Viral Isolation: FAQs

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 patient's exposure history.

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.

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 two weeks. However, the result may take more than two 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 two 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 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.

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.

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 patient's 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.

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."