Diagnostics

• Acute diarrhea

  • Testing is not required in the majority of cases.

  • Indications for testing include the presence of red flags in diarrhea and risk factors for specific etiologies.

• Persistent diarrhea: Testing is usually limited to stool studies for infectious gastroenteritis.

• Chronic diarrhea

  • Testing is initially broad with advanced follow-up testing as indicated. [20][24]

  • Repeat stool testing for infectious gastroenteritis.

Diagnostic testing is seldom indicated in acute diarrhea in the absence of red flag symptoms.

• Blood tests

  • CBC: To evaluate for anemia, leukocytosis, and/or leukopenia

  • CMP: Assessment of laboratory findings in dehydration (e.g., electrolyte abnormalities, AKI) and/or liver disease

  • CRP, ESR: Markers of inflammation

  • Blood cultures: Identification of bacteremia

• CT abdomen or MRI abdomen: Identification of inflammation, structural disease, and/or ischemia [11]

• Endoscopic studies [11][17]

  • Upper endoscopy or enteroscopy with biopsy (± aspiration): Diagnosis of mucosal disease (e.g., celiac disease), infection, or malignancy in the small intestine

  • Colonoscopy with biopsies: Diagnosis of inflammatory disease, infection, or malignancy

Endoscopic studies have limited diagnostic value in acute diarrhea but are commonly needed for the workup of chronic diarrhea to evaluate for the presence of inflammatory or neoplastic diseases. Their utility in persistent diarrhea is uncertain.

Stool diagnostic studies

• Stool culture: can be indicated if red flags in diarrhea are present (e.g., suspected invasive bacterial enteritis)

• Stool tests for CDI: if C. difficile infection is suspected

• Stool osmotic gap [10][15][17]

  • An equation used in patients with watery diarrhea to differentiate between osmotic, secretory, and functional diarrhea

  • Calculation: 290 - [2 × (stool sodium + stool potassium)]

  • Normal stool osmotic gap: 50–100 mmol/L