After studying this chapter, you should be able to:
Explain the importance of laboratory tests in clinical and veterinary medicine.
Explain what is meant by the reference range for the results of a test.
Explain the difference between the precision and accuracy of an assay method, and explain the sensitivity and specificity of an assay method.
Explain what is meant by the sensitivity, specificity, and predictive value of a laboratory test.
List techniques that are commonly used in a diagnostic lab carrying out biochemical tests and explain the principle of each method.
Explain why high plasma concentrations of enzymes are considered to be indicators of tissue damage.
Describe in outline the different requirements for measuring an enzyme in a plasma sample and using an enzyme to measure an analyte.
THE IMPORTANCE OF LABORATORY TESTS IN MEDICINE
Various laboratory tests are an essential part of medicine and veterinary practice. Biochemical tests can be used for screening for disease, for confirmation (or otherwise) of a diagnosis made on clinical examination, for monitoring progression of a disease and the outcome of treatment. Blood and urine samples are most commonly used; occasionally feces, saliva, or cerebrospinal fluid (CSF) may be used, and on rare occasions, tissue biopsy samples. Most of our knowledge and understanding of the underlying causes of metabolic diseases and of the effects of disease on metabolism has come from analysis of metabolites in blood and urine, and from measurement of enzymes in blood. In turn, that knowledge has permitted advances in the treatment of disease and the development of more effective drugs.
Advances in technology mean that many tests that were formerly carried out only in specialist laboratories can now be performed at the bedside, in the doctor’s office, or veterinary practice, sometimes even at home by patients themselves, with automated machines or “dipsticks” that are simple to use. Other tests are still conducted in hospital laboratories or by private clinical chemistry laboratories, with samples sent in by the referring physician. Some tests that are less commonly requested and may be technically more demanding are performed only in specialist centers. These often involve specialist techniques to study rare (and sometimes newly discovered) metabolic diseases. In addition, testing of samples from athletes and race horses for performance-enhancing drugs and other banned substances is normally carried out in only a limited number of specially licensed laboratories.
CAUSES OF ABNORMALITIES IN LEVELS OF ANALYTES MEASURED IN THE LABORATORY
A great many different conditions can lead to abnormalities of the results of laboratory tests. Tissue injury that results in damage to cell membranes and an increase in the permeability of the plasma membrane leads to leakage of intracellular material into the bloodstream (eg, leakage of creatine kinase MB into the bloodstream following a myocardial infarction). In other cases, the synthesis of proteins and hormones is increased or decreased (eg, ...