Laboratory tests for viral infections
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:
- Herpes simplex virus (HSV) types 1 and 2 (cause of herpes simplex infections including genital herpes)
- Varicella zoster virus (VSV) (cause of chickenpox and shingles)
In addition, a viral swab from oral skin mucosa can detect:
- Epstein-Barr virus (EBV) (cause of infectious mononuclosis)
- Coxsackie virus A16 (cause of hand, foot and mouth disease)
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:
- Full blood count: viral infection may raise or reduce the white cell count; atypical lymphoctyes may be reported
- C-reactive protein (CRP)is elevated but usually <50 in viral infections. CRP is a marker of inflammation anywhere in the body and is not a specific test for viral infections.
- Pro-calcitonin is negative: this is a blood test marker for generalised sepsis due to bacterial infection
- Serology: two samples of blood tested 10 days apart to determine immune response to a particular organism
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:
- Herpes simplex virus (HSV)
- Varicella zoster virus (VZV)
- Orf virus (a parapox virus from sheep/goats that causes orf)
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:
- Varicella zoster virus (VSV)
- Human immunodeficiency virus (HIV)
- Hepatitis B and C virus (in serum), the main causes of viral hepatitis
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.