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Good for Them, Not for Us???

Dec 2, 2001

I understand that blood donors are tested very thouroughly for blood born diseases such as HIV; Nat, P24 Antigen, and Antibody tests used together reduces the window period by 50. Stats show that if a person seroconverted by 22 days, the blood screening would pick it up by 11 days...12 weeks down to 6 and so on. Now, I realize that it would not be good for us to rely on blood donation to be tested-cost effectiveness as well as moral reasons; however, wouldn't it be reasonable to offer such sensitve testing to the rest of us who are pulling our hair out waiting for that "window period"? If it is good enough to protect the blood supply-why isn't it good enough for us?

Response from Mr. Kull

P24 antigen testing is primarily used to screen the nation's blood supply. The p24 antigen test detects the presence of a protein that is found on the outer shell of HIV (the "p" standing for protein, and "24" standing for its molecular weight). P24 is referred to as an antigen because it provokes the body's immune response, causing it to generate antibodies. Since HIV is present in the body in very high levels before antibodies are produced, the p24 test may be useful in detecting infection when antibody tests cannot; at its best, the p24 test probably reduces the window period by about a week. This is why the test is used to diagnose acute infection, infected blood donations, and newborns.

The problem with p24 tests is that they are insensitive when levels of p24 antigen are low; this is true for the middle stage of infection. Levels of virus and antigen in blood will vary, while levels of antibodies, once detectable, will remain detectable. Levels of antigen are highest during early and late infection. One study found that the antigen tests could not identify 75% of infected seronegative blood donors.

NAT (nucleic acid testing) is not indicated for diagnosing HIV infection and is primarily used for monitoring viral load. Tests used to determine viral load have a higher rate of false positives than antibody or antigen tests. This is clearly not desirable in a clinical setting.

If you think you are at recent and significant risk for infection, you should talk with your medical provider about the use of the p24 and other testing technologies for early detection. Otherwise, the antibody tests remain the most reliable tool for diagnosing infection.


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