Laboratory tests that can be helpful in guiding antimicrobial therapy include antimicrobial susceptibility testing, determination of bacterial beta-lactamase activity, assay of serum inhibitory and bactericidal activity, and assay of specific antibiotic levels in serum. When any microorganism is isolated from a normally sterile body fluid in a patient with clinical evidence of infection, susceptibility studies should be performed. The standardized disk test provides results that should be comparable from laboratory to laboratory but has the disadvantage of yielding results expressed only as susceptible, intermediate, or resistant. In contrast, dilution methods allow determination of the minimal inhibitory concentration of an agent, which can be correlated with blood, urine, and other body fluid levels of the antimicrobial agent. Accurate measurement of serum concentrations of antimicrobial agents is important when the margin between therapeutic and toxic levels is narrow, such as for aminoglycosides or vancomycin, and when a patient has renal failure and may have accumulation of high levels of antimicrobial agents that would normally be excreted by the kidneys.