Laboratory Diagnosis of Infectious Diarrhea

Abstract

The microbiology laboratory, in conjunction with the medical staff, must determine a reasonable approach to the evaluation of diarrheal stools since the cost to rule out all potential pathogens is prohibitive and control of the use of laboratory services is now a major focus in all institutions. All stool cultures should be examined for Campylobacter, Salmonella, and Shigella, the most common causes of inflammatory bacterial diarrhea in the United States. Special media for other pathogens should be added only if there is high regional endemicity or significant clinical suspicion. If a child has bloody diarrhea, a search for E coli O157:H7 is indicated. For patients with a history of raw seafood ingestion or foreign travel, the laboratory should be asked to screen specimens for Vibrio and Plesiomonas species. The report from the laboratory should specifically state what enteropathogens have been excluded, for example, "No Salmonella, Shigella, or Campylobacter isolated." A report of "negative" or "no enteric pathogens" is not very useful. Diagnosis of viral and parasitic enteritis and antibiotic-associated diarrhea require a variety of additional tests. Clinicians are encouraged to discuss these issues with the pathologist or microbiologist at their local laboratory and be familiar with community microbiology practice, particularly which organisms require a special request for the laboratory to attempt identification.

Diagnosis and treatment of bacterial diarrhea

“…However, it is a relatively expensive test with a low yield. Studies of stool culture yields have typically resulted in positive rates below 10% and as low as 1.5% [22,23]. The cost per positive culture from these stud- Can cause colitis and proctitis in persons engaging in receptive anal intercourse Neisseria gonorrheae Chlamydia trachomatis ies ranges from $136 to $1200.…”

“…Routine stool culture in most US laboratories consists of selective and differential agar plates capable of isolating Salmonella, Shigella, and, if all non-E. coli gram negatives are routinely identified, Aeromonas and Plesiomonas. Most laboratories also include a Campylobacter-selective medium incubated in microaerophilic conditions to detect Campylobacter species [20,23]. Other organisms require special media for culture diagnosis.…”

“…Rectal swabs are generally inferior. If stool cannot be plated within 2 hours, it should be refrigerated or placed in a transport medium [20,23]. Routine stool culture in most US laboratories consists of selective and differential agar plates capable of isolating Salmonella, Shigella, and, if all non-E. coli gram negatives are routinely identified, Aeromonas and Plesiomonas.…”

Approach to the Patient with Diarrhea

“…Fewer than 10% of stool specimens are positive for pathogens [105,108], so the cost of a single positive stool culture is high (more than $1000) if stools are cultured indiscriminately [108 -111]. However, the cost can be reduced to $30 per culture if only Campylobacter, Salmonella, and Shigella are sought and if only liquid stools are cultured.…”

Infectious enteritis

“…Serious diarrhea is suggested if the patient is more than 70 years old, is immunocompromised, and had stool frequency of more than 6 per day, duration of diarrhea of more than 48 hours, abdominal pain, tenesmus, fever, bloody or mucoid stools, or dehydration [1••]. Laboratory evaluation of clinically serious diarrhea should initially include stool studies for occult blood, lactoferrin, or leukocytes, which, if present, suggest an inflammatory enteritis [1••, [13][14][15]. Many (but not all) such patients have diagnostic routine stool cultures, and regardless of culture results, these are likely to benefit from empiric antimicrobial therapy [1••, 13, 16•].…”