IBTS - Virology Testing Flashcards
What are the minimum legal requirements that blood be tested for?
Individual donations must be tested for antibodies to:
- HIV type 1/2
- hepatitis C virus
- hepatitis B surface antigen (HbsAg)
How do we ensure transfusion safety?
Careful selection of donor -> travel history, high risk activity etc
Laboratory screening
Viral inactivation (SD plasma) and leukodepletion
Appropriate Blood Use
Haemovigilance
- Transfusion reaction reporting
- Guidelines
- Audit and review
What are the main sources of risk in the transfusion service
(6)
The infectious window period
Immunosilent infection (no antibodies)
Variants/mutations of known agents
Laboratory error
New agents for which no test is available
Unknown agents
Why are we concerned with the infectious window period?
The blood donor is infectious but has not yet developed antibodies or a detectable viral load
What requirements do we have for our screening assays
High sensitivity and specificity
High throughput and automated
Rapid turnaround time
High level of traceability
Low cost
Talk about the sensitivity of our assays?
Need highly sensitive assays that dont need to be extremely specific
- we want to detect the smallest amount of viruses
- not worried about typing the virus
How do we screen for viruses?
We use the Abbott Alinity system
Chemiluminescent immunoassay technology
They determine the presence of specific antigens and antibodies
List the immunoassays we use
HIV Ag & Ab
Anti-HCV
HbsAg
Anti-HBcore
Syphilis
Anti-HTLV I/II
Anti-CMV IgG
How does the chemiluminescent immunoassay technology work?
(5)
Paramagnetic particles are coated in recombinant viral antigen
These are used to capture virus specific antibodies in the donor sample
The mixture is washed
AHG/AHM acridinium-labelled conjugate is added to create a reaction mixture and incubated
Following a wash cycle, pre-trigger and trigger solutions are added and the resulting light emission is captured and measured
What is the anti-human IgG/IgM labelled with in the immunoassays?
Acridinium-labelled conjugate
How do we manage positive viral testing results?
(5)
Reactive samples are first duplicated
Any products which have a repeat reactive result are discarded
Samples are then referred to a reference lab
A follow-up donation is requested
The donor is notified and deferred for a speicific period of time depending on what virus has been detected
True positive donors are then offered medical counselling
What might cause a false positive
(2)
Transient non specific reactions e.g. cross reacting antibodies e.g. flu vaccination programme
Reactivity that persists from sample to sample -> antibodies specific to a component of the assay e.g. anti mouse antibodies
Why do we test with two HIV assays
HIV is a retrovirus with two primary types: HIV1 and HIV2
HIV1 is more virulent and relatively easily transmitted
HI2 is less transmissible and is largely confined to West Africa
Write about HIV
HIV infects and kills CD4 lymphocytes
HIV infection is often asymptomatic
523 diagnoses of HIV in Ireland in 2018
Transmission through sexual contact, infected blood/blood products/IV drug use
Write about HIV screening in the IBTS
Testing using both serology and NAT on all blood donations
CMIA detects the presence of HIV-1 p24 antigen
The test detects antibodies to HIV-1 and antibodies to HIV-2
What is the window period for HIV screening
23 to 90 days after exposure
Inclusion of HIV1 p24 antigen has decreased the window period to approximately 15 days
What is the risk of HIV transfusion
1 in 6.5 million donations
What is the eclipse phase?
This is where the virus replication is restricted to tissue sites e.g. lymph nodes so there is no detectable viraemia
Write about Hepatitis B
An infection which causes inflammation of the liver and can lead to cirrhosis
Most adults clear HBV within 6 months but infants can develop chronic infection
498 cases of HBV in 2018
Why do we test for HBcore
0.4% of the Irish population are core positive - we want to detect these as well
Anti-HBcore was used to identify donors in later stages of infection or in those with latent infections
Write about HBV screening
HbsAg assay as well as an anti-Hbcore assay
NAT was also introduced as it reduces the HBV window period to 20.6 days
Talk about NAT HBV screening
It takes several weeks before the viral load reaches concentrations detectable by NAT
NAT is carried out to detect HBV DNA in serologically negative donors during the early stages of infection
What is Hepatitis C (HCV)?
HCV is an infection which causes inflammation of the liver
About 25% of people with an infection will clear the virus within one year but 75% will develop chronic infection
474 cases in Ireland in 2019 -> mostly associated with IV drug use and migrants
Write about our HCV screening
CMIA to detect anti-HCV
Window period is 56 days
NAT also used for HCV RNA which reduces the window period to 4.9 days
