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The advice given to the clinician is generally supported by a User Handbook,

prepared by senior laboratory personnel, that includes a description of each test

offered, instructions on sample collection and storage, normal laboratory working

hours and the approximate time it will take the laboratory to undertake each test.

This turnaround time will vary from less than one hour to several weeks depending

upon the speciality of the test. The vast majority of biochemical tests are carried

out on serum or plasma derived from a blood sample. Serum is the preferred matrix

for biochemical tests but the concentrations of most test analytes are almost the

same in the two fluids. Serum is obtained by allowing the blood to clot and removing

the clot by centrifugation. To obtain plasma it is necessary to add an anticoagulant to

the blood sample and remove red cells by centrifugation. The two most common

anticoagulants are heparin and EDTA, the choice depending on the particular

biochemical test required. For example, EDTA complexes calcium ions so that calcium

in EDTA plasma would be undetectable. For the measurement of glucose, fluoride-

oxalate is added to the sample not as an anticoagulant but to inhibit glycolysis during

the transport and storage of the sample. Special vacuum collection tubes containing

specific anticoagulants or other additives are available for the storage of blood

samples. Collection tubes are also available containing clot enhancers to speed

the clotting process for serum preparation. Many containers incorporate a gel with

a specific gravity designed to float the gel between cells and serum providing a

barrier between the two for up to 4 days. During these 4 days, the cellular component

will experience lysis, so any subsequent contamination of the serum will include

intracellular components. Biochemical tests may also be carried out on whole blood,

Table 16.2Examples of biochemical analytes used to support clinical diagnosis

Type of analyte Examples

Foodstuffs entering
the body

Cholesterol, glucose, fatty acids, triglycerides

Waste products Bilirubin, creatinine, urea

Tissue-specific
messengers

Adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH),

luteinising hormone (LH), thyroid-stimulating hormone (TSH)

General messengers Cortisol, insulin, thyroxine

Response to
messengers

Glucose tolerance test assessing the appropriate secretion of insulin; tests of

pituitary function

Organ function Adrenal function – cortisol, ACTH; renal functionKþ,Naþ,, urea, creatinine;
thyroid function – free thyroxine (FT 4 ), free tri-iodothyronine (FT 3 ), TSH

Organ disease
markers

Heart – troponin-I, creatine kinase (CKMB), AST, lactate dehydrogenase (LD);

liver – ALT, AP,g-glutamyl transferase (GGT), bilirubin, albumin

Disease-specific
markers

Specific proteins (‘tumour markers’) secreted from specific organs – prostate-

specific antigen (PSA), CA-125 (ovary), calcitonin (thyroid),a 1 fetoprotein (liver)

631 16.2 Clinical measurements and quality control