occult blood a fecal occult blood test (fobt) is a non invasive test that detects the presence of...
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Occult blood
A fecal occult blood test (FOBT) is a non invasive test that detects the presence of hidden (occult) blood in the stool,not apparent to patient.
Hidden blood in stool is often the first, and, warning sign that a person has colorectal disease, including colon cancer.
Occult gastrointestinal bleeding:
It is usually identified only by tests or, it may manifested as iron deficiency anemia.
It is the bleeding that is clinically evident but from an obscure source.
Causes of gastrointestinal bleeding:
Mass lesion: carcinoma – adenoma>1cm at any site.Inflammation: erosive oesophagitis ,ulcer at any site,
erosive gastritis, ulcerative colitis and crohn`s disease.
Vascular disorder: vascular actasia at any site, portal
hypertension gastropathy or colonpathy, varicoces ,haemangioma...
Infectious diseases:
worms infestation ,T.B. enterocolitis,
amoebiasis Bleeding:
hemoptysis ,oropharyngeal bleeding…
Other cause: hemophalia…
Normally 0.5 - 1.5 ml blood per day is lost in stools amounting to ~ 2 mg hemoglobin per gram of stool
4-6 mg/gm, an amount that is typically not detected by occult blood tests.
Hema- screen test will detect 10 mg of hemoglobin/gm of stool.
Occult blood is commonly detected in the stool using a variety of FOBT.
Factors affecting the detection of faecal occult blood
Anatomical level of lesion. Stool transit time. Stool mixing. hemoglobin degradation in lumen. features of bleeding (intermittent or
continuous bleeding).
In screening programs an estimated 1-5% of the tested population have a
positive fecal occult blood test. (1-2% false positive)
Causes for a positive test are:2-10%: cancer (colorectal cancer,
gastric cancer) .20-30% adenoma or polyps .Bleeding peptic ulcer .Angiodysplasia of the colon.
The results of occult blood cannot be considered conclusive evidence of presence or absence of gastrointestinal bleeding or pathology.
The test is preliminary screen not replace any other diagnostic methods (proctosigmoidoscopy, barium enema…)
Intra-luminal haemoglobin degradation and faecal occult blood tests:
In the upper gastro-intestinal tract, haemoglobin is cleaved to form heme and globin .
Heme is converted to porphyrin and iron has been termed the "intestinal converted fraction" of haeme.
intestinal converted fraction This fraction is not detected by
guaiac-based tests but is detected by heme-porphyrin assay (HemoQuant test), which measures both heme and porphyrins and is therefore a highly accurate indicator of bleeding, regardless of the level.
Three types of tests are used to detect faecal occult blood. these include:
Guaiac based tests: HemeOccult (Hema-screen)
Heme-porphyrin tests: HemoQuant test
Immunochemical tests: HemeSelect test FlexSure OBT
Occult blood test 1. Guaiac based test
Hema-screen test
PrincipleHema-screen is composed of guaiac
impregnated paper (light tan in color) enclosed in a cardboard frame which permit stool sample application to one side ,and develoment and interpretation on reverse side.
The hematin portion of hemoglobin can catalyzing oxidation of guaiac resulting in production of blue color.
Guaiac-based tests detect the pseudo-peroxidase activity of heme, either as intact hemoglobin or as heme.
Such activity converts colourless guaiac to a blue colour in the presence of an oxygen donor like hydrogen peroxide (H2O2) (developing reagent).
It detect only hemoglobin released upon hemolysis of RBC by hydration
specimens :Card with thin stool smear or
5 g stool Container.Collection card or stool transport
vial without preservative.
Negative result positive result
+ve &-ve monitor ( act as quality control) this provide assurance that guaiac paper and developer are reacting properly.
Patient Preparation Patient should not receive vitamin C
(ascorbic acid), NSAIDs for 3 days prior to occult blood testing by guaiac.
A high bulk, red meat free diet. restriction of peroxidase-rich vegetables
(turnips, horseradish, artichokes, mushrooms, radishes, broccoli, bean sprouts, cauliflower, oranges, bananas, cantaloupes, grapes), has been recommended 3 days prior to guaiac testing, and during testing.
Therapeutic iron causes false-positives with guaiac tests in more than half of healthy subjects.
Special diet may be omitted initially, with dietary restriction imposed upon retesting of all positive results.
Because gastrointestinal lesions may bleed intermittently and blood in feces is not distribute uniformly ,all positive tests regardless of diet , follow-up diagnostic procedures should be done.
Recommendations for testing occult blood in stool using guaiac-based tests
Smear two area from each stool sample.
Test 3 successive evacuation, close to each other as possible.
Test all samples within 4 days of collection.
Do not rehydrate slide prior to development.
Even one positive test should be considered a positive even without dietary restriction
False-positive results:
Intake of foods that contain peroxidase activity (uncooked fruits and vegetables).
Drugs: Topical iodine, aspirin, NSAIDs. Non-human hemoglobins: red meat. Rehydration
False-negative results:
Vitamin C (Ascorbic acid) intake. Storage of slides. Improper sampling or developing. Lesion not bleeding at time of stool
collection. Hemoglobin degradation by colonic
bacteria.
Guaiac-based testAdvantage:
Readily available. Convenient. Inexpensive. Can be performed in physician's office.
Disadvantage Dependant upon degree of hydration.Affected by storage. Affected by site of bleeding. Dietary restrictions required.
Intestinal converted fraction is the fraction of heme converted to porphyrin during fecal transit, a phenomenon which leads to diminished guaiac sensitivity for a pathology more proximal to colon.
2. Heme-porphyrin test The HemoQuant test includes
the measurement of total fecal hemoglobin or porphyrin derived from heme, intestinal converted fraction (ICF) which is the performed porphyrin probably by bacteria during enterocolic transit.
The heme-porphyrin test,
HemoQuant measures hemoglobin derived porphyrin spectro-fluorometrically and allows exact measurement of total hemoglobin in stool.
The amount of hemoglobin present in stools as determined by HemoQuant test can be graded as:
1. Normal < 2 mg/gram of stool.
2. Borderline 2- 4 mg/gram of stool.
3. Increased > 4 mg/gram of stool.
Heme-porphyrin test
Advantage:No interference with dietary peroxidases
or rehydration.Highly accurate indicator of bleeding,
regardless of level. Disadvantage:Needs laboratory setup.Time consuming (~90 minutes required) False positive results with non-human
haemoglobins.
Since 2001, there are a new class of occult blood tests called Fecal Immunochemical Tests(FIT).
These tests detect the globin in feces rather than heme. By detecting globin the tests are both more sensitive and specific but only for lower gastrointestinal bleeding.
Globin in the upper gastro-intestinal tract is digested by pepsin ,pancreatic and intestinal proteases and is, thus, not detected by the immunochemical faecal occult blood tests.
3. Immunochemical tests use antibodies directed against human
globin epitopes and can detect colonic blood at a level as little as 0.3 ml of blood.
not detect small quantities of blood from upper gastrointestinal tract.
The tests are limited by loss of globin antigenicity at room temperature and require processing in laboratory.
Tests used HemeSelect test: is similar to
enzyme linked immunosorbant assay (ELISA) and needs to be done in the laboratory,
FlexSure OBT : is a simpler immunoassay which can be done in the physician's office.
HemeSelect test (ELISA) A more complex reaction that uses
monoclonal and/or polyclonal antibodies that detect the intact globin portion of human hemoglobin.
If hemoglobin is present in the stool, the labeled antibody will attach to its antigens, creating a positive test result.
For technical and commercial reasons, this test have not found wide usage among physicians.
+
FlexSure OBT A long-handled brush is used to brush
the surface of the stool, thus trapping dislodged occult blood and water on the bristles.
The brush is then dabbed onto the test card where the water dries into a special pad. Once dry, the hemoglobin is considered stable and the card is sent to a laboratory for testing.
Once the test card is received at the laboratory, an immunochromatographic test strip consisting of a test line and a control line is inserted into the test pad of the card. Liquid is added, causing any hemoglobin present to migrate up the test strip.
If hemoglobin is present in the sample, it binds to a colloidal gold-conjugated polyclonal antibody against hemoglobin, and is labeled red.
The control line has an antibody that reacts with the conjugate antibody, which also causes a line to form indicating the test has worked correctly.
So positive result will produce two lines on the test strip, whereas a negative result produces only one line (the control line), The test is completed in approximately five minutes.
Immunochemical test: Advantage: Highly specific, as it detects only human
hemoglobin FlexSure OBT can be performed in
physician's office Dietary restrictions are not required Detects as little as ~0.3 mg/gram of
blood. Disadvantage: Needs laboratory setup (Hemoselect). Affected by storage (hemoglobin
degradation) Affected by site of bleeding.
Fecal DNA test (PreGen-Plus®):reported Four-Times More Sensitive
than Fecal Occult Blood Test for Colorectal Cancer Screening
PreGen-Plus works by isolating the human DNA from a single stool sample collected, and analyzing that DNA for alterations associated with, active colorectal cancer.
colorectal cancers result from chromosomal instability, with acquired mutations progressively accumulating in the adenomatous polyposis coli (APC) and p53 tumor suppressor genes, and in the k-ras oncogene.
PreGen-Plus consists of a panel of 23 individual tests each looking for the presence of DNA alterations in human DNA isolated from stool
Research has shown that DNA is a good marker: It is stable in the stool, it is shed
continuously.Through the use of amplification
tests, such as polymerase chain reaction(PCR), it can be detected in minute amounts.
PreGen-Plus does not require any special bowel preparation, stool handling or changes in diet prior to testing.
It also does not carry risks associated with invasive tests, such as colon perforation, hemorrhage, or infection, or sedation.
Ability to detect cancers proximal to the colon. aerodigestive cancers (e.g., lung, esophageal, stomach, pancreas, colorectal(
The tests detect target mutations with a high degree of sensitivity and specificity.
The other arises from malfunction to genes involved in DNA mismatch repair, manifested by microsatellite instability.
Colorectal cancer may also be detectable by DNA markers associated with disordered apoptosis.
CONCLUSIONS fecal blood appears to be a poor
marker for colorectal neoplasia. Most cancers and the vast majority of polyps will be missed. Hemoccult and HemoQuant are similarly insensitive.
