What is a Guaiac Test?

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  • Written By: J.M. Willhite
  • Edited By: Heather Bailey
  • Last Modified Date: 19 October 2019
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A guaiac test is a diagnostic tool used to evaluate fecal matter in an effort to detect abnormalities associated with gastrointestinal tract dysfunction. Evaluated for the presence of blood, fecal occult blood test (FOBT) samples may be collected either by the individual at home or by a health care provider in a medical setting, such as a doctor’s office or clinic. The test often requires preparatory measures by the individual and these should be discussed with a qualified health care provider when scheduling the test. The administration of a guaiac test does carry some risk for false results due to the potential of insufficient sample collection or contamination.

Generally, a guaiac test is conducted to check for bloody stool that often occurs in the presence of gastrointestinal problems. Abnormal results demonstrating the presence of blood can be indicative of colonic problems, including polyps and tumors, gastrointestinal infection, and inflammatory bowel disease. Other conditions that may trigger bloody stool include hemorrhoids, ulcers, and various colonic malignancies. Any indication of abnormalities usually necessitates additional testing to further evaluate the source of the bleeding.


Individuals scheduled for a guaiac test may be given specific instructions that frequently contain dietary restrictions. Certain raw vegetables, such as radishes and broccoli, should be avoided because of their ability to trigger both gas and diarrhea. The consumption of any red meat is commonly discouraged due to its natural blood content which may result in sample contamination. The duration and specific content of preparatory dietary changes varies and should be discussed with a health care provider during consultation.

The method of stool collection utilized is often dependent on the age of the individual and the environment in which the collection takes place. Home collection techniques are non-invasive, do not induce any discomfort, and take place over a period of time. Those who undergo a guaiac test during the course of a physical examination conducted in a medical setting may experience some discomfort. Due to the invasive nature of this type of sample collection, irritation of the lower gastrointestinal tract can occur.

When a guaiac test is performed at home, a special kit containing all necessary collection materials may be employed to obtain the stool samples. The use of plastic wrap or special tissues is frequently utilized to obtain the samples and prevent cross contamination. Sample collection that involves young children, especially those who are still in diapers, necessitates precautionary measures to reduce the risk of urine-contaminated samples. A strict adherence to the supplied instructions is essential to ensure proper sample collection and submission. Obtained samples are usually mailed to a laboratory or delivered to a medical facility for analysis.


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