Choice of susceptibility test method and agents

Choice of method depends on the organism to be tested, the techniques available, and the need for speed and accuracy. The choice of antimicrobial agents to be tested is based on the organism to be tested, its site of isolation, and local patterns of susceptibility and antimicrobial usage. Some antimicrobials, such as amikacin and the newer b-lactams, are tested only if resistance to older aminoglycosides or b-lactams is detected.

Because critically ill patients are often treated with combinations of antimicrobial agents, the interaction of these agents is important. Effective antimicrobial combinations are either synergistic or additive. These effects can be assessed in vitro by testing the rate of killing of an organism with drugs alone and in combination. Many drug combinations are known to be synergistic in vivo, particularly b-lactams and vancomycin with aminoglycosides. Combinations known to be antagonistic in vivo include penicillin G (benzylpenicillin) with tetracycline in pneumococcal meningitis and chloramphenicol with a b-lactam agent in meningitis caused by enteric Gram-negative bacilli.

Susceptibility of yeasts and molds to amphotericin B and itraconazole, and of yeasts to ketoconazole and 5-fluorocytosine, can be determined using techniques similar to MIC determinations for bacteria. However, these techniques are difficult to perform and are best done by specialist reference laboratories.

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