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Blood Alcohol Test

When can a blood alcohol test be done?

  • In drivers who cannot or have refused to do a breath alcohol test — to check if their blood alcohol concentrations (BAC) are above the legal limit.
  • In an alcohol poisoned person — to evaluate the severity of poisoning.
  • In a person in coma — to check for alcohol poisoning as a possible cause.
  • In a person included in an alcohol treatment program — to check for drinking.

How is a blood alcohol test done?

No special preparation is needed. A doctor or nurse draws blood from a vein, usually from your forearm. As little as 0.5 milliliters blood (one half of inch in the tube) may be enough, but up to about 10 milliliters (two tubes) may be taken.

How is the blood alcohol concentration expressed?

  • In the U.S.: grams of alcohol per 100 milliliters of blood (g/100 mL)
  • In the UK: milligrams of alcohol per 100 milliliters of blood (mg/100 mL)
  • In the continental Europe: grams of alcohol per liter or blood, or permilles (g/L)
  • In hospitals: millimols of alcohol per liter of blood (mmol/L)

What causes a positive blood alcohol test?

A driver has a positive blood alcohol test when his or her blood alcohol concentration exceeds the legal limit (0.08 g alcohol/100 mL blood in the U.S., Canada and UK; 0.05 g/100 mL in most European countries; 0.00 g/100 mL in Russia).

When you are tested in hospitals, workplaces and alcohol treatment programs, any amount of alcohol in the blood may be considered positive. Current tests can detect blood alcohol concentrations as low as 0.001 grams of alcohol per 100 milliliters of blood, which can result from consumption of less than 1 gram of alcohol, for example from one sip of 5% beer or, possibly, from five sips of a non-alcoholic beer (containing up to 0.5% alcohol) by a 160 lbs [73 kg] man [1].

  • 160 lbs (73 kg) person may reach BAC 0.08 after drinking four drinks in one hour (four 12 oz cans of 5% beer, four glasses [2 x 5 oz] 12% wine, four shots [2 x 1.5 oz] 40% spirit) and a 100 lbs (45 kg) person after only two drinks [1].
  • Drinking alcohol-containing mouthwash or denatured alcohol, or eating large amounts of alcohol-containing deserts can result in blood alcohol concentration higher than 0.08 g/100 mL.
  • Using AWOL or occupational exposure to alcohol (for example, inhaling alcohol vapors during its production) could potentially result in a positive blood alcohol test.*

What does NOT likely result in a positive blood alcohol test (BAC 0.08 g/100 mL):

  • Drinking non-alcoholic beer. A 160 lbs (73 kg) person would need to drink at least thirty-two 12 oz cans of nonalcoholic beer (containing up to 0.5% alcohol) and a 100 lbs (45 kg) person at least 16 cans in one hour to reach BAC 0.08 [1].
  • Inhaling alcohol vapors during work in the bar.
  • Rinsing the mouth or gargling using alcohol-containing mouthwash or eating few pieces of alcohol-containing foods
  • Drinking or inhaling rubbing alcohol (isopropanol) or methanol, or other substances found in gasoline, solvents, lacquers or cleansers.
  • Regular use of ethanol-containing cosmetics, hair sprays, hand sanitizers or other products applied on the body
  • Metabolic processes in the body, including those in individuals with diabetes, hepatitis or liver cirrhosis [2]

High or low hematocrit (a percent of red blood cells in the blood) does not significantly affect the blood alcohol concentration [3,4].

How accurate is a blood alcohol test?

In general, a blood alcohol test is more accurate than breath, urine and other alcohol tests and has less false positive or negative results.

Are false positive or negative blood alcohol test results possible?

False positive blood alcohol test:

  • In few Japanese individuals with severe systemic yeast infection, blood alcohol concentrations over 0.08 g of alcohol/100 mL blood after eating carbohydrate foods have been recorded (yeasts break down carbohydrates to alcohol) [5].

False negative blood alcohol test may, theoretically, result from a technical error.

Can you manipulate a blood alcohol test?

  • Drinking large amounts of water or taking diuretics (water pills) will not significantly lower your blood alcohol concentration, since any excessive water will be quickly excreted in the urine.
  • Heavy exercise or coffee after drinking may make you more alert, but they do not lower your blood alcohol concentration.

  1. Alcohol Problems in Intimate Relationships: Identification and Intervention  National Institute on Alcohol Abuse and Alcoholism
  2. Alcohol in the body  Lion Laboratories
  4. Rainey PM, 1993, Relation between serum and whole-blood ethanol concentrations  Clinical Chemistry
  5. Logan BK et al, 2000, Endogenous ethanol ‘auto-brewery syndrome’ as a drunk-driving defence challenge  PubMed

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