Police crime labs will test the "whole blood," and will produce a "blood alcohol content" figure on that basis. However, many hospitals and clinics test only the "blood serum," resulting in a blood alcohol content figure that can be 25% - 33% higher than a "whole blood" test result. A hospital may also use an alcohol swab before drawing the blood sample, which may contaminate the sample.
A blood alcohol test measures the amount of alcohol (ethanol) in your body. Alcohol is quickly absorbed into the blood and can be measured within 40 to 70 minutes after you have consumed any alcoholic drink. The amount of alcohol in the blood reaches its highest level about an hour after drinking. However, food in the stomach may increase the amount of time it takes for the blood alcohol to reach its highest level. About 90% of alcohol is broken down in the liver. The rest of it is passed out of the body in urine and your exhaled breath.
Factors that can interfere with your test and the accuracy of the results include:
- The use of rubbing alcohol to clean the skin before inserting a needle to draw blood.
- Elevated blood ketones, as in diabetic ketoacidosis.
- Cough medicines or herbal supplements that contain alcohol, such as kava or ginseng.
- Drinking other alcohols, such as isopropyl alcohol or methanol.