Guidelines for Obtaining Specimens for Post mortem Toxicological Analysis
The following guidelines have been written to assist pathologists and anatomical pathology technicians in selecting the appropriate specimens for toxicological analysis. If it is unclear which specimens are required for a particular analysis please contact the laboratory to discuss specimen requirements before collection.
Where individuals have died several hours or days after hospitalisation, there may be ante mortem specimens available. These are the ideal specimens to submit. It is important to ensure that the blood is pre transfusion and has not been collected from indwelling catheters. If only small volumes are available, post mortem specimens should also be submitted for screening purposes (Depending on the duration of hospital admission) and the ante mortem specimens will retained for quantification.
10-20mL blood should, preferably, be collected from the femoral or subclavian vessels, so that the effects of post mortem redistribution are minimal. The most consistent quantitative findings are derived from samples collected from these sites. If this is not possible, other sites for collection include the root of the aorta, the pulmonary artery, the superior vena cava or the heart. All blood is assumed to be femoral unless otherwise stated. An additional 5mL blood should be collected into a separate tube containing 2% sodium fluoride. This is to inhibit bacterial production of analytes such as alcohol, GHB and cyanide, and to prevent degradation of labile drugs.
10-20mL un-preserved and 5mL 2% sodium fluoride preserved urine should be submitted to the laboratory. Urine is useful for qualitative analysis and confirmation of blood results. As the bladder is a confined compartment, largely free from bacteria, it is of particular value to quantify alcohol, where the validity of the blood result is in question. Drugs and their metabolites are usually present in urine in much higher concentrations than in blood and are detectable for much longer periods following ingestion. Depending on the half-life of the drug and the sensitivity of the assay, it may be possible to detect the drug in urine for a few days or even a week after ingestion. However, this also means that the presence of the drug in the urine does not necessarily suggest the drug was in the blood and exerting a pharmacological effect. Analysis of drugs in urine is easier than blood as it is a much cleaner specimen, there is no protein binding to hinder extraction, and it is often free from putrefaction products that can interfere with analytical methods.
Ante mortem Samples
All vitreous humour should be collected into tubes containing 2% sodium fluoride preservative. As it is collected from a relatively contained compartment, vitreous humour offers a number of advantages over more commonly submitted specimens, particularly when some putrefaction of the body has occurred. Vitreous humour is particularly advantageous for confirmation of blood alcohol concentrations as it is well protected from bacterial infiltration and is, therefore, less prone to post mortem fermentation observed with blood. This sample matrix is also useful for biochemical tests such as glucose and lactate. Vitreous humour should be submitted in all cases of suspected heroin overdose. The effects of post mortem redistribution in vitreous humour are believed to be minimal and as it is largely free of protein and other complex substances, analysis is relatively straightforward. Currently, drug concentrations are difficult to interpret due to lack of reference data.
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