Читать книгу Fundamentals of Analytical Toxicology - Robin Whelpton - Страница 40

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^TM may 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

Blood & related fluids	Blood (‘whole blood’) is the fluid that circulates through the arteries, capillaries, and veins. The adult human body contains some 5–6 litres of blood. It is composed of plasma and blood cells. Normally venous blood is obtained (Section 2.2.3.2). If whole blood is to be analyzed, then the sample should be collected into an appropriate anticoagulant, mixed, and then frozen in order to lyse the cells before the analysis. Dried blood spots (DBS) are the residues remaining when whole blood has been allowed to dry on filter paper or another appropriate medium. Post-mortem blood is blood obtained after death, usually at autopsy. The site of sampling should always be recorded (Section 2.3.2.1). [N.B. Occult blood is altered blood found only in trace amounts particularly in faeces. It is not used as an analytical sample.]
Blood cells include red cells (erythrocytes) and white cells (lymphocytes, leucocytes, platelets, etc.). All may be harvested from freshly collected blood with appropriate procedures (Section 2.2.3.5)
Cerebrospinal fluid (CSF) is a plasma ultrafiltrate (i.e. its composition is that of plasma except that high Mr proteins are absent) that surrounds the elements of the central nervous system (CNS). It is obtained by lumbar puncture (needle aspiration from the spinal cord) and is usually collected into sterile tubes. Samples contaminated with blood should be discarded if microbiological assessment is required
Cord blood is blood obtained from the umbilical cord at parturition. Normally venous cord blood is obtained in order to reflect neonatal, as opposed to placental, blood. It may be possible to obtain plasma or serum depending on the volume available
Plasma is the liquid portion of blood (Section 2.2.3.4). It is obtained by centrifugation of anticoagulated whole blood
Serum is the pale yellow fluid remaining when whole blood has clotted. Its composition is generally the same as plasma except that fibrinogen and factors associated with the clotting process are absent (Section 2.2.3.3). It is obtained by centrifugation of whole blood that has been allowed to clot in the sample tube
Body fluids other than blood	Amniotic fluid is the fluid that surrounds the foetus in the amniotic sac
Aqueous humour is the watery fluid occupying the space between the cornea and the iris of the eye
Bone marrow aspirate is a sample of the spongy tissue found within bones that produces blood cells
Breast milk is the protein and fat-rich fluid produced by nursing mothers. The first expression of breast milk (colostrum) is especially rich in protein
Gastric aspirate is an acidic aqueous fluid containing digestive enzymes, food residues, etc. obtained by aspiration from the stomach
Intraosseous fluid is fluid derived from blood and interstitial fluid residing within bones
Interstitial fluid is the fluid between blood vessels and cells
Lymph is a yellowish fluid in the lymph channel secreted from the lymph glands
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.

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 anticoagulanta
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