Virology Flashcards
What are two types of viral rashes?
Widespread macularpapular rash - Has areas of confluence (Typical of measles)
Vesicular rash - Dermatomal distribution (Typical of shingles)
What is searched for in virology labs?
Infectious viruses and their components (virus isolation and electron microscopy)
Protein components (antigens) of the virus, e.g. p24 antigen in HIV, surface antigen in HBV
Genetic components of the virus (DNA or RNA) – quantitative or qualitative tests are available
The host response (e.g. antibodies and cell responses)
What are the diagnostic methods of a virology lab?
Antibody detection (serology – enzyme linked immunosorbent assays, EIA)
Antigen detection (immunofluorescence, IF and enzyme immunoasaay, EIA)
Genome detection (polymerase chain reaction, PCR)
Quantification of antibody or antigens
Serotyping (e.g. HIV)
Genome sequencing (genotyping, antiviral resistance testing)
Quantification of genomes – viral load (essential for diagnosis and monitoring HIV, HBV and HCV)
(Viral load is the amount of viral nucleic acid present in blood or other bodily fluids)
What are sensitivity and specificity?
Sensitivity: the test’s ability to correctly identify positive samples (less false negative results)
Specificity: the test’s ability to correctly identify negative samples (less false positive results)
What are the typical samples used in virology labs?
Throat swab, Nasopharyngeal aspirate (NPA), bronchoalveolar lavage (BAL), ET secretions – for
detection of respiratory viruses by (IF or) PCR
Stools – for rotavirus, adenovirus & norovirus antigen detection (EIA) or PCR
Urine – for BK virus & adenovirus PCR
CSF – for herpes viruses and enteroviruses PCR
Blood (clotted) - for serology (antibody detection)
Blood (EDTA) - for PCR / viral load testing
Saliva – for serology &/or PCR (eg measles)
What is searched for in serological samples for specific diagnoses?
HIV serology - (antibody + p24 antigen)
Hepatitis A - IgM and IgG
HBV - surface Ag/Ab, eAg/eAb, core Ab, core IgM
HCV - serology (antibody +/- core antigen)
CMV (Cytomegalovirus) and EBV - IgM & IgG
VZV - IgG
Measles, mumps, rubella - IgM & IgG
Parvovirus - B19 IgM & IgG
What is antibody avidity testing?
- IgM tests usually show low specificity – so they give rise to a high rate of false positive results
- One way of determining the significance of a positive IgM result is the antibody avidity test
- At the acute stages of infection, antibody avidity is low
- You then get maturation of the immune response, and antibody avidity increases over a few months
- A HIGH antibody avidity result indicates past infection
- A low result is consistent with recent/acute infection
How does HIV serology work?
HIV serology has evolved over the years. We are on the 4th generation of EIA: Ab + p24 Ag detection. All reactive samples undergo confirmatory testing in a second assay to exclude non-specific reactivity (false positives).
• Confirmed positives undergo typing (to identify whether the patient has HIV 1 vs. 2)
• Repeat blood sample and EDTA blood is required for HIV viral load from all new positive (also allows for
genotyping and baseline resistance testing)
What is the advantage of serology?
Serology allows us to identify infection at a slightly earlier stage, before antibodies have had time to reach
detectable levels. Serology is highly automated, allowing us to test more samples, quickly and cheaply
What is point of care testing?
Tests at the bedside or in the clinic
Provides a rapid, cost-effective adjunct to laboratory testing (hasn’t replaced lab testing, but its an addition). It provides results almost immediately. However these tests are less sensitive than lab tests (more false negatives)
What is virus isolation in cell culture?
Viruses are cultured on cell monolayers, in increasing concentrations of antiviral drugs
It is still useful for phenotypic antiviral susceptibility testing (e.g. in HSV)
Now only performed in specialised laboratories because it is slow and time consuming (and expensive)
When is electron microscopy used?
Only performed in a limited number of laboratories (time consuming and costly)
Sample types: stool and vesicle fluids
How is immunofluorescence used?
Antibodies for specific viral antibodies marked with fluorescent dyes used
Still sometimes used for the direct detection of viral antigens in clinical samples (e.g. respiratory viruses)
Used for out-of-hours respiratory virus testing, because it is quick and easy
Rapid and inexpensive but subjective and very dependent on technician skill and quality of the sample
What samples are used in respiratory tract infections?
Throat swab +/- nose swab • Nasopharyngeal swab • Nasopharyngeal aspirate (NPA) • Bronchoalveolar lavage (BAL) • Endotracheal tube (ET) secretions
These samples are investigated using multiplex PCR assay – for dozens of respiratory virus. The term ‘multiplex’means multiple viruses (3-4) tested per tube
What is used to try and detect CNS viruses?
CSF is the sample of choice to investigate for meningitis and encephalitis (sometimes blood sample)
- CSF for PCR (HSV, VZV, enterovirus)
- Stools and throat swab for enterovirus detection (i.e. by PCR)
- Blood for serology and PCR for West Nile and Japanese Encephalitis virus infection & other arboviruses
