Stool guaiac test
Encyclopedia
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). Fecal occult blood
is blood
present in the feces
that is not visibly apparent.
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.
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.
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.
Vitamin C
can cause analytical false negative
s due to its anti-oxidant properties inhibiting the color reaction.
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.
(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.
One major use of stool testing for blood is detection of colorectal cancer
. However, other possible positive results include: gastroesophageal cancer, GI bleeds, diverticulae, hemorrhoids, anal fissures, colon polyps, ulcerative colitis
, Crohn's disease
, celiac disease, GERD
, esophagitis
, peptic ulcers, gastritis
, inflammatory bowel disease
, vascular ectasias, portal hypertensive gastropathy
, aortoenteric fistulas, hemobilia, endometriosis
, and trauma
.
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 sensitivty 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.
Fecal occult blood
Fecal occult blood refers to blood in the feces that is not visibly apparent. A fecal occult blood test checks for hidden blood in the stool...
(FOB). Fecal occult blood
Fecal occult blood
Fecal occult blood refers to blood in the feces that is not visibly apparent. A fecal occult blood test checks for hidden blood in the stool...
is blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
present in the feces
Feces
Feces, faeces, or fæces is a waste product from an animal's digestive tract expelled through the anus or cloaca during defecation.-Etymology:...
that is not visibly apparent.
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
Resin
Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...
of Guaiacum
Guaiacum
Guaiacum, sometimes spelled Guajacum, is a genus of flowering plants in the caltrop family Zygophyllaceae. It contains five species of slow-growing shrubs and trees, reaching a height of approximately but are usually less than half of that...
trees.
Methodology
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
Feces
Feces, faeces, or fæces is a waste product from an animal's digestive tract expelled through the anus or cloaca during defecation.-Etymology:...
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
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
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
Quinone
A quinone is a class of organic compounds that are formally "derived from aromatic compounds [such as benzene or naphthalene] by conversion of an even number of –CH= groups into –C– groups with any necessary rearrangement of double bonds," resulting in "a fully conjugated cyclic dione structure."...
. 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.
Analytical interpretation
The guaiac test can often be false-positiveType I and type II errors
In statistical test theory the notion of statistical error is an integral part of hypothesis testing. The test requires an unambiguous statement of a null hypothesis, which usually corresponds to a default "state of nature", for example "this person is healthy", "this accused is not guilty" or...
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.
Vitamin C
Vitamin C
Vitamin C or L-ascorbic acid or L-ascorbate is an essential nutrient for humans and certain other animal species. In living organisms ascorbate acts as an antioxidant by protecting the body against oxidative stress...
can cause analytical false negative
Type I and type II errors
In statistical test theory the notion of statistical error is an integral part of hypothesis testing. The test requires an unambiguous statement of a null hypothesis, which usually corresponds to a default "state of nature", for example "this person is healthy", "this accused is not guilty" or...
s due to its anti-oxidant properties inhibiting the color reaction.
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.
Clinical application
The article fecal occult bloodFecal occult blood
Fecal occult blood refers to blood in the feces that is not visibly apparent. A fecal occult blood test checks for hidden blood in the stool...
(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.
One major use of stool testing for blood is detection of colorectal cancer
Colorectal cancer
Colorectal cancer, commonly known as bowel cancer, is a cancer caused by uncontrolled cell growth , in the colon, rectum, or vermiform appendix. Colorectal cancer is clinically distinct from anal cancer, which affects the anus....
. However, other possible positive results include: gastroesophageal cancer, GI bleeds, diverticulae, hemorrhoids, anal fissures, colon polyps, ulcerative colitis
Ulcerative colitis
Ulcerative colitis is a form of inflammatory bowel disease . Ulcerative colitis is a form of colitis, a disease of the colon , that includes characteristic ulcers, or open sores. The main symptom of active disease is usually constant diarrhea mixed with blood, of gradual onset...
, Crohn's disease
Crohn's disease
Crohn's disease, also known as regional enteritis, is a type of inflammatory bowel disease that may affect any part of the gastrointestinal tract from mouth to anus, causing a wide variety of symptoms...
, celiac disease, GERD
Gerd
In Norse mythology, Gerðr is a jötunn, goddess, and the wife of the god Freyr. Gerðr is attested in the Poetic Edda, compiled in the 13th century from earlier traditional sources; the Prose Edda and Heimskringla, written in the 13th century by Snorri Sturluson; and in the poetry of skalds...
, esophagitis
Esophagitis
Esophagitis is inflammation of the esophagus. It may be acute or chronic. The acute esophagitis can be catarrhal or phlegmonous, whereas the chronic esophagitis may be hypertrophic or atrophic.-Infectious:...
, peptic ulcers, gastritis
Gastritis
Gastritis is an inflammation of the lining of the stomach, and has many possible causes. The main acute causes are excessive alcohol consumption or prolonged use of nonsteroidal anti-inflammatory drugs such as aspirin or ibuprofen. Sometimes gastritis develops after major surgery, traumatic...
, inflammatory bowel disease
Inflammatory bowel disease
In medicine, inflammatory bowel disease is a group of inflammatory conditions of the colon and small intestine. The major types of IBD are Crohn's disease and ulcerative colitis.-Classification:...
, vascular ectasias, portal hypertensive gastropathy
Portal hypertensive gastropathy
Portal hypertensive gastropathy refers to changes in the mucosa of the stomach in patients with portal hypertension; by far the most common cause of this is cirrhosis of the liver. These changes in the mucosa include friability of the mucosa and the presence of ectatic blood vessels at the surface...
, aortoenteric fistulas, hemobilia, endometriosis
Endometriosis
Endometriosis is a gynecological medical condition in which cells from the lining of the uterus appear and flourish outside the uterine cavity, most commonly on the ovaries. The uterine cavity is lined by endometrial cells, which are under the influence of female hormones...
, and trauma
Trauma (medicine)
Trauma refers to "a body wound or shock produced by sudden physical injury, as from violence or accident." It can also be described as "a physical wound or injury, such as a fracture or blow." Major trauma can result in secondary complications such as circulatory shock, respiratory failure and death...
.
The stool guaiac test was originally the principal colon cancer screening technology available, but modern tests which look for globin
Globin
Globins are a related family of proteins, which are thought to share a common ancestor. These proteins all incorporate the globin fold, a series of eight alpha helical segments. Two prominent members of this family include myoglobin and hemoglobin, which both bind the heme prosthetic group...
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
Globin
Globins are a related family of proteins, which are thought to share a common ancestor. These proteins all incorporate the globin fold, a series of eight alpha helical segments. Two prominent members of this family include myoglobin and hemoglobin, which both bind the heme prosthetic group...
rather than the heme
Heme
A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...
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 sensitivty 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
Heme
A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...
breakdown is less than globin
Globin
Globins are a related family of proteins, which are thought to share a common ancestor. These proteins all incorporate the globin fold, a series of eight alpha helical segments. Two prominent members of this family include myoglobin and hemoglobin, which both bind the heme prosthetic group...
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.