Measurement of Alcohol in Urine and Breath

1. Urine Ethanol

Direct Measurement

• During the postabsorptive phase, urine alcohol concentration is roughly 1.3 times that in blood.
• However, using this ratio to estimate blood alcohol concentration (BAC) is discouraged because:
  • The 1.3 ratio is highly variable between individuals.
  • Urine alcohol more accurately reflects an average BAC over the entire period in which urine accumulates in the bladder (not the current blood level).
• Detection window: alcohol in urine reflects ingestion within the previous 8 to 12 hours.
• Analytical methods: the same ADH (alcohol dehydrogenase) enzymatic method used for serum/plasma performs well on urine. Gas chromatography (GC) with flame ionization detection (FID) is the reference/confirmatory method.

Ethanol Biomarkers (Extended Detection in Urine)

• >1000 ng/mL - heavy drinking in past 48 hours, OR light drinking same day
• 500-1000 ng/mL - heavy drinking past 3 days, light drinking past 24 hours, or intense incidental exposure within 24 hours
• <500 ng/mL - heavy drinking past 3 days, light drinking past 36 hours, or recent incidental exposure

• Can be produced in vitro after specimen collection (post-collection artefact).
• Certain E. coli strains (beta-glucuronidase activity) and upper respiratory infections can lower EtG levels.
• EtS is more stable and not affected by these issues - monitoring both EtG + EtS improves sensitivity.
• Incidental ethanol exposure (hand sanitizers, mouthwash) can produce positive results - interpreting cut-offs carefully is important.

2. Breath Ethanol

Physiological Basis

• The true ratio may be closer to 2300:1, and is variable.
• In the United States, evidential measurements legally use 2100:1.
• The lower ratio (2100) predicts a slightly lower than actual BAC - this is intentional because it is not prejudicial to the subject.
• End-expiratory air equilibrates with arterial blood (not venous), so breath more closely reflects arterial BAC.

Pre-Test Requirements

• A deprivation period of 15 minutes is mandatory before testing - to allow clearance of any residual "mouth alcohol" (from very recent drinking, alcohol-containing mouthwash, or vomiting of alcohol-rich gastric fluid).
• Duplicate tests 5-10 minutes apart, with results within 20 mg/dL (0.02%) of each other, are used as an additional safeguard against mouth alcohol contamination.

Timing Considerations

• Arterial blood alcohol is initially higher than peripheral venous blood.
• In the postabsorptive phase, the reverse is true.
• Because breath equilibrates with arterial blood, breath measurements during absorption may give a slightly higher reading than simultaneous venous blood samples.

Analytical Methods for Breath Alcohol Analyzers

1. Infrared (IR) absorption spectrometry - Most common; ethanol absorbs IR radiation at specific wavelengths.
2. Dichromate-sulfuric acid oxidation-reduction (photometric) - Chemical oxidation of ethanol, measured colorimetrically.
3. Gas chromatography (GC) - With flame ionization (FID) or thermal conductivity detection (TCD); most specific method.
4. Electrochemical (EC) oxidation / fuel cell - Ethanol is oxidized at an electrode; generates a measurable current proportional to ethanol concentration.
5. Metal oxide semiconductor sensors - Resistance changes in the presence of volatile organic compounds including ethanol.

3. Widmark Equation - Estimating BAC

N = W × ρ × [C_t + β × t] / (d × Z)

• N = number of standard drinks
• W = body weight (kg)
• ρ = volume of distribution: 0.68 L/kg (males), 0.55 L/kg (females)
• C_t = measured BAC (kg/L)
• β = elimination rate: 0.15 g/L/h (range 0.10-0.30 g/L/h)
• d = specific gravity of alcohol (0.8)
• Z = ethanol per standard drink (15 mL)

4. Gas Chromatography - The Reference Method

• Distinguishes ethanol from other alcohols (methanol, isopropanol) and acetone - important to rule out other toxic alcohol ingestions.
• Detection limit as low as 10 mg/dL (0.01%).

• Direct injection - Sample diluted with aqueous internal standard solution then injected.
• Headspace analysis - The volatile alcohol partitions into the gas phase above the specimen; the headspace gas is injected. This reduces column contamination from biological matrix.

Summary Table