Differential Diagnosis for an 80-year-old Female with Darker Colored Stools

The patient presents with a 4-day history of darker colored stools, recent use of Pepto-Bismol, and is on anticoagulation therapy with Eliquis for atrial fibrillation. Here is a categorized differential diagnosis:

• Single Most Likely Diagnosis
  • Pepto-Bismol-induced stool discoloration: The patient's recent use of Pepto-Bismol, which contains bismuth subsalicylate, is a common cause of black stools due to the oxidation of the bismuth component.
• Other Likely Diagnoses
  • Upper gastrointestinal bleeding: Given the patient's age and use of anticoagulant medication (Eliquis), there is an increased risk of gastrointestinal bleeding, which could manifest as melena (black, tarry stools).
  • Iron supplement-induced stool discoloration: Although not mentioned, if the patient is taking iron supplements, these can also cause dark stools.
• Do Not Miss Diagnoses
  • Upper GI malignancy: Although less likely, gastrointestinal cancers can cause bleeding leading to dark stools. Given the patient's age, this is a critical diagnosis not to miss.
  • Esophageal varices: In patients with a history of liver disease or cirrhosis, esophageal varices could cause upper GI bleeding, presenting as melena.
  • Angiodysplasia: Vascular malformations in the GI tract, more common in the elderly, can cause chronic or acute bleeding.
• Rare Diagnoses
  • Ingested blood: Although rare, if the patient has been consuming blood (e.g., from nosebleeds), this could potentially cause dark stools.
  • Certain foods or medications: Other foods or medications can cause stool discoloration, but these are less common causes of dark stools compared to the other listed diagnoses.

Workup from the Emergency Department

• Complete Blood Count (CBC): To assess for anemia or evidence of blood loss.
• Coagulation studies: Given the patient's anticoagulation therapy, checking INR or aPTT (depending on the anticoagulant) can provide insight into the risk of bleeding.
• Electrolyte panel and renal function tests: To evaluate for any electrolyte imbalances or renal issues that could be contributing to or resulting from the gastrointestinal issue.
• Stool tests: A stool guaiac test can help confirm the presence of blood in the stool.
• Endoscopy: If an upper GI source of bleeding is suspected, an upper endoscopy may be necessary to visualize the esophagus, stomach, and duodenum.
• Imaging studies: Depending on the clinical suspicion, abdominal imaging (e.g., CT scan) might be ordered to evaluate for other causes of bleeding or to assess for complications.

This approach ensures a thorough evaluation of the patient's symptoms, considering both common and potentially life-threatening causes of dark stools.