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The laboratory diagnosis of infectious diseases involves two main approaches: one is the bacteriologic approach, in which the organism is identified by staining and culturing the organism, and the other is the immunologic (serologic) approach, in which the organism is identified by detection of antibodies against the organism in the patient’s serum.

In the bacteriologic approach to the diagnosis of infectious diseases, several important steps precede the actual laboratory work, namely, (1) choosing the appropriate specimen to examine, which requires an understanding of the pathogenesis of the infection; (2) obtaining the specimen properly to avoid contamination from the normal flora; (3) transporting the specimen promptly to the laboratory or storing it correctly; and (4) providing essential information to guide the laboratory personnel.

In general, there are three approaches to the bacteriologic laboratory work:

  1. Observing the organism in the microscope after staining.

  2. Obtaining a pure culture of the organism by inoculating it onto a bacteriologic medium.

  3. Identifying the organism by using biochemical reactions, growth on selective media, DNA probes, or specific antibody reactions. Which of these approaches are used and in what sequence depend on the type of specimen and organism. After the organism is grown in pure culture, its sensitivity to various antibiotics is determined by procedures described in Chapter 11.

A general approach to the diagnosis of a bacterial infection is described in Table 9–1. This approach emphasizes the importance of performing a Gram stain and obtaining a “pure culture” of the organism. However, sometimes the organism is not recovered by culturing, and other techniques must be used. Table 9–2 describes some approaches to making a diagnosis when the cultures are negative. One approach that is commonly used is serologic testing, which determines the presence of antibodies specific for the organism. In most cases, a fourfold rise in antibody titer between the acute- and convalescent-phase serum samples is considered to be significant.

TABLE 9–1General Approach to the Diagnosis of a Bacterial Infection

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