Page 78 - Review of Medical Microbiology and Immunology ( PDFDrive )

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CHAPTER 9 Laboratory Diagnosis
67
PEARLS
• The laboratory diagnosis of infectious diseases includes bac-
teriologic, immunologic (serologic), and molecular (nucleic
• Immunologic (serologic) tests can determine whether anti-
acid–based) tests.
bodies are present in the patient’s serum as well as detect
the antigens of the organism in tissues or body fluids.
Bacteriologic Tests
• Bacteriologic tests typically begin with staining the patient’s • In these tests, the antigens of the causative organism can be
detected by using specific antibody often labeled with a dye
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specimen and observing the organism in the microscope. This
such as fluorescein (fluorescent antibody tests). The presence
is followed by culturing the organism, typically on blood agar,
of antibody in the patient’s serum can be detected using anti-
then performing various tests to identify the causative
gens derived from the organism. In some tests, the patient’s
serum contains antibodies that react with an antigen that is
organism. Obtaining a pure culture of the bacteria is essential
to accurate diagnosis.
not derived from the causative organism, such as the VDRL test,
in which beef heart cardiolipin reacts with antibodies in the
• Blood cultures are useful in cases of sepsis and other diseases
serum of patients with syphilis.
in which the organism is often found in the bloodstream, such
as endocarditis, meningitis, pneumonia, and osteomyelitis.
serum, an acute and convalescent serum sample is obtained,
• Throat cultures are most useful to diagnose pharyngitis
caused by Streptococcus pyogenes (“strep throat”), but they are
convalescent samples must be found for a diagnosis to be
also used to diagnose diphtheria, gonococcal pharyngitis, and
made. The reason these criteria are used is that the presence of
thrush caused by the yeast Candida albicans.
antibodies in a single sample could be from a prior infection, so
• Sputum cultures are used primarily to diagnose the cause of and at least a fourfold increase in titer between the acute and
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a significant (fourfold or greater) increase in titer is used to
pneumonia but also are used in suspected cases of
indicate that this is a current infection. IgM antibody can also
tuberculosis.
be used as an indicator of current infection.
• Spinal fluid cultures are most useful in suspected cases of
meningitis. These cultures are often negative in encephalitis,
brain abscess, and subdural empyema.
• Stool cultures are useful primarily when the complaint is
• Molecular tests can detect the presence of bacterial DNA or
bloody diarrhea (dysentery, enterocolitis) rather than watery
RNA in patient specimens. These tests are both sensitive and
specific, and results are available within a clinically useful time
diarrhea, which is often caused by either enterotoxins or viruses.
frame. They have become the diagnostic “gold standard” for
• Urine cultures are used to determine the cause of either
many infections.
pyelonephritis or cystitis.
• Genital tract cultures are most often used to diagnose gonor-
chain reaction (PCR) to detect C. trachomatis and N. gonorrhoeae
rhea and chancroid. Chlamydia trachomatis is difficult to grow, • Nucleic acid amplification tests (NAAT) utilize the polymerase
in urine samples in sexually transmitted disease (STD) clinics.
so nonbacteriologic methods such as ELISA and DNA probes are
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These tests are also used to identify M. tuberculosis in sputum
now used more often than are cultures. The agent of syphilis has
samples.
not been cultured, so the diagnosis is made serologically.
• Wounds and abscesses can be caused by a large variety of
RNA probe to bind to DNA or RNA present only in the bacteria
organisms. Cultures should be incubated both in the presence
to be identified.
and in the absence of oxygen because anaerobes are often
involved.
SELF-ASSESSMENT QUESTIONS
the presence of which one of the following bacteria?
1. If the venipuncture site is inadequately disinfected, blood cultures
(A) Neisseria meningitidis
are most often contaminated with which one of the following 2. The main purpose of performing a throat culture is to detect
(B) Staphylococcus aureus
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bacteria?
(C) Staphylococcus epidermidis
(A) Escherichia coli
(D) Streptococcus pneumoniae
(B) Haemophilus influenzae
(E) Streptococcus pyogenes
(C) Pseudomonas aeruginosa
(D) Staphylococcus epidermidis
(E) Streptococcus pneumoniae
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