Points to Consider when Choosing Tests for Viral Detection
Conventional tube cultures (e.g. comprehensive viral cultures) offer the advantages of high specificity and detection of multiple viruses at one time from a single specimen. The disadvantage is that tube cultures are slow and can have a limited impact on clinical decision-making.
Shell vial or multiwell plate cultures decrease the time required for detection of viruses in culture. However, they detect only one or a few viruses at a time and are normally less sensitive than conventional culture systems.
Use and relative importance of cell culture systems for viral isolation is declining with the continued development of rapid and accurate immunologic and molecular tests. As such, our laboratory no longer offers cell culture for viral identification.
Immunologic tests for direct detection of viral antigens in clinical material are now commercially-available for many viruses and the assays are routinely used in most clinical laboratories. The tests are rapid, inexpensive, simple to perform and do not require viable virus for detection. They have the disadvantage of usually being less sensitive than viral culture or molecular amplification techniques.
Conventional nucleic acid hybridization assays have limited utility for diagnosis of viral infections. For the most part, the assays are slow, relatively insensitive, cumbersome to perform, and expensive. However, this type of testing is particularly well suited for detecting human papilloviruses.
Molecular amplification methods are extremely sensitive and now play an increasing important role in rapid and accurate detection of viral illnesses; the potential applications of PCR and other amplification assays for viral diagnosis and monitoring are unlimited and the technology is revolutionizing the approach to and significance of viral diagnosis. Quantitative measures of viral nucleic acids (e.g. CMV, EBV, HHV-6, BKV, HCV, HBV and HIV) provide useful information about disease progression, prognosis, transmission, response to therapy, and development of drug resistance in chronically-infected and immunocompromised hosts.
Electron microscopy offers the main advantage of speed when doing negative staining of liquid samples (i.e. examining stools for viral agents of gastroenteritis); major limitations include the high cost of the instrument, the requirement for specialized expertise, and the overall lack of sensitivity and specificity. This procedure is seldom available in Virology Laboratories in the United States.
Direct cytological or histological examinations of stained clinical material are some of the fastest and oldest methods of detecting viruses. The tests are relatively insensitive compared with direct antigen or nucleic acid detection methods and their specificity is often low. Tzanck preparations, for example, are extremely insensitive and limited by their inability to distinguish herpes simplex virus from varicella-zoster virus infections. The sensitivity of histological staining can be increased somewhat by using immunohistochemical or in situ hybridization techniques.
Serological assays provide an indirect diagnostic approach by detecting specific antibody responses to viral infections. Detection of virus-specific IgM or demonstration of a seroconversion from a negative to a positive IgG antibody response can be diagnostic of primary viral infection. Detection of virus-specific IgG in a single serum specimen indicates exposure to a virus at some time in the past. Negative antibody titers may exclude viral infection. Results of serological tests must be interpreted with caution, as measurements of antibody responses to viral infections can be complicated by numerous factors.
Molecular genotyping assays using nucleic acid amplification of specific viral genes and direct sequencing of the amplified products can be used to identify mutations that confer resistance to antiviral drugs and for recognition of genetic variants that may be refractile to antiviral agents.
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