Certain factors can hinder the diagnosis of an infectious disease by culture and biochemical techniques. These factors include the inability to cultivate an organism on artificial media, such as with Treponema pallidum, the agent of syphilis, or the fragility of an organism and its subsequent failure to survive transport to the laboratory, such as with respiratory syncytial virus and varicella-zoster virus. The fastidious nature of some organisms, such as leptospira or Bartonella, can result in long incubation periods before growth is evident, or administration of antimicrobial therapy before a specimen is obtained, such as with a patient who has received partial treatment, also can impede diagnosis. In these cases, detecting a specific product of the infectious agent in clinical specimens is very important because this product would not be present in the specimen in the absence of the agent. This chapter discusses the direct detection of microorganisms in patient specimens using immunochemical methods and the identification of microorganisms by these methods once they have been isolated on laboratory media. Chapter 10 discusses the diagnosis of infectious diseases by using some of these same immunochemical methods to detect antibodies produced in response to the presence of an infecting agent in patient serum.
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