The microbiology laboratory serves as a valuable ally to clinicians in the diagnosis and treatment of infection. In particular, the isolation of bacteria from clinical samples yields useful information that is translated directly into therapeutic strategies for the patient. While fungi, viruses, and parasites are also identified in the laboratory, the technology to perform susceptibility testing is not routinely available. Thus, discussion of antimicrobial susceptibility testing usually addresses bacterial isolates.
The responsibility of the clinical microbiology laboratory is to identify the bacterial isolate and to determine antibiotic susceptibility patterns. However, not all bacteria isolated from clinical specimens have antibiotic susceptibility performed; some of the factors that influence this decision include:
- Whether the antibiotic susceptibility of an isolate is predictable based upon the genus and species
- Whether the organism is likely to represent normal flora in which case susceptibility testing is not routinely done
- Whether sufficient numbers of colonies are present; if colony numbers are low (eg, urine, intravenous catheter tip), susceptibility tests are frequently not done.
Host factors also play a role in whether or not susceptibilities should be performed. If clinicians notify the laboratory that a patient is a transplant recipient or another immunocompromised host, all organisms will be identified and analyzed for susceptibility to antibiotics. The same organisms recovered from an immunocompetent patient may not be tested.
More detailed susceptibility testing is required for certain serious infections. As an example, determination of the minimum inhibitory concentration (MIC) may be necessary to choose optimal therapy for infections, such as streptococcal endocarditis  and pneumococcal meningitis .