Clinical Microscopy and Parasitology Rotation
1. Tabulate the different tests used with their corresponding principles in
determining fecal occult. List down the advantages and disadvantages.
Type of FOBT (with
principle used)
Guaiac-based FOBT
(gFOBT)
Explanation of the
Principle
Based on
pseudoperoxidase
activity of hemoglobin
resulting in production
of the blue color
Advantage
●
●
●
●
Most frequently
used screening test
for fecal blood (GI
bleeding and
Colorectal cancer)
Inexpensive
Non-invasive
Relatively simple to
perform
Disadvantage
●
●
●
●
●
Immunochemical
FOBT (iFOBT) Hemoccult ICT
Employs a unique
combination of
monoclonal or
polyclonal antibodies to
selectively identify the
globin component of
human hemoglobin in
fecal specimens
●
●
●
●
●
More sensitive to
lower GI bleeding
More clinically
sensitive than
gFOBT
Suitable for
population screening
and can be
automated
○ Increasing
reproducibilit
y and
throughput
Does not require
dietary or drug
restrictions
Can be used for
patients who are
taking aspirin and
other
●
●
Low sensitivity for
polyps
Relatively low
specificity for
cancer
Detects peroxidase
activity of heme
and not specific for
human hemoglobin
(require dietary or
drug restrictions)
Cannot be used for
patients who are
taking aspirin and
other
anti-inflammatory
medications
Subjective and
difficult visual
reading of test
results that
requires significant
expertise
More expensive
than gFOBT
Detects only lower
GI bleeding
Porphyrin-based
FOBT - Hemoquant
Fluorometric test for
hemoglobin based on
the conversion of heme
to fluorescent porphyrins
which cannot be
detected by guaiac
●
●
●
anti-inflammatory
medications
Most sensitive to
upper GI bleeding
Measures amount of
stool blood present
Not affected by
presence of
reducing or oxidizing
substances or the
water content of
fecal specimen
●
●
●
Expensive
Time-consuming
procedure
False-positive with
non-human
hemoglobin
2. Explain the stool antigen test in detecting C. difficile infection. Include the steps
used in the procedure.
Glutamate dehydrogenase (GDH) is test a screening test that detects the
presence of Clostridium difficile antigen in high amounts, both toxin and non-toxin
producing strains, by latex agglutination or immunochromatographic assays. This is
considered to be very sensitive, but it is not very specific for toxin-producing C. difficile,
which means that it only indicates if the bacteria are present, not if the bacteria are
producing toxins. Follow-up positive screening results must be combined with toxin
testing to confirm diagnosis.
Procedure:
1. Specimen collection
a. Collect sufficient quantity of feces (around 1-2 g or mL for liquid sample).
b. Stool samples should be collected in clean and dry containers with no
preservatives or transport media.
c. The samples can be stored in the refrigerator (2-8ºC/36-46.4ºF) for 7 days prior
to testing.
d. For longer storage the specimen must be kept frozen at –20ºC/-4ºF. In this case,
the sample will be totally thawed, and brought to room temperature before
testing.
2. To process the collected stool samples:
a. Use a separate swab or stick, dropper and testing tube or vial for each sample.
b. Dispense exactly 1mL of the buffer into a testing tube.
c. Introduce the swab or stick in different parts of the fecal specimen to pick up the
sample and put into the testing tube or vial in order to assure good sample
dispersion.
d. For liquid stool samples, aspirate the fecal specimen with a dropper and add
approx. 125 µL into the testing tube or vial with buffer.
3. Test Procedure:
a. Allow the tests, stool samples and buffer to reach room temperature
(15-30ºC/59-86ºF) prior to testing. Do not open the sealed pack until ready to
perform the assay.
b. Use GDH Strip as soon as possible when opening the tube.
c. Extract some liquid from the top side with a dropper and dispense 150μL into a
testing tube.
d. Use a separate test strip for each sample. Leave the test strip to stand vertically
taking care of not surpassing the limit of immersion indicated with the arrows.
Leave it for 1-3 minutes and place on a flat surface. Start the timer.
e. Read the result at 10 minutes after dispensing the sample.
References:
https://education.questdiagnostics.com/faq/FAQ136
https://labtestsonline.org/tests/clostridium-difficile-and-c-diff-toxin-testing
https://www.mayoclinic.org/diseases-conditions/c-difficile/diagnosis-treatment/drc-20351697
http://www.atlas-medical.com/upload/productFiles/207004016/Clostridium%20difficle%20Packag
e%20Insert.pdf