Description of the test
All stool samples (and anal smears) as well as pure isolates are first cultured. The presence of Shiga toxin genes [stx, the three variants of stx1 (a, c, d) and the seven variants of stx2 (a to g)] is detected by PCR on bacterial growth. If positive, individual colonies are then examined. Unfortunately, in about 30% of positive cultures it is not possible to demonstrate an individual positive colony.
In HUS patients, the number of cultivable STEC present in the stool sample is often low and, moreover, these patients are often treated with antibiotics beforehand, which makes culture no longer possible. To make a diagnosis in these cases, samples from HUS patients are additionally subjected to DNA extraction directly from the faeces sample followed by PCR for Shiga toxin genes. This test is also performed on anal smears (non-validated sample type).
Purpose of the test
The aim of the test is to isolate Shiga toxin-producing Escherichia coli strains that can be further characterised.
Incidentally, for HUS cases, PCR directly on sample aims to confirm the presence of STEC when isolation of colonies is unsuccessful.
Criteria for performing this test in the context of reference activities
STEC can be detected in the following cases:
- Patients with haemolytic uraemic syndrome (HUS), including those who have not experienced a prior episode of diarrhoea (especially in children, this may be absent). Please systematically send a stool sample to the reference laboratory (or an anal smear if the patient has no stool, but in that case, send the first stool for analysis afterwards as well).
- Bloody diarrhoea, haemorrhagic colitis
- Epidemic of diarrhoea
- Other cases of diarrhoea if epidemiological elements suggest STEC infection
All STEC strains cultured in other laboratories are accepted for confirmation.
Instructions for samples
Specimens
Note: it is very important to indicate whether or not it is an HUS case.
- Stool sample (preferred sample)
- Anal smear (accepted for HUS cases only; always send first stool sample later then)
- Primary reisolation on enteric agar medium
- Pure cultures of STEC
Storage
At 4°C, maximum one week.
Instructions for transport
Clinical samples should preferably be transported to the NRC within 24 hours without refrigeration. Material from the in-house laboratory can be used for this purpose for transport according to UN3373.
Faecal samples should be collected in a leak-proof sterile container. Rectal swabs are preferably sent on transport medium. The NRC does not distribute material.
The strains are transported to the NRC without refrigeration. Material (transport medium or solid media) from the in-house laboratory can be used for this purpose for transport according to UN3373.
The NRC does not distribute material.
Unacceptable requests
Indications
- Repeat requests (except in justified cases, e.g. providing evidence of a negative test result in contact with vulnerable persons)
- Asymptomatic patients (except in the context of investigation of epidemics in consultation with the health inspector, Sciensano and the NRC)
- Samples without a completed application form.
Sample collection
Sampling is preferably done before initiating antibiotic therapy.
Turn around time (and frequency of analysis)
Culture is plotted daily and PCR every working day. An initial result is usually communicated within 2 working days.
In case of a positive result, individual colonies are then searched and confirmed and characterised (see ‘Systematic typing of STEC isolates’). A complete result is reported within 7 days.
Reporting of test results
- By secure electronic mail and by post for an initial positive result
- By post for additional results of positive samples
- By post for negative results
- By Medibridge for laboratories and prescribers with a connection to the UZ Brussel.