Laboratory tests used to guide antimicrobial therapy.

Laboratory tests that can be helpful in guiding antimicrobial therapy include antimicrobial susceptibility testing, determination of bacterial beta-lactamase production, assay of serum inhibitory and bactericidal activity, and assay of specific antibiotic levels in serum. Susceptibility studies should be performed on any microorganism that is isolated from normally sterile body fluid (blood, cerebrospinal fluid, pleural fluid, synovial fluid) in the presence of clinical evidence of infection. The standardized disk test provides results that should be comparable from laboratory to laboratory. Dilution methods, however, allow determination of the minimum concentration of an agent which inhibits growth (MIC), and this value can be correlated with blood, urine, and other body fluid levels of the antimicrobial agent. Determination of serum bactericidal activity is, in effect, an assay of the activity of antimicrobial-containing serum; it indirectly measures the combined effects of susceptibility of the test organism and serum concentration of the antimicrobial agent. Accurate measurement of serum concentrations of antimicrobials may be important when treatment includes agents that have a narrow margin between their therapeutic and their toxic levels such as the aminoglycosides (especially gentamicin) or in patients with renal failure, who may accumulate unusually high levels of antimicrobials normally excreted by the kidneys.