Diagnosing Infectious Diarrhea Using Molecular Panels
The Infectious Diseases Society of America (IDSA) published an update to their 2001 guidelines on the diagnosis and management of infectious diarrhea in a recent issue of Clinical Infectious Diseases. For the first time, the guidelines include recommendations and cautions about the use of newer molecular panels that test for multiple disease-causing microbes at the same time. The panels detect the genetic material (DNA, RNA) of bacteria, toxins, viruses and parasites and are more sensitive than traditional tests such as cultures.
Diarrhea is a common symptom of digestive tract illness. It usually resolves without specific treatment, but severe acute diarrhea and diarrhea that lasts several weeks can be dangerous, especially to very young children and the elderly, causing dehydration and/or malnutrition. Though there are non-infectious causes of diarrhea, diarrhea is frequently caused by infections from various bacteria, viruses and parasites. Infectious diarrhea is a significant health problem that leads to 500,000 hospitalizations and over 5,000 deaths in the United States each year.
Most cases of diarrhea in previously healthy people with normal immune systems do not warrant laboratory testing, according to the guidelines. Cases that do warrant laboratory testing include diarrhea accompanied by fever, stools with blood or mucus, severe abdominal cramping or tenderness, or signs of sepsis. Testing is also recommended in the context of a possible outbreak of infectious diarrhea and in immunocompromised people. A table within the guidelines summarizes the types of available testing—including the new sensitive molecular panels.
Traditional testing for infectious diarrhea has relied on labor-intensive and time-consuming tests, such as stool cultures, to identify the microbe responsible for the illness and microscopic examination for parasites. Molecular panels, however, can simultaneously test for several different pathogens in a single patient sample without the time and effort needed to grow microbes in culture. These panels are types of tests called “culture-independent diagnostic tests” or CIDTs.
The molecular panels’ short time to results could help healthcare practitioners quickly differentiate cases of diarrhea that require medical intervention, such as treatment with antibiotics, from cases that do not. CIDTs could also shorten the time needed for identifying the cause of an outbreak and implementing isolation measures for certain infections. Because the molecular tests are more sensitive—they can detect microbes present in very low numbers—they can identify more cases of infectious diarrhea than traditional cultures. They can also help detect the cause of outbreaks, such as viruses, where routine testing did not detect the cause of the diarrhea.
However, the authors of the IDSA guidelines caution that because these CIDTs detect the genetic material of microbes, they do not determine whether the microbe identified is viable (alive) and is causing the current illness. Healthcare practitioners need to factor in clinical considerations, such as a patient’s signs and symptoms, when interpreting the results and making a diagnosis.
Despite the evolution of sensitive CIDTs, stool cultures remain important lab tests and the IDSA guidelines recommend following up a positive CIDT with traditional cultures in certain cases. Cultures are needed when antibiotic susceptibility testing is required to help determine the appropriate treatment and track changes in antimicrobial resistance occurring in stool pathogens.
Cultures are also needed to confirm suspected outbreaks. Further characterization of the microbe allows public health laboratories to identify a microbe’s strain or subtype, resistance pattern, or other characteristics that link that microbe to the current outbreak. Identifying the source of the outbreak (food, water, or animal) enables appropriate intervention by authorities and prevents further exposure to the contaminated product. Having the unique characterization of pathogens causing infectious diarrhea allows public health authorities to track and control future outbreaks.
Shane, A. L., et al. (19 October 2017) 2017 Infectious Diseases Society of America Clinical Practice Guidelines for the Diagnosis and Management of Infectious Diarrhea. Clinical Infectious Diseases. Available online at https://doi.org/10.1093/cid/cix669. Accessed on December 20, 2017.
October 19, 2017. Press Release. IDSA Infectious Diarrhea guidelines recommend when to test, when to treat. Infectious Diseases Society of America. Available online at https://www.eurekalert.org/pub_releases/2017-10/idso-iid101817.php. Accessed December 20, 2017.
(30 July 2016) Centers for Disease Control and Prevention. Culture-Independent Diagnostic Tests: What are CIDTs and How Do They Affect Foodborne Illness? Available online at www.cdc.gov/foodsafety/challenges/cidt.html. Accessed on December 20, 2017.
McAdam, A. J. (3 May 2017) Unforeseen Consequences: Culture-Independent Diagnostic Tests and Epidemiologic Tracking of Foodborne Pathogens. Journal of Clinical Microbiology. Available online at https://doi.org/10.1128/JCM.00678-17. Accessed on December 20, 2017.