Robin Whelpton - Fundamentals of Analytical Toxicology

Clinical samples can be divided into (i) blood and related fluids, (ii) body fluids other than blood, (iii) excretory fluids/residues, and (iv) other clinical specimens ( Table 2.2). A range of additional specimens may be collected for toxicological purposes. Special precautions will be needed with unstable analytes. Most compounds measured in urine can be considered stable for at least a few hours at room temperature because the urine may already have been held at body temperature for some time before it was voided. Nevertheless, stability testing should be included in any method validation work.

Arterial blood is normally collected by an experienced medical practitioner – it is a relatively dangerous procedure – for the measurement of blood gases and is rarely submitted for toxicological analysis. Capillary blood, which closely approximates to arterial blood, can be obtained by pricking the heel, finger or ear lobe; this procedure is most often performed on small children. Applying pressure to encourage blood flow will dilute the sample with tissue fluid.

Venous blood is obtained by venepuncture of (usually) the median cubital vein of an arm remote from any infusion site. Either a hypodermic needle and syringe (1–50 mL) or a commercial vacuum-sampling system such as a Vacutainer TMmay be employed. Care should be taken to ensure that 2-propanol or other alcohols are not introduced into the sample when using swabs to clean/disinfect the skin. A tourniquet can be used to distend the vein prior to venepuncture, but should be released immediately prior to sampling.

For repeated sampling, a small cannula may be inserted into, for example, a vein in an arm or hand that allows venous access via a rubber septum. However, maintaining patency may be a problem, and there may be risks of (i) inducing haemolysis and (ii) of specimen contamination due to use of anticoagulant or local anaesthetic solutions with such devices. The use of heparin anticoagulant solutions that contain phenolic preservatives should be avoided.

Following venepuncture, blood should be transferred into an appropriate container as soon as practicable. Some basic analytes and quaternary ammonium compounds, e.g. tricyclic antidepressants (TCAs) and paraquat, and aluminium bind to glass. Plastic tubes are thus preferred and are also less likely to shatter than glass, especially if frozen. On the other hand, if volatile compounds such as solvents or anaesthetic gases are to be analyzed, glass containers are preferred because of the risk of loss of analyte if plastic tubes are used ( Section 2.3).

Table 2.2 Some clinical sample types

If blood has been collected into a syringe, it is essential that the syringe needle is removed and the blood allowed to flow gently into the collection tube in order to prevent haemolysis. This should be followed by gentle mixing to ensure contact with the anticoagulant if one is being used. Even mild haemolysis will invalidate a serum iron or potassium assay, and may invalidate plasma or serum assays for other analytes concentrated in red cells such as chlortalidone.