Blood Test to Check for Alcohol in a Patient's System
For detecting recent alcohol use, order whole blood phosphatidylethanol (PEth), which is the most sensitive and specific biomarker available, capable of detecting a single drinking episode for up to 12 days and chronic heavy use for up to 6 weeks. 1
Testing Algorithm Based on Clinical Need
For Recent Alcohol Use (Past 3-5 Days)
- First-line: Urine ethyl glucuronide (EtG) - detects alcohol use for up to 3 days (up to 80 hours depending on intake level) with 89% sensitivity and 99% specificity 2, 3
- Alternative: Urine ethyl sulphate (EtS) - similar detection window to EtG 1
- Immediate detection: Blood or breath ethanol - only useful within 2-12 hours of consumption 1, 4
For Chronic or Heavy Alcohol Use (Past 2-6 Weeks)
- Gold standard: Whole blood PEth - detects single episode for 12 days, chronic use for 6 weeks 1
- Threshold ≥20 ng/mL indicates positive test
- 20-200 ng/mL suggests moderate consumption (20-60 g/day)
200 ng/mL (typically >500 ng/mL) indicates heavy daily drinking (>50-60 g/day)
- Directly correlates with amount consumed
- Not affected by liver disease, making it superior in patients with cirrhosis 2
For Long-Term Monitoring (Past 3-6 Months)
- Hair ethyl glucuronide (hEtG) - each 1 cm segment reflects approximately one month of use 3
- <7 pg/mg = abstinence
- 7-30 pg/mg = social drinking
30 pg/mg = chronic excessive consumption (>60 g/day)
Tests to Avoid as Primary Markers
Do not rely on traditional indirect markers alone - they have poor sensitivity and specificity 1, 3:
- Gamma-glutamyl transferase (GGT) - elevated in many conditions, loses specificity in advanced liver disease
- AST/ALT ratio - neither sensitive nor specific, especially in cirrhosis
- Mean corpuscular volume (MCV) - non-specific
- Carbohydrate-deficient transferrin (CDT) - requires 50-80 g ethanol/day for 1-2 weeks to become positive, inaccurate in advanced liver disease, sensitivity only 25-50% 2, 3
Critical Clinical Considerations
PEth is now the standard in US and European liver transplant centers for monitoring alcohol use because of its reliability and lack of interference from liver disease 1. The test costs more than traditional markers but provides far superior accuracy.
Important caveat: All biomarkers should be used as adjuncts to clinical history, standardized questionnaires (like AUDIT), and collateral information - never in isolation 1. PEth only reflects recent use and does not account for historical patterns.
For pregnant women or situations requiring detection of very low consumption: Urine EtG can detect as little as 5 g of alcohol, though this may cause false positives from incidental exposure (mouthwash, food) 3. Use higher cutoffs (>0.5 mg/L) to avoid this issue.
Renal failure prolongs detection times for urinary EtG and EtS, potentially causing false positives 2.