Most people would think that if a person's blood is tested in a hospital as part of a DUI investigation, the test would be checking for the presence of ethanol, the type of alcohol we consume in alcoholic beverages.
Most people would be wrong.
The typical hospital blood test is what is called an enzymatic immunoassay test. It doesn't test for the presence of alcohol, but rather for the presenze of an enzyme, NADH (nicotinamide adenine dinucleotide (NAD) + hydrogen (H)) which is produced through the testing process. The amount of NADH present in the sample is correlated to the presence of ethanol in the blood and a specific blood alcohol concentration (BAC).
The trouble is that NADH does not appear in the blood solely because of ethanol consumption. It will also appear if an alcohol based antiseptic is used to swab the skin prior to removing a blood sample because the alcohol in the antiseptic will react to the NAD inserted into the sample.
Patients undergoing a CT scan will normally have a contrast media in their system to allow the CT scan to yield clearer results. However, that contrast media will react with the NAD as well, producing NADH which the blood test reads as alcohol.
Even more commonly, the presence of lactate in a person's blood will lead to an increased measurement of NADH. Lactate will appear in the blood for a variety of reasons, including strenuous exercise or trauma to they body's hard or soft tissues. As well, the use of Lactated Ringers solution during medical treatment - normally used to quickly increase the fluid levels in the patient's body - will substantially increase the presence of lactate. That added lactate, regardless of the source, will cause a hospital blood test to measure a false level of ethanol in the blood when, in fact, there may be absolutely no ethanol present at all.
So, the next time you hear about a hospital blood test showing that someone had a high BAC, don't trust that conclusion. Hospital blood tests are not reliable sources of determining a person's BAC. The most reliable testing method is the dual column gas chromatographic method with flame ionization detection. This form of testing is reliable - when done properly on a properly maintained instrument - and is available at a reasonable cost to any law enforcement agency through a variety of sources.