After the diagnosis is made and the clinician determines that antibacterial treatment is warranted, the infectious etiology needs further consideration. The etiology for some infections may be established on the basis of clinical presentation, whereas further diagnostic testing is needed in other cases. In considering the need for etiologic diagnosis, clinicians must consider the difficulty of obtaining an appropriate specimen, laboratory costs, the likelihood of finding the etiology, the likelihood of serious outcome or complication with inappropriate therapy, and often, knowing how the etiology will alter treatment. For example, culture and pathogen identification is generally not needed for acute otitis media, acute rhinosinusitis, or uncomplicated urinary tract infections. In cases of simple cellulitis, the difficulty of obtaining uncontaminated specimens and low yield precludes routine culturing. The role of sputum cultures with community-acquired pneumonia is controversial for the same reasons. Although blood cultures are quite specific, sensitivity is low. The value of culture to determine specific etiology is quite clear for some infections, such as blood cultures in suspected sepsis or endocarditis, cerebrospinal fluid cultures in suspected meningitis, or bone biopsy in osteomyelitis. Defining exactly when culture is required and when susceptibility testing should be performed is beyond the scope of this chapter. However, a potential pathogen in situations of clinically apparent infection may not be cultured despite appropriate culturing methods. In the case of community-acquired pneumonia, sputum and blood cultures identify a specific etiology in less than half the cases (18). When cultures are negative, empiric therapy may be appropriate, depending on the clinical entity and likelihood of a bacterial pathogen. Susceptibility testing should clearly be performed for invasive infections and for clinical isolates in which resistance has been documented with relative frequency. Given modern resistance problems, susceptibility testing is usually indicated any time a culture is taken and a probable pathogen is isolated. The susceptibility results may not always influence therapy in the individual patient, but the results are also important for local susceptibility surveillance.

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