Guaiac testing - What it is and what it is not

 ·   · 

  • First, how does guaiac testing work and why was it developed?

    • A small amount of heme present in the stool reacts with hydrogen peroxide to oxidize the paper

    • In other words, a small amount of blood in the stool causes the paper to turn blue

    • It was developed to allow physicians to detect small amounts of blood that could signify an otherwise asymptomatic malignancy, allowing for further testing and earlier detection

  • Why do we use it in the ED?

    • To determine whether there is blood in the patient’s stool

    • We then utilize this information to determine whether there may be a GI bleed

  • What are the problems with guaiac testing?

    • The sensitivity of a guaiac test is very poor, even for its intended purpose of screening for malignancy

      • As a result, the correct specimen collection protocol is to have the patient take home multiple cards and collect samples from 6 different BMs

      • Immunochemical assays are now preferred for malignancy screening due to its increased sensitivity, allowing an accurate test with only 1 sample

    • The sensitivity and specificity of the test decrease with sample collected by digit rectal exam rather than those collected from normally passed stool

    • There are many sources of false positives, including but not limited to:

      • Epistaxis

      • Digested foods such as meat products containing nonhuman heme and certain vegetables including broccoli

      • Subjective nature of the test itself (what looks blue and positive to one practitioner may be read as negative by another)

    • There is minimal data evaluating the use guaiac testing in the emergency department, as this is not its intended purpose

      • 1 study looked at guaiac tests in patients with known upper GI lesions and found that the test was positive in only 11 of 42 patients

      • Given this lack of compelling data, the majority of gastroenterologists do not recommend guaiac testing in the emergency department, and do not rely on guaiac results from the ED to alter management

  • In summary…

    • The guaiac test was developed to detect small amounts of blood in the stool in order to screen for malignancy, not for use in the ED

    • The test has poor sensitivity, particularly when utilized after a digital rectal exam, meaning that many patients with a GI bleed may have a falsely negative guaiac result

    • There are many reasons a patient may have a false positive, even his diet the night before

  • So go out and do your digital rectal exams to document the color of the stool (is there melena, is there bright red blood), but perhaps you shouldn’t rely on the guaiac to help determine clinical management