Fundamentals of Analytical Toxicology. Robin Whelpton

but also contains some digestive enzymes. Oral fluid is the preferred term for the mixture of saliva, gingival crevicular fluid (fluid from the tooth/gum margin), cellular debris, blood, mucus, food particles, and other material collected from the mouth Semen is produced by the testes and the prostate gland, and consists of seminal fluid, which may be obtained from semen by centrifugation, and spermatozoa Synovial fluid is the clear, viscous, lubricating liquid that fills the synovium (the membrane that surrounds a joint and creates a protective sac) Tears are the clear watery secretion of the tear ducts of the eye Vaginal fluid is the viscous secretion of the vagina Vitreous humour is the transparent, viscous fluid contained behind the lens in the eye Excretory fluids/residues Bile is the thick yellow-green fluid secreted by the liver via the gall bladder into the intestine Exhaled (expired) air generally contains less oxygen and more carbon dioxide and water vapour than ambient air, but may contain other volatile and non-volatile compounds Faeces are the brown, semi-solid residues of the digestive process (Section 2.2.6) Sweat is the aqueous fluid excreted by the pores of the skin Meconium is the dark green substance forming the first faeces of a newborn infant Urine is the yellow/yellow-green fluid produced by the kidney. It consists mainly of water, salts, urea, creatinine, and other metabolic products (Section 2.2.4) Other samples ‘Cavity blood’ is the fluid remaining in the body cavity once the organs have been removed Bezoars are stones or concretions found in the alimentary tract of animals Bronchoalveolar lavage is obtained by washing the bronchi/alveoli with an appropriate solution and aspirating the resulting fluid Calculi (‘stones’) are hard crystalline deposits formed in various body cavities such as the kidney Dialysis fluid (extracorporeal or peritoneal) is the fluid remaining or recovered after dialysis has been performed Gastric lavage (stomach wash-out, SWO) is a specimen obtained by washing the stomach with an appropriate solution and aspirating the resulting fluid Hair (head, axilliary, or pubic) is sometimes used to assess exposure to poisons such as drugs or toxic metals Nails or nail clippings (finger or toe) are sometimes used to assess exposure to drugs or heavy metals Nasal swabs are fluid collected onto cotton swabs from inside the nose Stomach contents may be (i) gastric aspirate, (ii) gastric lavage, (iii) vomit, or (iv) the residue in the stomach at autopsy (Section 2.2.5) Tissue specimens are obtained either surgically, or at post-mortem. Tissue obtained from an aborted foetus and/or placenta may sometimes be presented for analysis. A biopsy sample is a small sample of a tissue obtained by a specialist sampling technique Vomit reflects the composition of gastric aspirate

      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.

      Provided the analyte is stable, anticoagulated whole blood can be kept at room temperature or refrigerated (2–8 °C) for 2 days or so before harvesting plasma. However, leaving plasma or serum in contact with red cells can either cause changes as a result of enzymatic activity, or redistribution of an analyte between cells and plasma. Thus, in general, plasma or serum should be separated from the blood cells as soon possible. If necessary, whole blood can be stored at –20 °C or below, but freezing will lyse most cell types.

A range of anticoagulants is available for in vitro use (Table 2.3). Sodium citrate tubes may contain 0.5 or 1 mL of an aqueous solution of anticoagulant and are unsuitable for quantitative work. Furthermore, citrate has strong buffering capacity and dilution of the sample may reduce the degree of plasma protein binding and consequently the plasma:red cell distribution of an analyte. It should be ensured that lithium heparin anticoagulant is not used if plasma lithium is to be measured (Arslan et al., 2016). Heparin too has been known to interfere in drug analysis.

Table 2.3 Anticoagulants for in vitro use

Anticoagulant	Concentration (mL^–1 blood)	Comment
Lithium heparin	10–20 units	General biochemistry
Sodium heparin	10–20 units	General biochemistry
Sodium fluoride (with either EDTA or oxalate)	1–2 mg	Glucose (inhibits glycolysis)
	6–10 mg	General anticoagulant^a
Sodium citrate	3 mg	Clotting studies – not recommended for other purposes because an aqueous solution dilutes the specimen
EDTA	2 mg	Haematology (stabilizes readily oxidized compounds)

      ^aca. 2 % w/v fluoride is recommended to attempt to stabilize analytes such as cocaine (Section 2.3) and to prevent fermentation of glucose to ethanol, for example

       2.2.3.3 Serum

      When whole blood is allowed to stand (15 min, room temperature) in a plain tube (no anticoagulant) a clot normally forms that will retract sufficiently to allow serum to be collected after centrifugation. For many analyses, serum is preferred to plasma because it produces less precipitate (of fibrin) on freezing and thawing.