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HIV-testing

Dear Doctor,

First of all, thank you for such a great site providing us reliable and professional information. Now, I have some concerns and a couple of questions about HIV-testing. To say the truth; some doubts about HIV-testing. As everybody knows, HIV is a retrovirus which mutates at a very high rate creating many different versions / strains of the virus and thus, having a very high genetic variability.

1-) HIV tests (All types of Antibody testing, PCR testing, Antigen testing etc.) are based on the known strains (known subtypes or sub-subtypes or CRF's) of HIV virus. Even for the known strains of HIV, the sensitivity of these tests are almost never 100%. They are mostly in the range of 99% - 100%. Anyway, no lab testing has 100% accuracy. But, if we come back to the new / unknown strains issue: I don't think that HIV-tests are being improved as fast as the mutation of the HIV virus which will cause a considerably high rate of false negatives. They can not be though, because the new strains are not yet discovered by the scientists. So, how can the manufacturers of these tests claim that the sensitivity of their tests are above 99%? More importantly, how can we make sure that we are HIV-negative after getting a negative result from these tests?

2-) If the new and unknown strains of HIV are not detectable by the present tests, how are the new strains discovered then? If a person is infected with a new strain of HIV which can not be detected by the current tests, then he/she is sent to home rest assured that he/she is HIV-negative and nobody will further investigate whether this person is really negative or is infected with a strain which is not detectable with the current tests. In this case, how is this new strain discovered at some day so that we see in the news that a new HIV-strain has been discovered?

Thank you very much for your time.

Kind regards,
Sam.
1 Responses
239123 tn?1267651214
MEDICAL PROFESSIONAL
Welcome to the forum.  I'll try to help.

You have learned a lot about these issue and your questions are thoughtful.  But I think you failed to understand some basic aspects of HIV evolution and diagnosis.

The genetic and antigenic differences between strains are great in one sense, but the core genes and the proteins for which they code are in fact quite stable.  There is no material difference across strains for p24, gp120, or gp41, the main antigens used as the basis for the routine HIV antibody tests.  These determinants were chosen as the basis for testing in part because they are very consistent across strains -- or if there are differences, they are not sufficient to materially influence their ability to bind to antibodies directed at them, which is the biochemical basis for antibody testing.

To the extent that evolutionary changes of such a degree that the current tests didn't work might someday evolve, it has not yet happened to any significant degree -- and to my knowledge has never been found to have happened in industrialized countries or anywhere in the world except where HIV originally evolved (and continues to evolve), i.e. parts of tropical Africa.  Ongoing surveillance in those areas almost certainly will be successful in identifying such HPV types and strains long before they are likely to be imported to other parts of the world.

Finally, in contrast to the assumption behind question 2, it isn't difficult to detect new strains when they occur.  As already suggested, the standard antibody tests can be expected to detect such infections.  Second, when there is serious clinical suspicion of HIV, both antigen detection tests and assays for HIV RNA/DNA in blood are done, and these are largely independent of strain differences.

For all these reasons, people concerned about HIV risks and the reliability of the current tests should have absolutely no worries.  Follow safe sex guidelines and, if the need for testing arises, trust the tests.  They work just fine.

I hope this helps.  Best wishes--  HHH, MD
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