Laboratory tests for viral infections

Author: Natasha Lee BSc (Hons), Visiting Medical Student, University of Leeds; Chief Editor: Hon A/Prof Amanda Oakley, Hamilton, New Zealand. August 2014.

Why do we need laboratory testing for viral infections?

Various tests are carried out in a laboratory to establish or confirm the diagnosis of a viral skin infection. Although a thorough history and examination of the patient are vital, laboratory tests help the clinician to make a diagnosis.

What is a viral skin swab?

A viral skin swab is a sterile implement lightly rubbed against a visible skin lesion or vesicle. The swab is then sent to the laboratory in viral transport medium for further viral cell culture and virus identification.

What organisms can viral swabs detect?

A viral swab from an external skin lesion or mucosal surface can detect:

In addition, a viral swab from oral skin mucosa can detect:

What is a viral cell culture?

Viral cell culture places samples into suitable cell cultures that the virus being tested can infect. When the cells show particular changes the culture is positive. Viral cell culture can identify HSV, VSV, morbillivirus (cause of measles) and other viruses.

What blood tests are needed?

Blood tests for the investigation of viral infections include:

Polymerase chain reaction (PCR)

PCR involves isolating and amplifying lengths of DNA and then detecting known genetic sequences of microorganisms. It is useful in identifying:

Enzyme-linked immunosorbent assay (ELISA)

ELISA can test for specific organisms either by detecting the antigen during a current infection or more commonly, antiviral antibody. The detection of the antibody confirms contact with the virus at some time but it is not necessarily the reason for a current infection.

Most commonly detected viral antibodies are to:

What is electron microscopy?

Electron microscopy involves directing a beam of electrons at a sample producing an image. It can provide much higher resolution images than a standard light microscope. It is rarely used in practice. However it can be useful to identify atypical and rare viral infections in the immunosuppresed patients presenting with unusual skin lesions.

What is resistance testing in viruses?

Resistance testing in viruses is done to aid the clinician as to what anti-viral medications may or may not be effective for an infection. As viruses have the potential to mutate rapidly, much faster than bacteria or fungi, this presents a therapeutic challenge for the clinician, as treatment with anti-viral medications may start to fail. The most noteworthy is HIV anti-retroviral (ARV) resistance, which confers a great deal of risk to the patient if the medications stop working.

Sequence analysis is performed most commonly at the genomic level, which looks directly at the HIV genome to see to which antiretroviral drugs that strain of HIV is already resistant and therefore which drugs will not work. Less commonly, this analysis is performed at the phenotypic level, where the patient's HIV virus is subjected to antiretroviral medication in the laboratory to see how well it responds.

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