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Blood

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  p309-310

2026-10-27

54

Blood

Historically, blood-alcohol testing was the first method employed to determine a subject’s alcohol consumption or intoxication (Jones 1996). Early methods involved distillation of alcohol from the blood, followed by oxidation and titration. These methods have been replaced almost entirely with enzymatic and gas chromatographic methods. Enzymatic methods have the advantage that they can be automated readily, and can allow an alcohol test to be added to a panel of other blood chemistry tests, particularly in a hospital setting.

Clinical alcohol tests are carried out on plasma or serum. It is important to know which matrix has been used for testing because plasma contains more water than whole blood and correspondingly more alcohol. Hence a conver sion factor must be applied to convert a plasma level into a blood alcohol level. If enzymatic testing is to be carried out, measurement using blood as the test matrix is to be preferred, particularly for evidential purposes. By far the most popular method for deter mining blood alcohol is by gas chromatography (GC). This technique has the advantages of being semi-automatable, extremely reproducible and compatible with multiple sample types. Both packed column and capillary column procedures are acceptable for forensic blood alcohol analysis, although most modern GCs operate with capillary columns. Headspace GC methods are normally applied  because ethanol is volatile and hence a complex sample clean-up procedure to extract the ethanol is not required. Some form of standard with a known value for ethanol concentration is normally analysed alongside test samples to enable the accuracy of the analytical result to be verified. This is exceedingly important for legislative purposes, particularly in view of the fact that a successful prosecution may result in a driver losing their means of making a living, or in the case of charges of dangerous driving may result in lengthy imprisonment. Certified standards can be purchased which are used in the determination of blood alcohol concentrations. These standards are analysed alongside test samples. If the result for the certified reference material is within the permitted tolerance, then results for samples can be accepted as accurate. The disadvantage of using blood for testing is that a police surgeon, forensic medical examiner or other clinically qualified person is normally required to take a blood sample. Very few, if any, police departments have their own medical examiner and hence the norm is for the examiner to be called to the station to take a sample. It may take some time before they arrive, during which time metabolism and excretion of alcohol will be occurring. Many persons have an aversion to needles and this can also give rise to sampling problems. Caution must also be exercised with blood samples to minimise possibilities of microbial contamination (as this may give rise to extraneous production of alcohol) and to prevent volatilisation of alcohol from improperly sealed containers (as this will result in an incorrect test value). A preservative is normally added to blood samples to prevent microbial growth. When blood is sampled for determination of alcohol concentration, there is also an associated delay in obtaining the test result because the sample is sent to a specialist laboratory for analysis. This delay in knowing the test result can be stressful for the driver suspected of DUI and delays the police in taking action to prosecute impairment. For the above reasons, many police forces prefer breath testing.