The blood-alcohol level is usually determined in the living by means of a venous blood sample collected from the cubital fossa (anterior aspect of the elbow). However, any blood specimen can be used. It is important to prevent contamination with alcohol-containing solutions, especially when the skin is cleaned.
The specimen must be collected in a special test tube containing sodium fluoride (NaF) and potassium oxalate. The former is an enzyme inhibitor that inhibits the production and transformation of alcohol in the specimen. It is also used for post-mortem specimens and for the analysis of carbon monoxide (carboxyhaemoglobin).
Breath-alcohol analysis is at present widely used by law-enforcement agencies. It is based on the principle that a certain amount of alcohol will be eliminated via the lungs. Although technical factors (eg condensation in the mouthpiece, the effect of temperature, etc) often had an effect on the functioning of the older measuring instruments, modern equipment is more sensitive and accurate. This method is often used as a screening procedure at roadblocks, and the level of alcohol intoxication is then confirmed with a blood analysis.
Alcohol levels are routinely measured in all individuals older than 16 years who die unnaturally. A blood or vitreous humour (eye fluid) specimen is tested. It is important to collect the specimen as far away as possible from the stomach and liver. Post-mortem diffusion of ingested alcohol from the stomach into adjacent tissues can lead to a false measurement. Therefore specimens from the heart are not acceptable. The same applies to the liver, where post-mortem fermentation of sugars by micro-organisms can produce alcohol. The specimen is therefore collected from the femoral vein in the groin, as the inferior vena cava has valves which will prevent backward diffusion to the abdominal region.
Most forensic laboratories use vitreous humour. This fluid has certain advantages. The eye is protected quite well by bony structures. Even in severe mutilation eye fluid will still be available. Then the eyes are also microbiologically quite sterile. Post-mortem alcohol production is therefore less common. Lastly eye fluid is in a way chemically isolated from the body and not so susceptible to biochemical changes with death.
Urine-alcohol levels only confirm alcohol ingestion. Urine-alcohol concentra-tion is influenced by the volume of urine, and is therefore not a reliable indication of the amount of alcohol ingested.
If the specimen is analysed by a gas chromotograph other alcohols will also be indicated on the graph. If an alcohol such as butanol is produced during decomposition it will thus be indicated. The absence of other alcohol peaks therefore further confirms the accuracy of the blood-alcohol level, as it indicates that there was no post-mortem or post-collection production of alcohol.
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