- Whenever possible, samples for bacterial culture should be taken before instituting antibiotic therapy. Just one or two doses even of the wrong antibiotic may hinder or prevent completely the isolation of an infecting pathogen. Therefore if a patient fails to respond to empirical treatment, there is little prospect that investigation at that stage will be productive.
If a patient is already on antimicrobials and bacteriological investigation is warranted, consideration should be given to suspending therapy (the patient's clinical condition permitting) for 48 - 72 hours before taking samples and restarting on a different regime.
If samples are taken with the patient already on antibiotics, it is helpful to indicate this on the request form so that the laboratory may take this into account - it may be possible to neutralise the drug, or compensate by prolonging the time the cultures are incubated.
- Samples should be taken as close as possible to, if not actually from, the anatomical focus of the infection. The further away from the site of infection, the lower the numbers of the pathogen become (eg. this dilution effect may be apparent in failure to detect small intestinal parasites in stool); some fastidious organisms will not even survive in or on adjacent tissues (eg. the gonococcus is rarely isolated from high vaginal swabs even when the infection is obvious on a cervical swab).
In addition, irrelevant colonising bacterial flora may be construed as having a pathogenic role if the infected site is not sampled directly (eg.overgrowth of potential pathogens often occurs on the surface of infected wounds, ulcers or discharging sinuses but they are seldom responsible for the underlying infection).
The practicalities of taking the optimum samples from different sites are discussed in more detail under the headings under Bacteriology General Information.
- Having taken a sample, commonsense should indicate if it is a poor one or not. Just looking at it may reveal gross inconsistencies (eg. a rectal swab without faecal staining, or the frothy salivary 'sputum' from a patient who is coughing foul pus into the pot on the bedside locker).
- Having obtained a proper sample its quality should not be jeopardised by putting it in the wrong container (all too often precious surgical material taken in theatre arrive at the laboratory in formaldehyde), allowing it to leak out of its container, or failing to get it to the laboratory within a reasonable time.
- For many patients the collection of microbiological specimens is a passive exercise. They should be extended the courtesy of an explanation of what the sample is for and how it might affect their management. Also, their active participation (based on understanding what is required) may be crucial to obtaining a good quality sample : this applies particularly to urine collection but also to faeces and sputum. Clear and simple instructions should be given, and help offered when necessary.