Page 72 - Review of Medical Microbiology and Immunology ( PDFDrive )

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mebooksfree.com mebooksfree.com mebooksfree.com mebooksfree.com C H 9 E R mebooksfree.com
A
P
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Laboratory Diagnosis

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mebooksfree.com mebooksfree.com mebooksfree.com Immunologic Methods mebooksfree.com mebooksfree.com
CHAPTER C ONTENT S

Approach to Laboratory Diagnosis
Bacteriologic Methods
Identification of an Organism with Known Antiserum
Identification of Serum Antibodies with Known
Blood Cultures
Antigens
Throat Cultures
Nucleic Acid–Based Methods
Sputum Cultures
Spinal Fluid Cultures
Self-Assessment Questions
Stool Cultures
Practice Questions: USMLE & Course Examinations
Urine Cultures
Genital Tract Cultures Pearls
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Wound & Abscess Cultures

APPROACH TO LABORATORY
and in what sequence depend on the type of specimen and
DIAGNOSIS
organism. After the organism is grown in pure culture, its
sensitivity to various antibiotics is determined by proce-
The laboratory diagnosis of infectious diseases involves two
main approaches: one is the bacteriologic approach, in
which the organism is identified by staining and culturing
A general approach to the diagnosis of a bacterial infec-
the organism, and the other is the immunologic (sero-
tion is described in Table 9–1. This approach emphasizes
logic) approach, in which the organism is identified by dures described in Chapter 11.
the importance of performing a Gram stain and obtaining
detection of antibodies against the organism in the patient’s
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mebooksfree.com mebooksfree.com mebooksfree.com niques must be used. Table 9–2 describes some approaches mebooksfree.com
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a “pure culture” of the organism. However, sometimes the
serum.
organism is not recovered by culturing, and other tech-
In the bacteriologic approach to the diagnosis of infec-
tious diseases, several important steps precede the actual
to making a diagnosis when the cultures are negative. One
laboratory work, namely, (1) choosing the appropriate
approach that is commonly used is serologic testing, which
specimen to examine, which requires an understanding of
determines the presence of antibodies specific for the
the pathogenesis of the infection; (2) obtaining the speci-
organism. In most cases, a fourfold rise in antibody titer
men properly to avoid contamination from the normal
between the acute- and convalescent-phase serum samples
flora; (3) transporting the specimen promptly to the labo-
is considered to be significant.
ratory or storing it correctly; and (4) providing essential
Obtaining a pure culture involves culturing the organ-
information to guide the laboratory personnel.
ism on bacteriologic agar. Initially, blood agar is used
In general, there are three approaches to the bacterio-
because it supports the growth of many bacteria and the
logic laboratory work:
type of hemolysis can be observed.
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(1) Observing the organism in the microscope after
Blood agar contains red blood cells, but it should be
staining.
noted that viruses and obligate intracellular bacteria, such
as Chlamydia and Rickettsia, will not grow on blood agar.
(2) Obtaining a pure culture of the organism by inocu-
lating it onto a bacteriologic medium.
(3) Identifying the organism by using biochemical reac-
therefore, are incapable of supporting the growth of either
viruses or the obligate intracellular bacteria.
tions, growth on selective media, DNA probes, or specific
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