Presumptive Diagnostic Tests

Presumptive diagnostic tests are procedures that, when properly performed, may indicate the presence of a particular agent or closely related agents. Presumptive diagnostic tests for biologic and chemical agents are more complex and require more time to perform than do the simple tests described above. Most bacterial and viral agents can readily be grown in culture media or cell culture if the appropriate conditions are met. These conditions include the appropriate temperature, pH, and a source of energy (e.g., glucose). Inhibitory substances, such as antibiotics, are often placed into the growth media to prevent the growth of unwanted organisms. Growth of organisms in cell culture or artificial media takes 24 hours to 30 or more days, depending on the organism.

Once organisms are detected on or in growth media, various techniques are used to determine the genus and species of the organism. For bacteria, the differential growth on select media, metabolism of various carbohydrates and other chemicals, and the presence of selected enzymes are often used for identification. Special stains that incorporate florescent dyes coupled with antibodies to selected organisms are used for identification of bacteria and viruses.

Laboratories use immunologic assays to identify many biologic agents and for the subspecies typing that is needed for epidemiological purposes. Immunologic assays generally involve the use of a specific antibody that can attach to or react with the outer-surface structures of an agent. Other immuno-logic assays are used to detect antibodies in biological materials after infection with biological agents. After the isolation and identification of many organisms, drug-susceptibility testing determines the organism's susceptibility to drugs that are used to treat infections with the organism.

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