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5.1 What is the window period?
The window period is the period between the onset of the infection (e.g. time of transfusion or injection) and the appearance of the detectable infectious agent or antibodies to the viral agent.
During the window period there is an eclipse phase when the donation is not infective. At the end of the eclipse, just before the NAT or viral marker test becomes positive, the donation may be infectious but the risk is extremely low.
5.2 What serological tests are performed for detection of HBV, HCV, and HIV contamination of blood donation?
Blood donors are screened for
- HBsAg
- Anti-HCV
- Anti-HIV 1 & 2
In addition to these tests blood donors are screened by NAT for
- HCV RNA
- HIV RNA
- HBV DNA
NAT is performed in a single multiplex assay
5.3 Why was nucleic acid test (NAT) introduced?
Introduction of NAT further reduced the risk of transmission of infectious agents through blood transfusions. This is because viral nucleic acid appears first in the blood after infection before the viral antigen markers or the host antibody markers appear.
5.4 Which blood component is more likely to transmit bacterial infections and why?
Platelets components are more likely to transmit bacterial infections because platelets are stored at room temperature i.e. 22°C rather than in refrigerated conditions.
5.5 During what time period is the test required for malaria?
ELISA tests are used to screen donors for malaria antibodies between 6 and 12 months after return from a malarial endemic area.
This provided time for the donor to form antibodies if exposure has occurred. Donors without antibodies can be reinstated as donors.
5.6 Why are tests for antibodies to malaria and Chagas performed on selected donors?
These parasites are found predominantly in the tropical and subtropical regions of the world. For other countries/regions donors are selected for testing based on their travel history. The concern is that travelers returning from these tropical areas may have been infected and therefore could pose a TTI risk.
5.7 What is the causative agent of vCJD and how is it transmitted to humans?
Variant CJD is caused by an abnormal prion protein which was transmitted through contaminated meat products and has resulted in over 200 deaths worldwide, mostly in the UK. The abnormal prion is also transmitted through blood transfusion.
5.8 Why are internal and external controls used?
Each batch of tests includes the manufacturer’s internal positive and negative controls. Correct results for these controls shows that the sample test results are valid. s
In addition, an external run control {such as the British Working Standard (BWS)} is included in each batch of tests to demonstrate that an acceptable level of sensitivity is achieved. By ‘sensitivity’ we mean that the test is able to detect any infected sample down to the lowest limits permitted by the test system.
5.9 Why is pre-acceptance testing performed?
Pre-acceptance testing is performed to show the batch of test kits/reagents received meets pre-defined criteria such as sensitivity and specificity and has not deteriorated during transportation.