The antimicrobial drug used initially in the treatment of an infection is chosen on the basis of clinical impression after the clinician is convinced that an infection exists and has made a tentative etiologic diagnosis on clinical grounds. On the basis of this “best guess,” a drug that is likely to be effective against the suspect agent(s) can be selected . Before this drug is administered, specimens are obtained for laboratory detection of the causative agent. The results of these examinations may allow for narrowing of antibiotics to targeted therapy (as opposed to broad gram-positive and gram-negative coverage for sepsis). The identification of certain microorganisms that are uniformly drug-susceptible eliminates the necessity for further testing and permits the selection of optimally effective drugs based on the organism’s known susceptibility profile. When the organism resistance profile is varied, tests for drug susceptibility of isolated microorganisms will guide optimal drug choice . Disk diffusion susceptibility tests measure the ability of bacteria to grow on the surface of an agar plate in the presence of paper disks containing antibiotic drug. The drug diffuses out into the surrounding agar, inhibiting bacterial growth in a circular area surrounding the disk. The diameter of this zone of growth inhibition is measured, and correlates with the susceptibility of the isolate being tested. The choice of drugs to be included in a routine susceptibility test battery should be based on the susceptibility patterns of isolates in the laboratory, the type of infection (community-acquired or nosocomial), the source of the infection, and cost effectiveness analysis for the patient population. The Clinical and Laboratory Standards Institute (CLSI) (Wayne, PA) pro vides recommendations for which agents to test based on the organism recovered and the specimen type, and interpretive criteria (susceptible, intermediate, or resistant) based on the measured zone size.

The sizes of zones of growth inhibition vary with the pharmacologic characteristics of different drugs. Thus, the zone size of one drug cannot be compared to the zone size of another drug acting on the same organism. However, for any one drug the zone size can be compared to a standard, provided that media, inoculum size, and other conditions are carefully controlled. This makes it possible to define for each drug a diameter of inhibition zone that distinguishes susceptible from intermediate or resistant strains.

The disk test measures the ability of drugs to inhibit the growth of bacteria in vitro. The results correlate reasonably well with therapeutic response in those disease processes in vivo when body defenses can eliminate infectious microorganisms, but may be less well correlated with response in immunocompromised patients. The selection of appropriate antibiotic therapy depends on clinical as well as bacterial factors, such as use of bactericidal rather than bacteriostatic drugs for endocarditis, or drugs that will penetrate the blood–brain barrier for central nervous system infections. Minimum inhibitory concentration (MIC) tests measure the ability of organism to grow in broth culture in the presence of various dilutions of antibiotics. It measures more exactly the concentration of an antibiotic necessary to inhibit growth of a standardized inoculum under defined conditions. A semiautomated microdilution method is used in which defined amounts of drug are dissolved in a measured small volume of broth and inoculated with a standardized number of microorganisms. The end point, or MIC, is considered the last broth cup (lowest concentration of drug) remaining clear, ie, free from microbial growth. The MIC provides a better estimate of the probable amount of drug necessary to inhibit growth in vivo and thus helps in gauging the dosage regimen necessary for the patient. Guidelines available from CLSI provide interpretive criteria, defining strains as resistant, intermediate, or susceptible to a certain drug based on the MIC.

The MIC only shows that bacterial growth is inhibited at that drug concentration; there may still be viable bacteria that can recover when the drug is removed. Bactericidal effects can be estimated by subculturing the clear broth from MIC testing onto antibiotic-free solid media. The result, eg, a reduction of colony-forming units by 99.9% below that of the control, is called the minimal bactericidal concentration (MBC).

results of antimicrobial susceptibility tests are available, it is recommended by CLSI that laboratories publish an antibiogram annually that contains the results of susceptibility testing in aggregate for particular organism–drug combinations. For example, it may be important to know the most active β-lactam antimicrobial agent targeted against Pseudomonas aeruginosa among intensive care unit patients in a particular hospital. This allows the best therapy to be chosen based on clinical suspicion of the infecting organism and known locally circulating strains.

The selection of a bactericidal drug or drug combination for each patient can be guided by specialized laboratory tests. Such tests measure either the rate of killing (time-kill assay) or the proportion of the microbial population that is killed in a fixed time by patient serum (serum bactericidal testing). Synergy testing measures the ability of drugs to enhance bacterial killing when present in combination; drugs showing synergy may be more effective when given together to treat infection. Few clinical laboratories perform this type of specialized susceptibility testing.