HIV - laboratory testing Flashcards
What sample is needed to carry out lab testing for HIV?
Serum or plasma
What additive is added to carry out HIV testing?
Heprin
EDTA
What test is done in the lab to test for HIV infection?
Chemiluminescent magnetic immunoassay
How is the CMIA carried out?
An immune complex is formed
Streptavidin-coated magnetic microparticles are added that bind to the complex
Complexes bind to the magentic surfaces on the electrode
Application of voltage generates electrochemiluminescence in the ruthenium complex
What is the immune complex made up of?
Target antigen
Biotinylated antibody
Ruthenium-labelled antibody
What is biotinylated?
The process of covalently binding the vitamin biotin to a protein
How do the streptavidin-coated magnetic microparticles bind to the immune complex?
Via interaction with biotin
How do you measure the amount of target antigen present in the sample?
Light formed when voltage is applied to the complex via electrochemiluminescence
Measured by a photomultiplier
Used to calculate the amount of target antigen present in the sample
How long does the complete assay take?
29 minutes
How many CMIA tests can be run per hour?
150 per hour
What is the overall specificity of the CMIA?
99.7%
What is the HIV antibody sensitivity of the CMIA?
100%
What does a negative CMIA result indicate?
Assay has 100% specificity
A negative result where there has been no exposure in the last three months should rule out HIV infection
What if a patient has a negative CMIA but has been exposed to HIV in the last 3 months?
Test should be taken again
At 3 months post-exposure
What does a strong positive result CMIA result indicate?
Same test should be repeated
Additionally immunocomb test should be carried out to identify which of the two major HIV types the patient is infected with
What happens if a weak positive CMIA result occurs?
Different HIV test will be conducted
Using a different analyser
What is an example of an alternative HIV test that can be conducted?
VIDAS
What is the most common sub-type of HIV infection?
HIV-1
What are the two sub-types of HIV infection?
HIV-1
HIV-2
What is the commonest test performed to differentiate between the two sub-types HIV-1 and HIV-2?
ImmunoComb assay
Enzyme immunoassay
What does the ImmunoComb assay do?
Qualitative differential detection of antibodies to type 1 and type 2
What sample is used for the ImmunoComb assay?
Serum
Plasma
How much time does an ImmunoComb assay take to produce results?
40 minutes
What is the sensitivity of the ImmunoComb assay?
100%
What is the specificity of the ImmunoComb assay?
99.4%
What happens once two tests have concluded the presence of HIV?
Third test needed after positive test is concluded from two tests
What is tested for triple check the diagnosis of HIV?
Two serum samples in gold-top SST tubes
One sample is re-tested to confirm the result and patient identity
The other sample will be tested for baseline serology
What is the EDTA tube test used for?
Used to measure the HIV viral load
Need for treatment and likelihood of progressing to AIDS
What is the colour of the EDTA test tube top?
Lavender/black top
What happens whilst the patient is being tested for HIV for the third time?
Patient needs to be treated by
- talking about options
- starting conversations about therapy
How is viral load measured?
RNA quantitative assays
How do quantitative assays differ from HIV diagnostic ELISA or POCT?
Measure how much virus a person is carrying
Why does the baseline serology need to be measured?
May have been exposed to other blood-borne viruses
Hepatitis B and C
Which tube is used to measure baseline serology?
EDTA tubes
