The Test Sample
What is being tested?
These tests detect the presence of disease-causing (pathogenic) Escherichia coli that produce Shiga toxins. E. coli bacteria commonly occur in nature and they are a necessary component of the digestive process in humans and most other mammals. Most strains of E. coli are harmless, but pathogenic E. coli can be responsible for inflammation of the stomach and intestines (gastroenteritis). There are multiple subtypes of E. coli that cause diarrheal illness, and they are classified by the mechanisms by which they cause disease. For example, some invade the lining of the intestines, causing inflammation, while others produce toxins.
E. coli that produce poisons called Shiga toxins are generally the only type of E. coli that are tested for in clinical settings from stool specimens. The Shiga toxins associated with these infections are so called because they are related to the toxins produced by another type of disease-causing bacteria, Shigella.
A strain of Shiga toxin-producing E. coli (STEC) called O157:H7 has been responsible for many outbreaks of gastrointestinal illness in the U.S, though there are also non-O157 strains of STEC that are gaining recognition in individual cases and outbreaks. For example, an outbreak of E. coli O104:H4, a non-O157 STEC, was associated with travel to Germany and resulted in 32 deaths in 2011 from eating contaminated sprouts. In 2007, STEC O157:H7 was responsible for 7% of gut-related diseases reported to all U.S. health agencies.
Outbreaks have been linked to the consumption of contaminated food, including undercooked ground beef, unpasteurized juice, raw milk, and raw produce such as leafy greens and alfalfa sprouts. STEC may also be transmitted through contaminated water, contact with farm animals or their environment, and from person to person.
In addition to symptoms of nausea, severe abdominal cramps, watery diarrhea, fatigue, or possible vomiting and low grade fever, STEC infections are often associated with bloody stools and can lead to serious complications, specifically hemolytic uremic syndrome (HUS). HUS is a result of the toxin entering the blood and destroying red blood cells (hemolysis). It can lead to kidney failure (uremia or the build up of nitrogen wastes in the blood) and can be life-threatening. Signs and symptoms include decreased frequency of urination (evidence of uremia), fatigue, and pale skin due to hemolytic anemia. HUS usually develops about a week after the onset of diarrhea.
Approximately 8% of people who are diagnosed with an O157 STEC infection develop HUS. Children, the elderly, and persons with weakened immune systems are at greatest risk. However, most healthy persons recover from a STEC infection within a week and do not develop HUS. Non-O157 Shiga toxin producing E. coli can cause the same symptoms and complications and likely account for 20-50% of STEC infections in the U.S. annually. Different testing techniques are required to identify O157 and non-O157 Shiga toxin-producing E. coli.
How is the sample collected for testing?
A fresh liquid or unformed stool sample is collected in a clean dry container. The stool sample should not be contaminated with urine or water. Once it has been collected, the stool should be taken to the laboratory immediately or refrigerated and taken to the lab as soon as possible. Some laboratories provide transport media to support the survival of the organism from the time of collection until delivery to the laboratory. STEC become difficult to detect in the stool after one week of illness, so the timing of sample collection relative to the onset of illness is important.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.