Diagnostics Flashcards
What is FOBT?
(Fetal Occult Blood Test)
How does it differ from FIT?
- for screening of colorectal cancer
- two main types of FOBT: guaiac-based FOBT (gFOBT) and immunochemical-based FOBT (iFOBT)
- A guaiac-based Fecal Occult Blood Test (gFOBT) uses the chemical guaiac (reagent derived from wood resin of Guajacum trees) to detect heme in stool. Heme is the iron-containing component of the blood protein hemoglobin. Heme contains pseudoperoxidase, a chemical which converts guaiac to the colour blue. Hydrogen peroxide is dropped onto the paper containing the smear of stool; and if trace amounts of blood are present, the paper will change color. This method works as hemoglobin has a peroxidase-like effect (pseudoperoxidase), rapidly breaking down hydrogen peroxide. The idea behind the gFOBT is that blood vessels at the surface of larger colorectal polyps or cancers are often fragile and easily damaged by the passage of feces. The damaged vessels usually release a small amount of blood into the feces, but only rarely is there enough bleeding to be noticeable in the stool. This test, however, cannot determine whether the blood is from the colon or from other portions of the digestive tract (such as the stomach). Therefore, if the test is positive, a colonoscopy is required to determine if there is a cancer, polyp, or other cause of bleeding such as ulcers, hemorrhoids, diverticulosis (tiny pouches that form at weak spots in the colon wall) or inflammatory bowel disease (colitis).
- Immunochemical Fecal Occult Blood Test (iFOBT) uses antibodies to detect human hemoglobin protein in stool. One example of the iFOBT test is the Auto FIT - uses antibodies to detect human hemoglobin protein in stool. Much like the gFOBT, immunochemical based testing detects the presence of blood in the stool, but the main difference is that the iFOBT/AutoFIT uses a different technology to detect the presence of gastrointestinal bleeding. For this reason, it may be a more accurate way to screen for blood in the stools than the traditional fecal occult blood test. This test reacts to part of the human hemoglobin protein (heme), which is found in red blood cells and it is also less likely to react to bleeding from parts of the upper digestive tract, such as the stomach. As with the gFOBT, immunochemical based testing may not detect a tumor that is not bleeding. If blood is detected, the patient will require follow-up testing such as colonoscopy, to determine the reason for the presence of blood in the stools.
Fecal Fat Test?
• Quantitative 2–6 g/d on an 80–100 g/d fat diet • 72-h collection time (refrigerate sample)
Aids in diagnosis of malabsorption, steatorrhea. Most fat normally absorbed in small bowel
Increased:
Pancreatic dysfunction (chronic pancreatitis, CF, Shwachman–Diamond syndrome), diarrhea with or without fat malabsorption (any diarrhea state alters fat absorption), regional enteritis (Crohn disease), celiac disease
FOBT?
- Normal: Negative
- Collection: Diet free of exogenous peroxidases (fish, horseradish, turnips), no vitamin C or medicines that irritate GI tract (eg, NSAIDS). Patient collects 2–3 consecutive stool specimens and uses a wooden stick to place sample on assay card. Rectal exam sample may also be used.
Annual FOBT reduces colorectal cancer deaths 15–33%. Test based on detecting stool peroxidase activity. Hemoccult II test entails use of guaiac-impregnatedpaper and developer to detect oxidation of a colorless indicator to a colored (blue) one in the presence ofhemoglobin pseudoperoxidase. More sensitive assays are immunochemical tests such as HemSelect(HS) and FlexSure (FS) in which anti-human hemoglobin antibodies are used to detect stool human hemoglobin.
Positive:
Colon or rectal polyps or cancer, hemorrhoids, anal fissures, esophageal or gastric cancer, peptic ulcers, ulcerative colitis, Crohn disease, GERD, esophageal varices, vascular ectasia
False-Positive:
Recent dental procedure with bleeding gums, eating red meat within 3 days of test, fish, turnips, horseradish, or drugs such as colchicines and oxidizing drugs (eg, iodine and boric acid)
False-Negative:
High doses of vitamin C
Helicobacter Pylori Antibody Titers?
• IgG < 0.17 = negative
Most patients with gastritis and ulcer disease have chronic H. pylori infection that should be controlled. Positive in 35–50% of patients without symptoms (increases with age). Use in dyspepsia controversial. Methods to test for H. pylori: noninvasive (serology, 13C or 14C urea breath test one of the most accurate noninvasive tests currently available, fecal assay [see Helicobacter pylori Antigen, Feces]) and invasive (“gold standard” gastric mucosal biopsy and Campylobacter-like organism test). The IgG subclass is found in all patient populations; occasionally only IgA antibodies can be detected. Serology most useful in newly diagnosed H. pylori infection or monitoring response to therapy. IgG levels decrease slowly after treatment and can remain elevated after infection clears.
Positive:
Active or recent H. pylori infection, some asymptomatic carriers
Helicobacter Pylori Antigen, Feces?
• Collection: 5 g of stool in a screw-capped, plastic container. Submit promptly to lab. Watery, diarrheal specimens or stool in transport media, swabs, or preservatives cannot be tested.
Uses: diagnosis of H. pylori and monitoring H. pylori clearing after therapy. Persons without symptoms should not be tested.
Positive:
H. pylori antigen present in the stool
Negative:
Absence of detectable antigen; does not exclude the possibility of infection by H. pylori
What are the tests available for fat malabsorption?
- fecal fat excretion (amount of fat excreted)
- Sudan III stain (stains fat in poop)
- 14C triolein breath test
What does fecal fat excretion tell you?
- average fat consumption 60-100g/24 hrs, of that <6 g and up is expected to be excreted /24 hrs in stool
- memory aid (poop pile with a hook looks like 6)
- test requires 1, 3 or 5 day stool collection
- if gross steatorrhea (fat containing stool) is obvious, no further evaluation is needed
- if excreting >6 g of fat, could be really high fat diet OR fat malabsorption
What does Sudan III stain tell you?
- special stain that binds to fat in poop
- but also binds to undigested muscle fibers - many false positives
- decent test for steatorrhea - fat in stool
What does 14C Triolein breath test tell us?
- **triolein **is a triglyceride
- (think tri - for triglyceride and 3 fatty oleic acids)
- when triolein is hydrolyzed (broken down), it releases CO2 present in breath
- so if you label C to C14, can measure the amount of 14CO2 in breath to see how well triolein is hydrolyzed
- since most hydrolization of triglycerides is done by pancreatic lipase, triolein breath test is a measure of pancreatic function -> low levels of 14CO2 indicate pancreatic insufficiency
- results can be influenced by lung disease, presence of colon bacteria, etc
- done extremely rarely “now only in anorexic patients”
What tests are available for carbohydrate malabsorption?
- lactose breath test/hydrogen breath test
- D-xylose test
How does lactose/hydrogen breath test works?
- lactose –break down –> galactose + glucose
- enzyme to break lactose is in intestine: mucosal cells make lactase, which breaks lactose to galactose and glucose - simple sugars easy to absorb
- no lactase -> lactose passes undigested to colon, where it gets digested by bacteria -> they make a lot of H2 when they digest lactose (because ferment to fatty acids and H2)
- anytime COLON BACTERIA involved = GAS
- increase in H2 in breath - likely deficiency in lactose metabolism
- can have false positives if bacteria overgrown
what does D-xylose test indicate *rarely used*?
- measures intestinal absorption area
- D-xylose is poorly metabolized, so absorbed in duodenum and jejunum and excreted in urine
- if urine has low levels of D-xylose - poor intestinal absorption sugesting inflammed mucosa -> celiac disease/tropical sprue
- poor sensitivity/specificity/rarely used
Discuss Shilling test?
- two stages:
- stage 1: given radio-B12 PO (cobalamin) and non-radio B12 IV (in case there is shortage, want to saturate B12, so body doesn’t just hord it)
- **urine collected over 24 hrs to assess how much B12 absorbed **
- if B12 lack in diet, B12 levels in urine would increase dramatically
- if abnormal: pernicious anemia - parietal cells destroyed, cannot make intrinsic factor needed for B12 absorption; pancreatic insufficiency - pancreatic enzymes detach B12 from R protein so B12 can attach to IF until absorption; loss of absorptive surface in terminal ileum or bacterial overgrowth (first and second stage abnormal)
- stage 2: since know it is not diet, give radioactive B12 with IF -> skips all the coupling and transition steps except pure absorption, if stage 2 negative, know it is absorption issue -> disease in distal ileum/surgical resection preventing absorption or bacterial overgrowth
- reminder: B12 ABSORBED IN DISTAL ILEUM (ALMOST BY COLON!!! THATS WHY NEEDS IF AND R PROTEIN TO PASS ALL THAT GI JUICE AND MAKE IT TO THE END)
What tests can detect bile salt malabsorption?
if steatorrhea is associated with ileal disease or resection, not usually helpful to test for bile salt malabsorption (obvious!)
- 14C glycocholic acid breath test
- selenium-75 labeled homotaurocholic acid test
- cholestyramide trial (bile salt binder)
What is 14-glycocholic acid breath test
- radiolabelled glycocholic acid (think acid, likely fatty acid, breath test - bacteria breaking down CO2, so fat making it to colon if bacteria -> shows bacterial overgrowth)
- if not absorbed in small intestine and reaches the colon, broken down by colonic bacteria, releasing 14CO2 (exhaled)
- can diagnose bacterial overgrowth, but high # of false positives/negatives
