Molecular-based diagnostic tests for enteric pathogens are highly sensitive but may require expert input to assess their clinical and public health implications, according to new guidelines from the Infectious Diseases Society of America.

“Differentiating colonization from active infection, obtaining antimicrobial susceptibility results, providing optimal management, and preventing transmission are areas in need of additional research as nonculture diagnostics replace traditional culture-based methods,” writes Andi L. Shane, MD, of Emory University and Children’s Healthcare in Atlanta, with her associates in Clinical Infectious Diseases. Performing a complete physical exam and taking a thorough exposure history remain crucial in this era of rapid molecular tests in order to identify and treat infectious diarrhea and interrupt the chain of transmission, the experts emphasize (Clin Infect Dis. 2017 Oct 19. doi: 10.1093/cid/cix669 ).

Infectious diarrhea accounts for about 500,000 hospitalizations and more than 5,000 deaths in the United States annually. Primary care providers are at the front lines of diagnosis and response, and rapid molecular tests can aid that process because they are faster and more sensitive than culture. However, these culture-independent diagnostic tests cannot distinguish live from dead or inactive pathogens and have not been validated as proof of cure. Also, because these tests are so sensitive, they can identify multiple and less common pathogens that may require input from infectious disease specialists.

The rise of culture-independent diagnostic tests also has important public health implications, the experts state. Pulsed-field gel electrophoresis and whole-genome sequencing are essential for rapidly detecting outbreaks, but they must be performed on clinical isolates. “Continuing to detect and respond to such outbreaks is a vital part of making our food and water systems safer,” the authors add. “As culture-independent diagnostic panels become used more frequently, public health departments may request that specimens be cultured in public health laboratories if unable to be cultured in the clinical diagnostic laboratory.” Clinicians should continue submitting isolates for subtyping of notifiable pathogens, such as Salmonella, Shiga toxin–producing Escherichia coli, Shigella, and Listeria, according to the guidelines.

The IDSA last updated its guidelines on infectious diarrhea in 2001. The current iteration includes 60 specific recommendations and five tables that stratify pathogens by exposures, clinical signs and symptoms, postinfectious sequelae, laboratory diagnostics, and antimicrobial therapy. Two additional tables list other sources of guidelines and provide detailed recommendations on rehydration therapy. Most patients with diarrhea do not need to be tested for infectious pathogens, with exceptions such as children younger than 5 years, the elderly, immunocompromised patients, and patients with bloody diarrhea, severe abdominal pain or tenderness, or signs of sepsis. Even when patients do not need to be tested, they should receive oral rehydration solution to correct mild to moderate dehydration or intravenous rehydration if they cannot tolerate oral therapy.

The World Health Organization defines diarrhea as loose or liquid stools occurring three or more times in 24 hours or more often than normal. Rapid molecular tests most often identify norovirus in these patients. Infectious diarrhea remains most common in children under age 5 years, but the advent of rotavirus vaccines over the past decade has decreased its incidence in this age group.

The work was funded by IDSA. Dr. Shane disclosed research grants from the Division of Microbiology and Infectious Diseases of the National Institute of Allergy and Infectious Diseases, salary support from the Gerber Foundation, honoraria from SLACK, and travel support from International Scientific Association for Probiotics and Prebiotics.