Most
specimens for virus isolation are tested by cell culture methods, including both standard
tube culture and shell vial, where appropriate. In addition, specimens may be evaluated by
electron microscopy for direct detection of viruses in the specimen. Molecular detection
methods are also used for certain viruses. Identification methods include serum
neutralization, fluorescent antibody testing, PCR, hemagglutination inhibition, and immune
electron microscopy.
Frequently
Asked Questions
How do I know what source of specimen to submit?
Refer to the Specimen Selection Based on Clinical
Symptoms table. In addition, the specimen of choice is influenced by seasonal or
regional epidemiological trends and the patients exposure history.
When should specimens be collected?
For optimal viral recovery, specimens should be collected as close to the time of onset
as possible. The length of viral shedding depends on the virus, the clinical presentation,
and the site sampled as well as the immunocompetency of the patient but generally declines
with time after onset.
How long does it take to obtain results?
Turnaround time for viral isolation results varies with the test(s) performed and the
rate of growth of the virus isolated.
If no virus is isolated, a result is generally available in 2 weeks. However, the
result may take more than 2 weeks if the specimen causes a reaction in cell cultures that
appears to be due to a virus which requires follow up testing, but the follow up testing
does not confirm the presence of a virus. In addition, the testing may take more than 2
weeks if the specimen has components that are toxic in cell cultures requiring additional
procedures to decrease the toxic effects.
If a virus is isolated, a result can be available within several days but may take
weeks due to the varying times needed for some identification test results and the ease
with which the virus grows.
Turnaround time for influenza surveillance results is similar to that of viral
isolation results.
Turnaround time for dengue isolation results is generally 7-10 days.
Turnaround time for electron microscopy is generally same day; however, if EM is
coupled with an isolation effort, the results are normally reported at the same time. Only
positive EM results are reported as the microscopy is completed.
Turnaround time for C. trachomatis isolation is 7 days.
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What is the significance of a "No virus isolated"
result?
A "No virus isolated" result does not rule out the presence of a virus.
Successful isolation of a virus depends on proper specimen collection including timing and
site selection, proper transport or storage of the specimen, use of susceptible host
systems in the laboratory, and lack of interference from contaminants or toxic components
in the specimen.
What is the significance of a "Viral agent
isolated:......." result?
The significance of a viral isolate must be interpreted based on clinical findings, the
site of isolation as it relates to the organs involved, and an understanding of the
pathogenesis of the virus recovered.
Viruses are not considered normal flora. If a virus is isolated, that virus has caused
an infection; however, it does not necessarily mean that virus is responsible for the
patients symptoms. A virus can be isolated when a latent virus is reactivated, when
virus is shed for long periods of time, or following immunization with a live virus
vaccine. In these situations, a positive isolation result must be interpreted with
caution.
If I receive a qualification statement on a report, what does it
mean?
It means that the test requested was performed but was not optimal for a reason which
is a part of the qualification statement. For example if our laboratory received a
specimen at ambient temperatures, a qualifying statement will be included which reads,
"Results may be invalid because specimen was received warm."
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Note: External links to other sites
are intended to be informational and do not have the endorsement of the
Texas Department of State Health Services. These sites may also not be accessible to
people with disabilities.
CDC Home Page
All the Virology on the WWW
Pan American Society for Clinical Virology
WHO Influenza Surveillance
US Food and Drug Administration
"Bad Bug Book"
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