Abstract

Laboratory tests that are usually considered helpful in guiding antimicrobial therapy include antimicrobial agent susceptibility tests, determination of bacterial production of beta-lactamase, and assay of specific antimicrobial levels in serum and other body fluids. Susceptibility tests should be performed primarily on clinically significant isolates from critical specimens (such as blood or other normally sterile body fluids or tissues) with use of standardized methods established by the National Committee for Clinical Laboratory Standards. Reporting of results should be selective so that clinicians are encouraged to use the least expensive but useful agent in a group--for example, first-generation rather than third-generation cephalosporins. Because standardized methods are not available for assays of serum inhibitory and bactericidal activity, the accuracy and clinical utility of these tests are as yet undetermined. Determination of bacterial resistance to antimicrobial agents is the most important goal of susceptibility testing. Special methods are needed to detect methicillin-resistant staphylococci, high-level aminoglycoside- and glycopeptide-resistant enterococci, and antimicrobial-resistant strains of Neisseria gonorrhoeae, Streptococcus pneumoniae, and Haemophilus influenzae. Accurate measurement of serum concentrations of antimicrobial agents is important to ensure that therapeutic levels have been obtained and to avoid excessive levels of potentially toxic agents such as aminoglycosides and glycopeptides, especially when renal function is compromised.