Susceptibility Testing

Some types of bacteria and fungi are known to be susceptible to certain antibiotics, so routine testing may not always be necessary. For example, it is well known that most streptococci, including the type that causes strep throat, can be treated with penicillin, so these types of infections may be treated without susceptibility testing. However, if the usual drug of choice fails to treat the infection, then susceptibility testing may be necessary to determine a more effective drug.

Other types of infections may require testing because the bacteria or fungi isolated from an infection site are known to have unpredictable susceptibility to the antibiotics usually used to treat them. Some examples include staphylococci (“staph”) and Pseudomonas aeruginosa. Sometimes there may be more than one type of pathogen isolated from an infected site. Susceptibility testing may be used to determine which antibiotic or antibiotic combinations will be most effective in treating the different types of bacteria causing the infection. This may be true, for example, with wound infections.

• Susceptible — likely, but not guaranteed to inhibit the pathogenic microorganism; may be an appropriate choice for treatment
• Intermediate — may be effective at a higher dosage, or more frequent dosage, or effective only in specific body sites where the antibiotic penetrates to provide adequate concentrations
• Resistant — not effective at inhibiting the growth of the organism; may not be an appropriate choice for treatment

If there is more than one pathogen, the laboratory will report results for each one.

Your doctor will choose an appropriate antimicrobial agent from those on the report that were categorized as "Susceptible." If there are no "Susceptible" choices, then the doctor may select one categorized as "Intermediate." This may mean a higher dosage of antimicrobial and may involve a longer duration of therapy and a higher risk for medication side effects. A pathogen may be "Resistant" to all of the antimicrobials that are usually used to treat that type of infection. If this is the case, then the doctor may prescribe a combination of antibiotics that work together to inhibit the bacteria when neither one alone will be effective. These drug therapies may be more expensive and have to be given intravenously, sometimes for extended periods of time. Some infections due to resistant bacteria have proven very difficult to treat.