What have the following got in common?
- E.coli resistant to nitrofurantoin
- E.coli resistant to fosfomycin
- Haemophilus influenzae resistant to ciprofloxacin
- Group B streptococcus resistant to penicillin.
- Coagulase negative staphylococci resistant to vancomycin
- Candida albicans resistant to fluconazole
In my area of the world anyway (New Zealand), the percentage resistance rates of the above micro-organism/antimicrobial combinations is less than 1%. i.e. the prevalence is very low.
And because the prevalence is very low, unless your susceptibility testing methods are very specific, the positive predictive value of the result will also be very low. Thus , there will generally be a large number of false positives amongst such results.
Such a result should therefore automatically trigger a double check of everything, with a close look at the audit trail leading to the result. In some circumstances, repeating the test or sending the isolate to a reference laboratory may be the best option even if the result looks genuine.
We always need to be very careful when reporting low prevalence results, because even though we would like them to be, our tests are generally not perfect…