than either alone. A moderate, non-linear correlation means the biomarker ranks patients roughly in the right order but cannot be converted into a consumption estimate for an individual.

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