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INTRODUCTION

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OBJECTIVES

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.

  • List causes that result in abnormalities in levels of analytes in blood.

  • 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.

  • Describe the main tests that can be used to assess kidney, liver, and thyroid function; describe markers of cardiovascular risk and gastro-intestinal function.

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THE IMPORTANCE OF LABORATORY TESTS IN MEDICINE

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Laboratory tests of one kind or another are an essential part of medicine. 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 (Table 48–1). Blood and urine samples are most commonly used; occasionally feces, saliva, or cerebrospinal fluid may be used. On rare occasions, tissue biopsy samples may be used. 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 better drugs.

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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 that are simple to use and require only a limited amount of training to provide reliable results. 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.

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Table Graphic Jump Location
TABLE 48–1Major Uses of Biochemical Tests With Selected Examples for Each

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