Laboratory results are a tool, not a diagnosis. Results should be interpreted in the context of the patient’s disease and clinical progression. If a patient is responding well to empirical therapy but pretreatment culture results indicate resistance to the chosen drug, a drug change may not be indicated. The patient should be reassessed, but changing treatment based solely on a culture report in a patient responding clinically may be counterproductive.
It is also important to remember that not all microbes must be killed. Antimicrobial therapy does not need to be directed against every isolated pathogen. Multiple bacteria are sometimes isolated from a diagnostic sample, which may complicate selecting an appropriate drug or drug combination effective against all identified bacteria. However, true co-infections are less common than mixed-growth culture results. Before choosing a complicated combination or undesirable drug (eg, vancomycin and other drugs that are expensive, toxic, and used to treat serious infections in humans) to address all isolates reported, consider the infection site (ie, which bacteria usually cause disease there and which are often contaminants), the level of reported growth (with low levels more likely to be contaminants), and the nature of the bacteria. Some species (eg, coagulase-negative staphylococci, Bacillus spp, enterococci) are common contaminants of limited virulence that can sometimes be ignored, particularly when present along with more convincingly pathogenic species.