Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (138 page)

   Tumor-associated calcitonin stimulates gut motility.

   Gastrinoma leads to increased gastric acid, which directly causes fluid secretion.

10. Infection:

   Refer to p. 624, Foodborne Infectious Illnesses, and see other sections on specific agents that cause diarrheal disease.

   Laboratory Findings

Endoscopy
: Lower endoscopy may help. One series has a 20% yield in identifying a pathologic diagnosis. In non–HIV-infected patients, the role of sigmoidoscopy versus colonoscopy is unclear. When clinically suspected, even if no gross abnormalities are noted, consider doing blind biopsies looking for lymphocytic and collagenous colitis. The yield of biopsy with no gross abnormalities ranges from 6% to 42%. Upper endoscopy is useful for making the diagnosis of sprue, Whipple disease, and other small bowel infiltrative processes.

Radiology
: An upper GI series with small bowel follow-through is most commonly used when evaluating for Crohn disease. Enteroclysis is superior, with 100% sensitivity and 98% specificity for small bowel involvement with Crohn disease.

Recommended laboratory tests of stool
:

   Fecal leukocytes.

   Stool for osmolality gap: The osmolality gap is calculated by the following formula: 2 (stool Na + K). The accuracy is fair in distinguishing between osmotic (if gap is 50) and secretory (if gap is >50) diarrhea.

   Stool for pH: For carbohydrate intolerance (e.g., lactose or sorbitol), one small study found the pH <5.6. For bile acid–induced diarrhea, the pH is usually over 6.8.

   Stool for fecal fat: This test is used to detect steatorrhea on the basis of malabsorption.

   Qualitative: Sensitivity is 97–100%, but the specificity varies from 56% to 86%.

   Quantitative: Based upon a 72-hour collection, the patient should be on a 75- to 100-g fat diet. A nutritional consult is advised to maximize compliance.

   Test(s) for infectious agents (e.g., stool culture, O&P examination, rotavirus detection) based on clinical presentation.

Other recommended tests
:

   
Nutrition indices
: CBC, albumin, and potassium (sensitivity of hypokalemia is 100% for pancreatic cholera or [VIPoma]) are routine studies in the evaluation of chronic diarrhea.