2. Virology Lab Flashcards
what does widespread maculopapular rash suggest?
areas of confluence are typical of measles
what does vesicular rash suggest?
dermatomal distribution is typical of shingles (variceller zoster)
what can be detected in a lab?
- infectious viruses and their components
- protein components (antigens)
- genetic components (DNA/RNA)
- host response (antibodies and cell responses)
what are the diagnostic methods?
- antibody detection (serology - enzyme linked immunosorbent assays)
- antigen detection (immunofluorescence)
- genome detection (PCR)
- quantification of antibody or antigens
- serotyping
- genome sequencing
- quantification of genomes (viral load - the amount of viral nucleic acid present in blood/other bodily fluids)
what are the limitations of laboratory tests?
- sensitivity: the test’s ability to correctly identify positive samples (less false -ve results)
- specificity: the test’s ability to correctly identify negative samples (less false positive results)
what are the typical samples used?
- throat swab, nasopharyngeal aspirate, bronchoalveolar lavage (for respiratory viruses)
- stools (for rotavirus, adenovirus, norovirus)
- urine (for BK virus and adenovirus)
- CSF (for meningitis, herpes, enterovirus)
- clotted blood (for serology - antibody detection)
- blood with EDTA (for viral load testing)
- saliva (for serology)
all samples can also be used for PCR
what does serology mean? give examples
testing for specific antibodies or viral antigens in serum or other body fluids
e.g. hep A = IgM and IgG, MMR = IgM and IgG, HIV = antibody + p24 antigen, shingles (VZV) = IgG
how are IgM and igG used in serology?
- IgM is a marker for recent/acute infection (lasts 3 months)
- IgG in the absence of IgM indicates infection at some point in the past or immunisation (lasts a lifetime)
what is the antibody avidity test?
how a positive IgM result is determined as IgM tests show low specificity (high rate of false positives)
at the acute stages of infection antibody avidity is low. as the immune response matures antibody avidity increases. a high antibody avidity result = past infection, a low result = recent infection
what happens when HIV serology gives a positive result?
- all reactive samples undergo confirmation testing to exclude false positives
- confirmed positives undergo typing to determine whether it is HIV 1/2
- repeat blood sample and EDTA blood is required for HIV viral load
what is point of care testing?
testing that occurs at the bedside/in the clinic - it provides a rapid, cost-effective addition to laboratory testing however they are less sensitive (more false negatives)
what is virus isolation in cell culture?
viruses are cultured on cell monolayers, in increasing concentrations of antiviral drugs
now only performed in specialised labs because it is slow and expensive
how is electron microscopy used in a virology lab?
viruses are too small to be seen by a LM but can be visualised using EM
sample types include stool and vesicle fluids
only performed in a limited number of labs because it is time consuming and expensive
how is immunofluorescence used in a virology lab?
- used for the direct detection of viral antigens in clinical samples
- used for out-of-hours respiratory virus testing because it is quick and easy
- dependent on the technicians skill
what does multiplex PCR mean?
more than one virus can be tested for in each tube saving time and money (limited to 3/4 per tube)
