Both square test areas in the upper area of the card show the intense blue color of a positive result. The lower two smaller circular areas on the orange stripe are analytical control reactions, positive on the left and negative on the right, that help assure that the card and developer bottle have been maintained in proper conditions and have not been damaged before the test is performed.
The stool guaiac test or guaiac fecal occult blood test (gFOBT) is one of several methods that detect the presence of fecal occult blood (FOB) which is blood in the feces that cannot be seen.
The term guaiac denotes the name of the paper surface used in the test which has a phenolic compound, alpha-guaiaconic acid, that is extracted from the wood resin of Guaiacum trees.
The stool guaiac test involves fasting from iron supplements, red meat (the blood it contains can turn the test positive), certain vegetables (which contain a chemical with peroxidase properties that can turn the test positive), and vitamin C and citrus fruits (which can turn the test falsely negative) for a period of time before the test. It has been suggested that cucumber, cauliflower and horseradish, and often other vegetables, should be avoided for three days before the test.
In testing, feces are applied to a thick piece of paper attached to a thin film coated with guaiac. Either the patient or medical professional smears a small fecal sample on to the film. The fecal sample is obtained by catching the stool and transferring a sample with an applicator. Digital rectal examination specimens are also used but this method is discouraged for colorectal cancer screening due to very poor performance characteristics.
Both sides of the test card can be peeled open, to access the inner guaiac paper. One side of the card is marked for application of the stool and the other is for the developer fluid.
After applying the feces, one or two drops of hydrogen peroxide are then dripped on to the other side of the film, and it is observed for a rapid blue color change.
When the hydrogen peroxide is dripped on to the guaiac paper, it oxidizes the alpha-guaiaconic acid to a blue colored quinone. Normally, when no blood and no peroxidases or catalases from vegetables are present, this oxidation occurs very slowly. Heme, a component of hemoglobin found in blood, catalyzes this reaction, giving a result in about two seconds. Therefore, a positive test result is one where there is a quick and intense blue color change of the film.
The guaiac test can often be false-positive which is a positive test result when there is in fact no source of bleeding. This is particularly common if the recommended dietary preparation is not followed, as the heme in red meat or the peroxidase or catalase activity in vegetables, especially if uncooked, can cause analytical false positives.
If the card has not been promptly developed, the water content of the feces decreases, and this can reduce the detection of blood. Although rehydration of stored samples can reverse this effect this is not recommended because the test becomes unduly analytically sensitive and thus much less specific.
Some stool specimens have a high bile content that causes a green color to show after applying the developer drops. If entirely green, such samples are negative, but if questionably green to blue, such samples are designated positive.
The package insert guidelines from the manufacturers, for example Hemoccult SENSA, recommend that nonsteroidal anti-inflammatory drugs (NSAID), such as ibuprofen and aspirin, and iron supplements be discontinued for at least several days before the tests. There is a concern that these agents may irritate the body and cause biologically positive tests even in the absence of a more substantial illness, but there is some doubt about how frequently this occurs with NSAID medication. Although both iron and bismuth containing products such as antacids and antidiarrheals can cause dark stools that are occasionally confused as containing blood, actual bleeding from iron is unusual.
There is no consensus on whether to stop warfarin before a guaiac test. Even when using anticoagulants a high proportion of positive guaiac tests were found to be due to diagnosable lesions, suggesting anticoagulants may not cause bleeding unless there is an abnormality.
The article fecal occult blood (FOB) provides an expanded consideration of the clinical application of FOB tests generally, including other clinical methods, and the comments here are those that relate specifically to the guaiac gFOBT method.
The stool guaiac test was originally the principal colon cancer screening technology available, but modern tests which look for globin or DNA are now also available. Several recent colon cancer screening guidelines have recommended replacing any older low-sensitivity, guaiac-based fecal occult blood testing (gFOBT) with either newer high-sensitivity guaiac-based fecal occult blood testing (gFOBT) or fecal immunochemical testing (FIT), which tests for globin rather than the heme detected by the guaiac method. The US Multisociety Task Force (MSTF) looked at 6 studies that compared high sensitivity gFOBT (Hemoccult SENSA) to FIT, and concluded that there were no clear difference in overall performance between these methods, and a similar recommendation was made by the National Guideline Clearinghouse (NGC).
Results of a single fecal sample should be interpreted cautiously, as there is a high rate of false negativity associated with the test. Using three cards, each on different days, is recommended to improve sensitivity. The Centers for Disease Control and Prevention (CDC) in a 2006–2007 survey found extensive inappropriate use of low sensitivity gFOBT and of single specimens; it is unclear if these widespread suboptimal approaches have since declined. The Current Procedural Terminology (CPT) coding was changed in January 2006 to include CPT code 82270, which indicates that consecutive collection of three stool samples has occurred, either as three single cards or a single triple card. Since January 2007, the US Medicare program reimburses for colorectal cancer screening with gFOBT only when this code is used.
The stool guaiac test method may be preferable to fecal immunochemical testing (FIT) if there is a clinical concern about possible gastric or proximal upper intestinal bleeding. However, although heme breakdown is less than globin during intestinal transit, false negative results can be seen with the stool guaiac tests due to degradation of the peroxidase-activity. This can cause false negative results in upper gastrointestinal bleeding sources, or in right colon adenomas and cancers that have comparable blood losses to positively testing left colon lesions. A positive gFOBT with subsequent negative colonoscopy may lead to an upper endoscopy. It is unclear whether this is an effective intervention if there is a positive gFOBT but no anemia. Endoscopy when there is a positive gFOBT along with iron deficiency anemia, or iron deficiency anemia on its own, has a higher rate of finding problems.
^Ahlquist DA, McGill DB, Schwartz S, Taylor WF, Owen RA (May 1985). "Fecal blood levels in health and disease. A study using HemoQuant". N. Engl. J. Med.312 (22): 1422–8. doi:10.1056/NEJM198505303122204. PMID3873009.
^Wells HJ, Pagano JF (1977). "Hemoccult test — reversal of false-negative results due to storage". Gastroenterology72: 1148.
^Macrae FA, St John DJ (May 1982). "Relationship between patterns of bleeding and Hemoccult sensitivity in patients with colorectal cancers or adenomas". Gastroenterology82 (5 Pt 1): 891–8. PMID7060910.
^Frame PS, Kowulich BA (December 1982). "Stool occult blood screening for colorectal cancer". J Fam Pract15 (6): 1071–5. PMID7142926.
^Grossman MI, Matsumoto KK, Lichter RJ (March 1961). "Fecal blood loss produced by oral and intravenous administration of various salicylates". Gastroenterology40: 383–8. PMID13709097.
^Pierson RN, Holt PR, Watson RM, Keating RP (August 1961). "Aspirin and gastrointestinal bleeding. Chromate blood loss studies". Am. J. Med.31: 259–65. PMID13735596.
^Norfleet RG (April 1983). "1,300 mg of aspirin daily does not cause positive fecal hemoccult tests". J. Clin. Gastroenterol.5 (2): 123–5. PMID6304182.
^Doubilet P, Donowitz M, Pauker SG (1982). "Evaluation for colon cancer in patients with occult fecal blood loss while taking aspirin: a Bayesian viewpoint". Med Decis Making2 (2): 147–60. PMID7167043.
^Bahrt KM, Korman LY, Nashel DJ (November 1984). "Significance of a positive test for occult blood in stools of patients taking anti-inflammatory drugs". Arch. Intern. Med.144 (11): 2165–6. doi:10.1001/archinte.144.11.2165. PMID6333854.
^Laine LA, Bentley E, Chandrasoma P (February 1988). "Effect of oral iron therapy on the upper gastrointestinal tract. A prospective evaluation". Dig. Dis. Sci.33 (2): 172–7. doi:10.1007/BF01535729. PMID3257437.
^Anderson GD, Yuellig TR, Krone RE (May 1990). "An investigation into the effects of oral iron supplementation on in vivo Hemoccult stool testing". Am. J. Gastroenterol.85 (5): 558–61. PMID2186616.
^Imperiale TF (September 2010). "Continue or discontinue warfarin for fecal occult blood testing in 2010? Does the published evidence provide an answer?". Am. J. Gastroenterol.105 (9): 2036–9. doi:10.1038/ajg.2010.269. PMID20818354.
^Barada K, Abdul-Baki H, El Hajj II, Hashash JG, Green PH (January 2009). "Gastrointestinal bleeding in the setting of anticoagulation and antiplatelet therapy". J. Clin. Gastroenterol.43 (1): 5–12. doi:10.1097/MCG.0b013e31811edd13. PMID18607297.
^Collins JF, Lieberman DA, Durbin TE, Weiss DG (January 2005). "Accuracy of screening for fecal occult blood on a single stool sample obtained by digital rectal examination: a comparison with recommended sampling practice". Annals of Internal Medicine142 (2): 81–5. PMID15657155.
^Nadel MR, Berkowitz Z, Klabunde CN, Smith RA, Coughlin SS, White MC (August 2010). "Fecal occult blood testing beliefs and practices of U.S. primary care physicians: serious deviations from evidence-based recommendations". J Gen Intern Med25 (8): 833–9. doi:10.1007/s11606-010-1328-7. PMC2896587. PMID20383599.
^Centers for Medicare and Medicaid Services. Colorectal cancer: Preventable, treatable and beatable: Medicare coverage and billing for colorectal cancer screening. Medicare Learning Network Matters SE0710. www.cms.hhs.gov/MLNMattersArticles/downloads/SE0710.pdf Accessioned 2010 November 1
^Herzog P, Holtermüller KH, Preiss J, et al. (November 1982). "Fecal blood loss in patients with colonic polyps: a comparison of measurements with 51chromium-labeled erythrocytes and with the Haemoccult test". Gastroenterology83 (5): 957–62. PMID7117808.
^Allard J, Cosby R, Del Giudice ME, Irvine EJ, Morgan D, Tinmouth J (February 2010). "Gastroscopy following a positive fecal occult blood test and negative colonoscopy: systematic review and guideline". Can. J. Gastroenterol.24 (2): 113–20. PMC2852233. PMID20151070.