3.1 What affects appropriate sample selection?

Diagnosis requires a composite of information, including history, physical examination, radiographic findings and laboratory data. Specimens are selected based on signs and symptoms.

It is important that samples should be representative of the site of infection, and should be collected before antimicrobial agents are administered (otherwise the growth of organisms in the laboratory will be suppressed). You should also consider that the patient may have taken antimicrobials acquired outside the healthcare system before presenting.

As noted above, other types of infection also need to be considered. In some cases, sampling follows a protocol or algorithm, where patients with particular symptoms are initially tested for one type of infection (such as malaria, or Covid-19); if these are negative, or if the patients do not respond to initial treatment, you can then make further investigations for other infections or non-infectious conditions. Clinical microbiologists or infectious disease specialists can provide advice on prioritising tests or developing testing algorithms.