Page 303 - parasitology for medical and clinical laboratoryprofessionals

Laboratory Procedures for Identifying Parasitic Organisms and Their Ova 283

is passed through the anus and into the lower intestine histolytica. Tests for soluble antigens (components of the
to examine the sigmoid colon and the rectum. Aspirates parasites themselves) rather than antibodies to the para-
and scrapings from the device are used for diagnosing sitic antigens in clinical specimens do provide clinical in-
amebiasis or cryptosporidiosis. Immediate examination formation regarding current infection. But tests are only
by wet mount may yield trophozoites, and a part of the available currently for some of these organisms.
specimen is also placed in PVA fixative and fixed perma- Serological tests may not be useful unless the para-
nent slides are prepared by staining the specimen with site is of a type that invades the tissue, which provides
trichrome or other stains designed for this purpose. the greatest stimulation of antibody production. But
these tests may be useful where invasive procedures
can be avoided if the parasites are identified by immu-
Urine, Vaginal, and Urethral nologic means. Kits are not always available from com-
Specimens mercial vendors but the Centers for Disease Control
and Prevention (CDCP) may be of assistance by provid-
Urine sediment may reveal the trophozoites of Trichomo-
ing diagnostic methods to clinical laboratories. A direct
nas vaginalis in women and occasionally in men, and fluorescent procedure is available where an antibody
is a sexually transmitted infection. These trophozoites
against Giardia lamblia and Cryptosporidium antigens
are often extremely motile and are easily identified in may be obtained.
urine sediment during a routine urinalysis examination
in most cases. Ova of Enterobius vermicularis (pinworm
eggs) may be recognized, along with Schistosoma hema- Quality Assurance for
tobium, a fluke. T. vaginalis is also often identified by a Parasitology Procedures
wet mount of vaginal or urethral discharge, and can also
be cultured, unlike many other parasites, in a medium Reference samples are necessary for comparison when
that promotes growth and reproduction when incubated performing parasitological procedures. In addition,
°
at 37 C, a normal human body temperature. written reference materials and pictures are necessary
for those who seldom perform procedures for recover-
ing and identifying parasites. Most laboratories do not
Immunological Tests for have a dedicated laboratory professional to perform
Diagnosing Infections of or oversee the supplying and maintenance of the sec-
Parasites tion, and rotate various personnel through the depart-
ment when specimens arrive. The recovery rate suffers
Additional tests are currently being researched for the in these cases, at an estimate of the finding of twice as
development of serological tests for antibodies to para- many positive specimens when one person is in charge
sites. Several disadvantages exist to the use of these tech- of the department. The parasitology department should
niques that would prevent the use of the tests for routine also be enrolled in a proficiency program acceptable to
diagnosis, because they cast much doubt as to a current the facility’s accreditation requirements to ensure accu-
infection. These tests are not suitable for determining racy. And because size is important for stages such as
if antibodies that may yield a positive result were there the trophozoites, cysts, or ova of various species of par-
prior to the most recent illness, as would be the case in asites, a properly calibrated ocular micrometer should
a past exposure to the parasite in question. Antibodies be available for each objective on the microscope used
persist for years, and would be positive even with no cur- for parasitology.
rent infection. Also, there are a number of cross reactions
where similar antigens may give a positive result, limit-
ing any usefulness in diagnosing an infection of parasites. CALIBRATION OF MICROSCOPES
The cost for some of these tests may also be prohibitive, USING AN OCULAR MICROMETER
and the number of types of tests available commercially
is quite low. Some tests that are commercially available The purpose of calibrating the ocular micrometer is to
and that are quite useful are immunoassay or fluorescent correctly correlate ocular units of an entity being mea-
antibody tests for Toxoplasma gondii and Entamoeba sured to the number of microns represented by these