Shiga toxin-producing Escherichia coli (STEC) test

You do not need to do anything special before giving a sample. Your doctor or the laboratory will explain how to collect the sample correctly. 

You will need to collect a fresh stool (poo) sample into a clean, dry container. The sample must not contain urine or water. Take it to the laboratory as soon as possible or keep it in the fridge until you can. Some laboratories provide a special transport container to help preserve the sample during delivery. 

STEC become harder to detect after the first week of illness, so it is important to give a sample as early as possible after your symptoms begin. Infants may have a rectal swab taken instead of a stool sample. 

What the test measures 

STEC tests look for harmful strains of Escherichia coli bacteria that produce poisons called Shiga toxins. Most E. coli are harmless and live naturally in the gut. But some strains produce Shiga toxins that cause severe illness, including bloody diarrhoea and, in serious cases, kidney failure. 

The most common harmful strain in the UK is E. coli O157:H7. However, other strains (called non-O157 STEC) can cause the same symptoms and complications. The implementation of PCR assays targeting common enteric pathogens has detected an increasing number of non-STEC serotypes in the UK. 

Your laboratory may use one or more of the following tests: 

• Stool culture – a sample is grown in the laboratory to identify E. coli O157:H7 bacteria. This method is restricted to the detection of a limited range of STEC serogroups and will usually be used in conjunction with molecular methods (PCR test) that have a high sensitivity and specificity for all STEC serotypes. 
• PCR test – detects the genes that make Shiga toxins. This can find both O157 and non-O157 STEC. It is now the most commonly used first test in UK laboratories. 
• Toxin test (EIA) – detects the Shiga toxin directly in your stool. This method is less commonly used since PCR became available.

What your results mean 

If your result is positive 

A positive result means that STEC bacteria or their toxins were detected in your sample. This confirms that a STEC infection is likely causing your symptoms. Your doctor will work with you to decide on the right next steps, which may include monitoring for complications such as HUS. 

A positive result is also reported to the local public health team. This is a legal requirement, as STEC is a notifiable infection. The public health team may contact you to gather more information. This helps prevent the infection from spreading to others.

If your result is negative 

A negative result suggests that STEC bacteria were not found in your sample. However, a negative result does not always rule out infection. STEC become harder to detect in stool after the first week of illness. 

If your symptoms strongly suggest a STEC infection, your doctor may refer your sample to a specialist reference laboratory for further testing. Another cause for your symptoms, such as Salmonella, Shigella, or Campylobacter, may also be investigated.

Most people with STEC recover at home within a week. The main treatment is rest and drinking plenty of fluids to avoid dehydration. There is no specific medicine for STEC. 

Do not take antibiotics or anti-diarrhoeal medicines (such as loperamide) unless your doctor advises you to. These can increase the risk of developing HUS. If you have bloody diarrhoea or feel very unwell, contact your GP or call NHS 111 straight away. 

. To mitigate the risk of transmission, children aged 5 years old and under, and all those in risk groups (for example food handlers, carers of the elderly, immunocompromised and those with underlying health conditions) are excluded from work, school and childcare settings until they are microbiologically clear. Your GP or public health team will advise you on this.

The timing of the sample is the most important factor. STEC are much harder to detect after the first week of illness. Giving your sample as soon as possible after symptoms start gives the best chance of an accurate result. 

If your stool sample is contaminated with urine or water, or is not kept at the right temperature, this can reduce the quality of the test. Follow any instructions given by your laboratory carefully. 

Standard stool cultures detect E. coli O157 but cannot detect non-O157 STEC. If your symptoms suggest a non-O157 infection, your sample may need to be sent to a reference laboratory for additional testing. 

If STEC is confirmed, or if your doctor is concerned about complications, they may arrange blood and urine tests to check how your kidneys are working. These are especially important if you develop signs of HUS, such as reduced urination, extreme tiredness, or pale skin. 

If another cause for your symptoms is suspected, your stool sample may also be tested for other bacteria such as Salmonella, Shigella, or Campylobacter. 

See also: Stool culture; OCP (ova, cysts and parasites) 

STEC infection is caused by strains of E. coli that produce Shiga toxins. It can cause severe diarrhoea, stomach cramps, and, in some cases, a serious kidney complication called HUS. Most people recover fully within a week. For more information, visit NHS inform: E. coli O157.