OCCULT BLOOD TESTING

Effective Date: 
Mon, 03/03/2014
Reviewed: 
Tue, 01/10/2017
Revised: 
Mon, 03/03/2014
Policy: 

CLIA Complexity:  Waived

Method:

  • Hemoccult Fecal Occult Blood Test

Principle:

  • Hemocult is  a screening  procedure for fecal  blood  and  consists  of a cardboard slide with  guaiac-impregnated  paper  in  a cardboard frame which  permits  sample application  to  one  side with development and interpretation on  the  reverse  side.  Test is  based on  the  oxidation  of  phenolic compounds present  in  the  guaiac  to quinones resulting  in  production  of the blue color.  When a fecal  specimen  containing  occult blood  is  applied to the test  paper,  contact  is  made  between  hemoglobin  and  the guaiac.  A  pseduoperoxidase  reaction will  occur upon the addition  of the developer solution, with a blue chromagen formed proportional  to the concentration of hemoglobins.  The color reaction will occur after 30  seconds.

Materials:

  • Hemoccult Slide(s) CAUTION:  Protect from heat,  sunlight, humidity, fluorescent light, U.V.  radiation,  excessive air flow  or volatile  chemicals;  store  at  15 -  30°C  Do not refrigerate or freeze.
  • Wooden applicator sticks
  • Hemoccult Fecal Occult Blood Developer <6% Hydrogen Peroxide and Denatured Alcohol WARNING:  FLAMMABLE.  VAPOR HARMFUL.  AVOID CONTACT WlTH EYES OR SKIN.  FOR  IN VITRO DIAGNOSTIC USE ONLY.
  • Stool collection  container  (for home collection)

Specimen:

  • A thin  smear of  stool is applied to  impregnated filter paper  (Sure-Vue  slide)  using  a wooden  applicator stick.   Specimens are usually collected  in groups of three on consecutive days.  A clinician  may also collect  a specimen  in  a patient examination. (See following  special  patient  instructions  for home  collection.)
Procedure: 
  1. Hemoccult slides may be collected several ways:
    a.) The slides may be given to the patient with the following instruction  sheet for home collection  (see occult  blood  instruction  for patients).
    b.) The  patient  may collect the specimen  in the  lab,  and  the  Clinical  Laboratory Scientist will apply  a thin  smear of stool  specimen  to the  filter paper test areas  on the Hemoccult slide(s)  using wooden applicator sticks.

    c.) The  clinician  may collect a  specimen  rectally and  apply it to the  slide.

  2. When the labeled slide(s) are returned to the laboratory, open slide cover labeled “Open Tab”
  3. If the specimen  is thick and/or has been  received after  more than a week  after collection, rehydrating  the  specimen  using  2-3  drops of saline  may  enhance the sensitivity  of the test.
  4. Open perforated  section  on  back  labeled  “FOR  LABORATORY  USE ONLY."
  5. Apply two  drops  of developing  solution to sample “A” (right  side).
  6. Read results after 30 seconds:
    a)   A trace of blue is positive for occult blood.
    b)   No indication of blue is negative for occult blood.
  7. Repeat step 4 for sample “B" (left side).
  8. For Quality Control, apply one drop of developing solution to each control spot (positive and negative  red rectangles at bottom of slide).   Read results after 30 seconds and within 2 minutes.
  9. Enter both patient and QC results as well as Hemoccult Lot#/Exp date in the appropriate fields in the LIS.

Quality Control:

  • Positive and negative controls are present on each slide (located in red rectangle) with  patient test  areas  “A"  and  “B”  Negative  control  must show no trace of blue color.   The  positive control  must produce a  blue  or blue-green color.   lf expected results are  not obtained,  the  slides  should  be discarded  and  patient testing repeated  using  a new slide.   Controls are  performed with  each slide  and  are recorded along with the patient results in the LIS. Testing two samples of the patient specimen serves as a procedural control for quality assurance since this is a CLIA- waived  procedure,  and commercial  controls  are not  required.  There are three unknown occult blood specimens annually included with the laboratory’s Proficiency testing  for additional  quality  assurance.

Limitations:

  • It is  important that the slides  be  read 30 seconds  after developer has been added and within 2 minutes.   Results obtained with this test cannot be considered conclusive evidence of the presence or absence of gastrointestinal bleeding  or pathology.  False negative tests may be obtained since most bleeding occurs intermittently.  Fecal occult blood tests are designed as a preliminary screen and  are  not intended  to replace other diagnostic procedures.   This  method will detect  only  hemoglobin  released upon  hemolysis of the  red  cell.  Should whole blood be applied to the test paper,  it is necessary to hemolyze the red cells by the addition of a drop of water before adding the developer.

Expected  Results:

  • The guaiac paper tests detect occult blood,  but they are not diagnostic for disease.   False positive results may result from interfering substances (see chart below) or from,  diverticulitis,  hemorrhoids,  or colitis.   Positive tests should be followed  by  a thorough diagnostic workup.  This test will  detect 10 mg  of hemoglobin  per  gram of fecal  material.  Clinical  studies  demonstrate that the guaiac impregnated  slide tests  yield  a  positive result 3-5%  in screening programs.  The false positive rate was between  1-2% during controlled  diet situations that were  medically supervised.

Interfering Substances:

Substance Source Comments
Ascorbic  acid Vitamin C in high doses
(> 250 mg / day)
False  negative result:
discontinue intake for 2 days prior and during the test period
Peroxidase Fruits and vegetables, such
as turnips,  broccoli,
horseradish,  cauliflower,
cantaloupe,  parsnips  and  red
radishes
False positive result: avoid these
foods 2 days  prior to and
continuing  through the test
period
Oral  medications Aspirin,  indomethacin,
reserpine,  phenylbutazone,
corticosteroids
May cause bleeding  or  irritation
of the  gastrointestinal tract and
should  be  discontinued for 7
days prior to and during the test period
Heavy alcohol
consumption
Alcoholic drinks May cause bleeding or irritation
Of the  gastrointestinal  tract and
should  be discontinued for 7
days prior to and during the test
period
Red meat --- May cause false positives:
eliminate  red  meat from the  diet
during the test period

 

Key Points: 

References:

  • Product Insert,  Hemoccult,  Beckman Coulter Inc., Brea, CA Sept 2009