In a 79-year-old patient receiving enteral tube feeding who presents with diarrhea, which stool test should be ordered first?

Stool Testing for Diarrhea in Tube-Fed Patients

In a 79-year-old patient on tube feeding with diarrhea, you should first order a Clostridioides difficile toxin test, as this patient has a known independent risk factor for CDI (gastrostomy/jejunostomy tube) and likely recent antibiotic exposure. 1

Primary Testing Approach

First-Line Test: C. difficile Testing

• Test for C. difficile infection (CDI) if the patient has ≥3 unformed stools in 24 hours, which is the preferred threshold for testing 1
• Patients with gastrostomy or jejunostomy tubes have an independent risk factor for CDI in addition to typical risk factors like recent antibiotic exposure and hospitalization 1
• Testing for C. difficile should be performed in patients with healthcare-associated diarrhea and those with a history of diarrhea following antimicrobial use 2
• Use a single diarrheal stool specimen (stool that takes the shape of the container) for optimal yield 2

Testing Strategy for C. difficile

• A highly sensitive test such as glutamate dehydrogenase (GDH) test is required initially, though this does not distinguish infection from carrier state 3
• Differentiation requires specialized nucleic acid amplification test (NAAT) for toxin B or enzyme immunoassays for toxin A or B 3

Secondary Diagnostic Considerations

Medication Review and Stool Osmotic Gap

• Before attributing diarrhea to the tube feeding formula itself, review all medications - particularly liquid formulations containing sorbitol, which are the most common cause of diarrhea in tube-fed patients (61% of cases) 4
• Calculate stool osmotic gap (using stool osmolality and electrolytes) to distinguish osmotic from secretory diarrhea - this correctly differentiates causes in all cases 4
• An osmotic gap >100 mmol/L indicates osmotic diarrhea (often medication-related), while a lower gap suggests secretory causes 4

Evidence on Causation in Tube-Fed Patients

• The feeding formula itself is responsible for diarrhea in only 6.5-21% of cases 4, 5
• Medication-associated diarrhea accounts for 61% of cases, with CDI representing only 9.7-17% 4, 5
• Antibiotic use is the most significant risk factor, especially when two or more antibiotics are used concurrently 6

Clinical Algorithm

1. Confirm true diarrhea: ≥3 unformed stools in 24 hours that are not clearly attributable to laxatives 1

2. Order C. difficile testing first given the patient's tube feeding status and likely healthcare exposure 1, 2

3. Simultaneously review medications for sorbitol-containing elixirs and other diarrhea-inducing agents 4

4. If initial testing is negative, obtain stool osmolality and electrolytes to calculate osmotic gap 4

5. Consider stopping suspected medications before modifying or discontinuing tube feeding 5, 6

Important Caveats

• Do not routinely discontinue tube feeding as the primary intervention - this is rarely the actual cause and risks nutritional compromise 4, 5, 6
• Fecal leukocytes and lactoferrin should NOT be used to establish the cause of acute infectious diarrhea 2
• Testing for C. difficile is not indicated if diarrhea is clearly attributable to laxatives, though testing may be warranted if symptoms persist after stopping such therapies 1
• Patients with baseline albumin <3 g/dL are at higher risk for developing diarrhea on tube feeding 5