The (f)utility of inpatient guaiac/fecal occult blood tests

Here’s a situation that most of us have probably encountered:

Intern: Mrs. Jones’s hemoglobin is drifting down…her baseline is 12 and today it’s 10. She’s hemodynamically stable and doesn’t show any signs of bleeding.

Attending: No signs of bleeding, eh? Let’s guaiac her.

Intern: *dies a little inside*

There is one, and only one appropriate indication for a stool guaiac, and that is for colon cancer screening in someone who for one reason or another cannot undergo colonoscopy. In a recent Australian study in the Journal of Internal Medicine, out of 461 inpatients who were guaiac’ed, only 1 patient was appropriately screened for colon cancer. 16% of the patients had their care delayed or somehow adversely affected because of a positive result that necessitated consideration for endoscopy. Another study shows that inpatient FOBTs are a quality improvement issue–if you’re not going to act on a positive result, why bother getting a test that the patient will get charged for?

Here are a few more fun facts:

  • We all shed about 0.5-1.5 ml of blood into stool over 24 hour period. A guaiac can detect an amount of blood as small as 2 ml to 10 ml of blood in the stool per 24hrs.
  • To reduce the rate of false positives, a patient should be on medication and diet restrictions: 72 hours off medications like NSAIDs or aspirin or vitamin C; off red meat, cantapoupe and other melons, grapefruit, turnips, broccoli, figs, radhish, horseradish, cauliflower, cabbage, cucumber, carrot, potato, pumpkin, zucchini, parsley.
  • A FOBT costs the patient anywhere from $20-60. And the discomfort of a rectal exam.
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