[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

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CHAPTER 2 Specimen Collection and Processing
Processing
Once a stool specimen has been received in the
laboratory, the analytic phase of laboratory
testing, also referred to processing, begins. In this
phase, samples are examined from two perspectives,
macroscopic and microscopic. A detailed
description of each perspective follows.
Macroscopic Examination. Stool specimens
submitted for parasitic study should first be
examined macroscopically to determine the consistency
and color of the sample. The specimen
should be screened and examined for the presence
of gross abnormalities. To perform this
macroscopic examination, the laboratory must
receive a fresh, unpreserved stool specimen.
Because most laboratories receive fecal specimens
already in fixative, this step is often skipped
because these macroscopic characteristics cannot
be determined. In such situations, a notation of
the gross appearance, either on the actual specimen
container or on the requisition form, is recommended
at the time of specimen collection.
The consistency or degree of moisture in a
stool specimen may serve as an indication of the
types of potential parasites present. For example,
soft or liquid stools may suggest the presence of
protozoan trophozoites. Protozoan cysts are
more likely to be found in fully formed stools.
Helminth eggs and larvae may be found in liquid
or formed stools.
The color of a stool is important because it
may indicate the condition of the patient, such
as whether a patient has recently had a special
procedure (e.g., a barium enema) or if the patient
is on antibiotic therapy. The range of colors
varies, including black to green to clay, and
colors in between. The color of normal stool is
brown. Unusual colors, such as purple, red, or
blue, typically suggest that the patient is on a
particular medication.
Gross abnormalities possibly found in stool
include adult worms, proglottids, pus, and
mucus. First, the surface of the stool should be
examined for parasites, such as pinworms
(Chapter 8), tapeworm proglottids, and adult
worms (Chapter 9). The sample should then be
broken up—a wooden applicator stick works
nicely for this task—and examined once more for
macroscopic parasites, especially adult helminths.
Samples containing adult worms may be carefully
washed through a wire screen. This process
allows for the retrieval and examination of the
parasites for identification purposes.
Other macroscopic abnormalities in the specimen
may have parasitic indications. For example,
blood and/or mucus in loose or liquid stool may
suggest the presence of amebic ulcerations in the
large intestine. Bright red blood on the surface
of a formed stool is usually associated with irritation
and bleeding.
A number of possible terms may be used to
describe the macroscopic appearance of a stool
specimen. A suggested list of possible consistency,
color, and gross appearance descriptions is
found in Table 2-2.
Quick Quiz! 2-3
Which of the following characteristics is observed
during the macroscopic examination of stool specimens?
(Objective 2-5)
A. Consistency
B. Color
C. Adult worms
D. All of the above
TABLE 2-2
Consistency
Terms
Macroscopic Examination of
Stool Specimens: Possible
Descriptive Terms
Possible
Colors
Gross
Appearance
Terms
Hard Dark brown Conspicuously fibrous
Soft Black Fiber scanty to
moderate
Mushy Brown Colloidal
(homogeneous)
Loose Pale brown Scanty mucus
Diarrheic Clay Much mucus
Watery, liquid Yellow Mucus with scanty
blood
Formed Red-brown Other (e.g., blood,
barium)
Semiformed Green,
other