Pericardial fluid is fluid present between the heart and the sac surrounding the heart known as the pericardium
Peritoneal fluid is the fluid that accumulates in the peritoneum
Saliva is the viscous, clear secretion of the mucous glands in the mouth. It is related in composition to plasma, 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.
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
