Using microbiological tests to select an appropriate antimicrobial

Review the results of microbiological testing promptly. The results may enable empiric therapy to be changed to directed therapy (ie use of a narrower-spectrum antimicrobial or therapy with a single agent).

Reviewing the stain (eg Gram stain) results, cell counts and interpretive comment can assist in interpretation of the culture results. The stain results may also be available before the final microbiology report and can be used to direct initial therapy. Presence of high numbers of leucocytes is indicative of inflammation or infection. Presence of organisms on appropriately collected samples and their growth may also guide decision making. In the absence of symptoms, the presence of some organisms (eg organisms found in chronic wounds or urine) may represent colonisation and not infection. Correlation of the clinical syndrome with the Gram stain or cell count is important.

If there is evidence of infection, assess the antimicrobial susceptibilities reported for the respective organism. Some laboratories use hierarchical reporting where only the narrowest spectrum agents for treatment are reported. Other laboratories report a standardised set of antimicrobials without providing hierarchical information. In these cases, review the antimicrobials reported as ‘susceptible’ or ‘susceptible at increased dose exposure’ and select the antimicrobial with the narrowest spectrum at the appropriate dose. For information about the standardised testing methods used by laboratories and definitions of susceptibilities, see Interpreting microbiological testing results.

If all the agents are listed as ‘resistant’ or the susceptible agents are not appropriate, seek expert advice from a clinical microbiologist, an antimicrobial stewardship (AMS) service, or an infectious diseases physician.

Where there are 2 or more organisms to be treated, consider selecting a single agent that treats all organisms.