Diagnosis of Viral Infections Flashcards
What is a key concept about diagnosing viral infection?
It is not always possible to diagnose a infection clinically. Often, we require a laboratory diagnostic test.
Other factors that aid diagnosis:
- history
- examination
- special investigations
Why is it good to get rapid diagnosis?
A rapid diagnosis of viral infections can reduce the need for unnecessary tests and inappropriate antibiotics.
It is also an important public health tool, as it has infection control implications.
What are the three types of laboratory tests with regards to viral infections?
- diagnostic tests
- monitoring tests
- screening tests
List some possible tests.
- Electron Microscopy
- Virus isolation (cell culture)
- Antigen detection
- Antibody detection by serology
- Nucleic acid amplification tests (NAATs, e.g. PCR)
- Sequencing for genotype and detection of antiviral resistance
How do we visualise viruses?
Viruses can be visualised with an electron microscope.
They have mostly been replaced by molecular techniques. However, they are possibly still useful for faeces and vesicle specimens.
They’re also useful in characterising emerging pathogens.
How does an electron microscope work?
The specimens are dried on a grid.
They can be stained with heavy metal e.g. uranyl acetate.
They can also be concentrated with application of antibody i.e. immuno-electron microscopy to concentrate the virus.
Beams of electrons are then used to produce images.
It uses a wavelength of electron beam that is much shorter than light, resulting in a much higher resolution than light microscopy.
List some advantages of using an electron microscope.
- Rapid
- Detects viruses that cannot be grown in culture
- Can visualise many different viruses
List some disadvantages of using an electron microscope.
- Low sensitivity - need 106 virions/millilitre to be able to visualise (may be enough in vesicle secretion/stool)
- Requires maintenance
- Requires skilled operators
- Cannot differentiate between viruses of the same virus family
Which two herpes viruses cause vesicles, and would an electron microscope be able to distinguish between them?
Herpes viruses that cause vesicles are:
- Herpes simplex
- Varicella zoster virus
An electron microscope cannot differentiate between these different viruses so, it depends on the clinical context, site of vesicle and symptoms present.
Describe virus isolation in cell culture.
Viruses require host cells to replicate and may cause a Cytopathic Effect (CPE) of cells when a patient sample containing a virus is incubated with a cell layer.
Thus is an old method, now replaced by molecular techniques, but still needed for research or for rare viruses. It led to discovery of hMPV and Nipha virus in last 20 years.
We use different cell lines in test tubes or plates. The selection of cell types is important as different viruses may have different affinities for different cells.
It’s slow, but occasionally is useful in anti-viral sensitivity testing.
What are some common methods used for antigen detection?
A variety of different methods can be used. The commonest methods are:
- Direct immunofluorescence
- Enzyme immunoassay
- Immunochromatographic methods
It is often used at point of care for rapid diagnosis.
Briefly, describe immunofluorescence.
The antigen (from infected host cells in sample) is bound to the slide.
A specific antibody (polyclonal or monoclonal) to that antigen is tagged to a fluorochrome and mixed with the sample.
It is then viewed using a microscope equipped to provide ultraviolet illumination.
Any cells that have the virus in them will fluoresce.
Briefly, describe the immunochromatographic method.
You get a bit of the patients blood, and you add it to the machine.
You’ll see a line appear on it where you get the binding on an antigen in the patient’s blood with an antibody in the particular kit, and it causes precipitation of heavy metals and you see a visible line.
This is called the immunochromatographic method.
Briefly, describe ELISA for antigen detection.
ELISA is the enzyme-linked immunosorbent assay.
There are three formats:
- Indirect
- Direct (primarily antigen detection)
- Sandwich
Describe how the ELISA test works.
- The plate is coated with a capture antibody.
- The sample is added and any antigen present binds to the capture antibody.
- An enzyme-conjugated primary antibody is added, which binds to the detecting antibody.
- After washing away the unconjugated sample, a chromogenic substrate is added, and is converted by the enzyme to a detectable form e.g. colour change.
The substrate only will change colour only if the enzyme-conjugated antibody and therefore also the antigen are present.
A negative result = NO colour change.