PRINCIPLES OF CLINICAL BIOCHEMICAL ANALYSIS
Basis of Analysis of Body Fluids for Diagnostic, Prognostic and Monitoring Purposes
Underlying most human diseases is a change in the amount or function of one or more proteins that in turn triggers changes in cellular, tissue or organ function. The dysfunction is commonly characterised by a significant change in the biochemical profile of body fluids. The application of quantitative analytical biochemical tests to a large range of biological analytes in body fluids and tissues is a valuable aid to the diagnosis and management of the prevailing disease state. In this section, the general biological and analytical principles underlying these tests will be discussed and related to the general principles of quantitative biochemical analysis discussed in Section 2.7.
Body fluids such as blood, cerebrospinal fluid and urine, in both healthy and diseased states, contain a large number of inorganic ions and organic molecules. While some of these chemical species exert their normal biological function within that fluid, the majority of these take a passive role and are only transported by the fluid. The presence of this latter group of chemical species within the fluid is due to the fact that normal cellular secretory mechanisms and the temporal synthesis and turnover of individual cells and their organelles within the major organs of the body both result in the release of cell components into the surrounding extra-cellular fluid and eventually into the blood circulatory system. This, in turn, transports them to the main excretory organs, namely the liver, kidneys and lungs, so that these cell components and/or their degradation products are eventually excreted in faeces, urine, sweat and expired air. Examples of cell components in this category include enzymes, hormones, intermediary metabolites, small organic molecules and inorganic ions. The concentration, amount or enzyme activity of a given cell component that can be detected in these fluids of a healthy individual at any point in time depends on many factors that can be classified into one of three categories, namely chemical characteristics of the component, endogenous factors characteristic of the individual and exogenous factors that are imposed on the individual.