[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

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CHAPTER 2 Specimen Collection and Processing
Fixatives for Preservation
A freshly collected stool sample, which is immediately
submitted to the laboratory, is the ideal
specimen for parasitic examination. When this is
not possible, the sample must be preserved to
maintain its integrity. Fixatives are substances
that preserve the morphology of protozoa and
prevent further development of certain helminth
eggs and larvae. Several preservatives are available
commercially (see later). The ratio of fixative
to stool is important for the successful
recovery of parasites and, whatever fixative is
used, the recommended ratio is three parts fixative
to one part stool. Commercial kits may
contain one or more vials, each containing an
appropriate preservative. These kits usually
contain vials with fill lines marked to indicate the
appropriate sample volume. It is also important
that the specimen be mixed well with the preservative
to achieve thorough fixation. Because the
patient is often responsible for collection of the
specimen and transfer to the fixative vials, it is
imperative that he or she be given detailed and
complete instructions. The specimen must be
fixed in the preservative for at least 30 minutes
before processing begins.
The choice of fixative(s) for O&P use depends
on the preference of the laboratory performing
the test. Because the laboratory ideally should
have the ability to perform all steps of the O&P
test, appropriate fixatives should be on hand to
accomplish these steps. Some fixatives are limited
to use in certain O&P laboratory procedures.
Thus, the laboratory technician must be familiar
with and understand the uses and limitations of
each fixative. Table 2-1 provides an overview of
the procedures that can be performed using specific
fixatives. Some laboratories prefer to use a
two-vial fixative system; others use a single-vial
system. In addition, if other tests are ordered,
such as a fecal immunoassay, the laboratory must
ensure that the fixative is compatible for use
with these techniques. Finally, some fixatives
contain mercury and disposal regulations for
these compounds could affect the laboratory’s
decision of which fixatives to use in their testing
protocols. A description of representative fixatives
used in O&P testing follows.
Formalin. Formalin has been used for many
years as an all-purpose fixative for the recovery
of protozoa and helminths. Two concentrations
of formalin are commonly used; a 5% concentration
ideally preserves protozoan cysts and a 10%
concentration preserves helminth eggs and larvae.
Formalin may be routinely used for direct examinations
and concentration procedures, but not
for permanent smears.
There are advantages and disadvantages to
using formalin as a fixative. There are three
primary advantages for the use of formalin: (1)
it is easy to prepare; (2) it preserves specimens
for up to several years; and (3) it has a long shelf
life. One of the biggest disadvantages of formalin
is that it does not preserve parasite morphology
adequately for permanent smears. Other disadvantages
include the fact that trophozoites
usually cannot be recovered and morphologic
details of cysts and eggs may fade with time.
It is important to note that because the use of
formalin is considered a potential health hazard,
OSHA has developed formalin handling regulations
for laboratories. Monitoring of vapors, use
of protective clothing, and a comprehensive,
TABLE 2-1
Stool Preservatives and Applicable Laboratory Procedures
Preservative Concentration Permanent Stain Antigen Tests
10% formalin + − +
SAF + + (iron hematoxylin) +
PVA ± + (trichrome or iron hematoxylin) −
Modified PVA (zinc) ± + (trichrome or iron hematoxylin) ±
Single-vial system + + (trichrome or iron hematoxylin) ±