Page 279 - Review of Medical Microbiology and Immunology ( PDFDrive )

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PART III Basic Virology
268
DETECTION OF VIRAL NUCLEIC
patient, the polymerase chain reaction can be used to amplify
ACIDS
the viral nucleic acids. Assays for the RNA of HIV and hepa-
titis C virus and the DNA of hepatitis B virus in the patient’s
Viral nucleic acids (i.e., either the viral genome or viral
mRNA) can be detected in the patient’s blood or tissues with
blood (viral load) are commonly used to monitor the course
complementary DNA or RNA (cDNA or cRNA) as a probe.
PEARLS of the disease and to evaluate the patient’s prognosis.
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Identification in Cell Culture
that has a characteristic appearance and is dangerous to grow
in culture.
• The presence of a virus in a patient’s specimen can be detected
by seeing a “cytopathic effect” (CPE) in cell culture. CPE is not
specific (i.e., many viruses cause it). A specific identification of
the virus usually involves an antibody-based test such as fluo-
• The presence of IgM can be used to diagnose current
rescent antibody, complement fixation, or enzyme-linked
infection.
immunosorbent assay (ELISA).
• The presence of IgG cannot be used to diagnose current
infection because the antibody may be due to an infection in
Microscopic Identification
the past. As a result, an acute and convalescent serum sample
should be analyzed. An antibody titer that is fourfold or greater
• Inclusion bodies, formed by aggregates of many virus parti-
in the convalescent serum sample compared with the acute
cles, can be seen in either the nucleus or cytoplasm of infected Detection of Viral Antigens & Nucleic Acids
sample can be used to make a diagnosis.
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cells. They are not specific. Two important examples are the
nuclear inclusions formed by certain herpesviruses and the
cytoplasmic inclusions formed by rabies virus (Negri bodies).
• Multinucleated giant cells are formed by several viruses,
notably certain herpesviruses, respiratory syncytial virus, and
hepatitis B surface antigen, is commonly used in diagnosis.
measles virus.
• The presence of viral DNA or RNA is increasingly becoming
• Fluorescent antibody staining of cells obtained from the
the “gold standard” in viral diagnosis. Labeled probes are highly
patient or of cells infected in culture can provide a rapid, spe-
specific, and results are rapidly obtained. Small amounts of viral
cific diagnosis.
nucleic acids can be amplified using reverse transcriptase to
produce amounts detectable by the probes. An important
• Electron microscopy is not often used in clinical diagnosis but
example is the “viral load” assay of HIV RNA.
is useful in the diagnosis of certain viruses, such as Ebola virus
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SELF-ASSESSMENT QUESTIONS
(C) Human papillomavirus
(D) Parvovirus B19
1. Regarding the diagnosis of viral infections in the clinical laboratory,
(E) Rubella virus
which one of the following provides the MOST specific diagnosis?
(A) Cytopathic effect produced by a virus that replicates on human
foreskin cells
(B) Cytoplasmic inclusion bodies produced by a virus that repli-
1. (D)
cates in the cytoplasm
(C) Multinucleated giant cells produced by a virus that replicates in
human skin cells
(D) Neutralization of infectivity using antibody against the viral 2. (B)
surface protein PRACTICE QUESTIONS: USMLE &
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COURSE EXAMINATIONS
(E) Intranuclear inclusion bodies produced by a virus that repli-
cates in the nucleus
Questions on the topics discussed in this chapter can be found
2. Seeing multinucleated giant cells in a Tzanck smear can be used
to make a presumptive diagnosis of infection by which one of the
Board) Practice Questions starting on page 720. Also see Part XIV:
following viruses?
USMLE (National Board) Practice Examination starting on
(A) Epstein–Barr virus
page 751.
(B) Herpes simplex virus

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