A driver arrested at midnight had a blood alcohol concentration of 240 mg/dL. He is detained and retested at 2 am. Assuming complete absorption and no further intake, the expected blood alcohol concentration at retesting is approximately:

• A 180 mg/dL
• B 240 mg/dL, since elimination follows first-order kinetics
• C 120 mg/dL
• D 210 mg/dL ✓

Explanation

Ethanol elimination follows zero-order kinetics because hepatic alcohol dehydrogenase is saturated at even low concentrations, so blood alcohol falls at a fixed rate of roughly 15 mg/dL per hour regardless of concentration. Over two hours this gives a fall of about 30 mg/dL, from 240 to approximately 210 mg/dL. First-order kinetics would predict a proportional percentage fall, which does not apply to ethanol at intoxicating levels.

Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.

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