2.2.2 Clinical sample types
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.
2.2.3 Blood and blood fractions
2.2.3.1 Arterial blood
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.
2.2.3.2 Venous blood
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 VacutainerTM 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 | |
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Saliva is the viscous, clear secretion of the mucous glands in the mouth. It is related
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