Diagnosis of Parasitic Organisms in Stool Specimens
The most common parasitic infections
in the United States are giardiasis (two
million infections per year), and cryptosporidiosis (300,000 infections per year).
In the absence of appropriate travel history or exposure, other parasites are rare.
Pinworm is relatively common, but is not
a cause of diarrhea and is not diagnosed
in fecal specimens.
The microscopic examination of fresh
fecal material is appropriate in certain
situations, but is time consuming and
expensive to test, and requires a highly
skilled technician. For giardiasis and cryptosporidiosis, the fecal antigen tests are
faster, less expensive and more sensitive.
For this reason, the antigen tests are the
first-line test, and in the absence of pertinent history, there is no reason to do an
O&P. For those patients that have traveled to endemic regions or have a history
of exposure it is then best to do the O&P.
If the O&P is negative, follow up with
the antigen tests. Three stool collection
specimens spread out over 10 days are
optimal for identification. In immunocompromised patients, if the preceding
tests are negative, it may be appropriate
to test for Microsporidia and Cyclospora
species. Both of these tests need to be
sent out to the Mayo Medical Labs.
Figure 1. Parasitic Investigation of Stool Specimens Algorithm
Patient has watery diarrhea and:
•
•
•
•
Patient has diarrhea and:
• History of travel or residence in a
third world country
•
• Possible worm segments observed
in stool
It is extremely helpful to submit supportive history for the situations that arise
when a true O&P exam is needed. We
realize that there are some immigrant
populations as well as people who travel
to endemic areas, but without their
patient histories, we may be wasting our
resources.
In order to simplify the process, we
have adopted an algorithmic approach
from the Mayo Clinic. The algorithm lists
situations where testing for cryptosporidia and Giardia are appropriate and when
O&P’s are appropriate (see figure 1).
Summary
• Giardiasis and cryptosporidiosis are the
predominant intestinal parasites in the
United States.
• A simple antigen test for giardiasis and
cryptosporidiosis is faster, less expensive and more sensitive, and should be
the first-line test in most situations.
• If a full stool O&P is ordered, please
submit appropriate history to support
performance of the test.
• It is recommended to submit up to
three specimens over 10 days if the first
specimen is negative.
— Jeff Pearson, MD
POSITIVE
NEGATIVE
POSITIVE
NEGATIVE
Stop testing
Consider testing for
Microsporidia and
Cyclospora species,
especially if immunocompromised
Stop testing
Consider testing for
giardia and cryptosporidium antigens
Two separate specimens
should be negative for
giardia before excluding.
Obtain at least two
more stool specimens
for testing over 10 days
Jeffrey
Pearson, MD