Guaiac testing of stool for occult blood is a commonly used screening test for colorectal cancer. Patient preparation is necessary to reduce the frequency of false-positive results.


Testing protocols:

(1) While a single test may be done without patient preparation, patient preparation is indicated before conducting a screening test series (usually 3 tests).

(2) If persistent positive results are found on a test series, then colonoscopy is usually performed.


Medicines to avoid for 72 hours prior to and during testing because of potential gastrointestinal irritation and/or increased bleeding:

(1) aspirin (low dose for cardiovascular disease prophylaxis can be continued)

(2) other NSAIDS

(3) oral iron, only if irritating

(4) cimetidine


Medicines or vitamins causing false negative results:

(1) ascorbic acid (reducing agent): discontinue 5 days prior to testing

(2) antacids


Foodstuffs to avoid because of peroxidase-like substances for 3-5 days before and during the test period (false positives):

(1) turnips

(2) horseradishes

(3) artichokes

(4) mushrooms

(5) radishes

(6) broccoli

(7) bean sprouts

(8) cauliflower

(9) apples

(10) melons

(11) oranges

(12) bananas

(13) grapes


Conditions requiring a delay in testing:

(1) hemorrhoids

(2) menstruation

(3) dental work with bleeding

(4) known upper GI bleeding


Foodstuffs containing blood should be stopped for 72 hours prior to testing and through test period:

(1) red meat

(2) black (blood) pudding


Suggested meat-free diet starting the day before and during test period:

(1) vegetables, raw and/or cooked: lettuce, spinach, corn

(2) fruits not listed above

(3) bran

(4) roughage with popcorn and peanuts


Sources of false positive for specimens collected from within the toilet:

(1) toilet bowl sanitizers

(2) blood in toilet from other sources


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