- Can you describe the screening tests and technologies used to detect TTIs in donated blood, and how they differ from one another in terms of sensitivity and specificity?
Ans: Screening tests for Transfusion-Transmissible Infections (TTIs) in donated blood typically include ELISA, nucleic acid testing (NAT) and antibodies screening methods. Serological tests detect antibodies or antigens produced by the infectious agent, while NAT directly detects the genetic material of the pathogen. Sensitivity and specificity vary depending on the specific tests used, but NAT generally has higher sensitivity and specificity compared to serological tests. For instance, an HIV antibody test may have a sensitivity of around 99%, meaning it can accurately detect 99% of HIV infections, but it may not detect very recent infections during the window period. On the other hand, nucleic acid testing (NAT) directly detects the genetic material of the pathogen and is highly sensitive and specific, making it effective at detecting infections even in the early stages. These screening tests have a broad reactive spectrum, are simple to perform and can be automated for handling large numbers of samples at a time.
- What is the recommended window period for different TTIs, and how do you account for this in your screening protocols?
Ans: The recommended window period, the time between infection and when a test can reliably detect the infection, varies for different TTIs. For HIV, it’s typically around 9-11 days for NAT and 22-25 days for serological tests. For hepatitis B, it’s about 60-180 days and for HCV, it is about 45-150 days respectively. Screening protocols account for these differences by using a combination of tests, such as initially performing NAT to detect early infections and following up with serological tests to cover a broader time range. This multi-test approach reduces the risk of false negatives during the window period and ensures the safety of the donated blood supply.
- Are nucleic acid testing (NAT) methods used in addition to serological testing for TTIs, and if so, which TTIs are screened using NAT?
Ans: Nucleic acid testing (NAT) methods are indeed used in addition to serological testing for certain TTIs, especially for HIV, hepatitis B, and hepatitis C. NAT is highly sensitive and can detect the genetic material of these viruses within a few days to weeks of infection, significantly shortening the window period compared to serological tests. For other TTIs like syphilis and human T-cell lymphotropic virus (HTLV), which do not have NAT assays routinely used in blood screening, serological tests are the primary method of detection. The choice of testing method depends on the specific TTI and the availability of suitable screening assays.
