Is a stool culture necessary when ordering separate stool bacterial testing?

From the Guidelines

A stool culture is not always necessary when ordering separate stool bacterial testing, as molecular assays such as nucleic acid amplification tests (NAAT) can detect bacterial pathogens without the need for culture.

Key Points

• The Infectious Diseases Society of America (IDSA) recommends that stool testing should be performed for specific bacterial pathogens, such as Salmonella, Shigella, Campylobacter, and STEC, in people with diarrhea accompanied by certain symptoms or risk factors 1.
• However, the yield of stool cultures can be low, with estimates ranging from 0.3% to 5.8% for various pathogens 1.
• The IDSA guidelines suggest that routine stool culture may not be necessary in all cases, and that molecular assays such as NAAT can be used to detect bacterial pathogens 1.
• In some cases, such as when a patient is suspected of having a nosocomial outbreak of diarrheal illness or when antimicrobial susceptibility testing is required, a stool culture may still be necessary 1.

Important Considerations

• The choice of diagnostic test should be based on the patient's symptoms, risk factors, and epidemiologic context 1.
• Molecular assays such as NAAT can provide rapid and sensitive detection of bacterial pathogens, but may not provide information on antimicrobial susceptibility 1.
• Stool culture can provide information on antimicrobial susceptibility and can be used for public health reporting and outbreak investigation 1.

From the Research

Necessity of Stool Culture

• The necessity of a stool culture when ordering separate stool bacterial testing is a topic of debate among medical professionals.
• A study published in 2022 2 found that metagenomics-based stool testing can provide diagnostic and public health information, and may be more routinely applied in clinical and public health laboratories as cost, speed, and analytics improve.
• However, another study from 2008 3 suggests that stool cultures can be important in guiding antimicrobial therapy for diarrhea, and that frequent use of antimicrobials without stool culture may indicate inappropriate antimicrobial use.

Predictors of Positive Stool Culture

• A study from 2002 4 identified clinical predictors of positive stool culture, including summer season, fever, duration of abdominal pain, and requirement of IV fluid therapy.
• These predictors can help physicians determine the necessity for stool requests.

Comparison of Stool Culture Methods

• A study from 2015 5 compared conventional stool culture with the BD MAX Enteric Bacterial Panel (EBP) and found that the EBP can result in significant time savings and a shorter time-to-result for patients with acute bacterial diarrhea.
• Another study from 2017 6 implemented a clinical decision support tool to decrease the number of unnecessary stool cultures and found that it resulted in a substantial reduction in the number of evaluations of stool cultures and a cost savings.

Stool Culture in Hospitalized Patients

• The 2017 study 6 also found that stool culture and parasitological examinations are of minimal to no value after 3 days of hospitalization, and that a clinical decision support tool can help reduce unnecessary testing.
• The study found that the intervention resulted in a reduction in the number of stool cultures ordered after 3 days of hospitalization, from 20.26% to 10.31%.