[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

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CHAPTER 2 Specimen Collection and Processing
in an appropriately collected and processed
specimen. Occasionally, however, standard laboratory
tests are not sufficient for the diagnosis of
a parasite. For example, in some parasitic infections,
the diagnostic stage is located deep in the
tissues of the host (e.g., toxoplasmosis; see
Chapter 7) and it may not be possible to detect
its presence or it may be dangerously invasive to
attempt it (e.g., echinococcosis; see Chapter 10).
In these situations, immunologic assays can be
used. Immunologic testing is usually considered
as an adjunct or supplement to standard laboratory
protocols.
Immunologic tests include methods for antigen
and antibody detection. Antigen detection
methods are more reliable and a positive test
result is indicative of a current infection. Some
antigen detection methods for intestinal pathogens
were described earlier in the discussion of
stool screening methods. These techniques allow
for the rapid detection of specific intestinal
pathogens. Tests that detect antibody in the
patient are more complex and must be interpreted
cautiously. The presence of an antibody
against a given parasite may not always indicate
a current infection, however. Because antibodies
remain with a host for many years, a positive test
result can occur from a past infection. The detection
of an antibody to a given parasite in a
patient with no previous exposure prior to travel
to an endemic area can be considered a positive
result.
There are a wide variety of immunologic tests
that have been developed over recent years. These
assays are not usually offered by routine laboratories
and specimens must be sent out to specialty
commercial or reference laboratories that perform
them. The Centers for Disease Control and Prevention
(CDC) also performs these assays on
request. Each laboratory must check with the
local public health laboratory to make arrangements
for these tests to be performed.
Table 2-8 contains a list of parasitic diseases
for which immunologic tests are available and
the type of assay used. This table is not intended
to be exhaustive in nature, but rather a representation
of certain diseases that may be diagnosed
by these tests. The principle of each type of
immunoassay is beyond the scope of this text.
The reader can refer to an immunology text to
review these features.
Nucleic acid tests have also been developed
for certain parasites and are primarily performed
in a specialized research or reference laboratory.
The only commercial molecular test available is
for the diagnosis of T. vaginalis. Further molecular
techniques will become available as manufacturers
develop automated systems that can
be used by the diagnostic laboratory. Studies
designed to incorporate new techniques in the
diagnosis of parasitic diseases are performed on
a regular basis. There are numerous methods
that will no doubt emerge over time and perhaps
eventually replace current standard techniques.
Quick Quiz! 2-13
The detection of an antibody to a given parasite in a
patient with no previous exposure prior to travel to
an endemic area can be considered a positive result.
(Objective 2-14)
A. True
B. False
REPORTING OF RESULTS AND
QUALITY CONTROL
Once the analytic phase of testing is completed,
the results are interpreted and reported. This is
considered the postanalytic phase of laboratory
testing. When reporting a positive specimen, the
report should state the scientific name (genus and
species), along with the stage that is present (e.g.,
cyst, trophozoite, larvae, eggs, adults). It is also
helpful to report the presence of certain cells in
the specimen. White blood cells should be
reported semiquantitatively—rare, few, moderate,
many.
The results of the O&P procedure should
include a comment indicating that this procedure
does not detect Cryptosporidium spp., Cyclospora
cayetanensis, and microsporidia; it will recover
the oocysts of Isospora belli (see Chapter 7 for