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Technical question on HIV combo tests
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Technical question on HIV combo tests

Hello Doctors,
Excuse my bad english.
First, thank you very much for all what you are doing.
I am a French guy established in Canada for a few months,
6 weeks and 2 days after a low risk sexual exposure, I remained very worried so I performed an HIV test "Elisa combo p24" that came back negative. I know that this result is conclusive according to your previous answers on this forum.
In my case, I first relied to the new recommendations of the French health authority that recommend the test "Elisa combo p24, with a detection sensitivity of Ag p24 lower than 50 pg/ml", and CONSIDERS a negative result 6 weeks after the exposure using this test as CONCLUSIVE.
(page 4, last section: http://www.has-sante.fr/portail/upload/docs/application/pdf/2008-10/recommandations_-_depistage_de_linfection_par_le_vih_en_france_-_modalites_de_realisation_des_tests_de_depistage_2008-10-22_11-55-8_316.pdf)

First question, I actually don't know which detection sensitivity of Ag p24 is used in the laboratory where I performed the Elisa combo p24 test, can it be higher than 50 pg/ml? I want to check if I respect the French recommendation.
- This raises another question, does all the Elisa combo p24 tests have the same reliability?
- In previous answers in this forum, you mention that starting from 4 weeks, the results are conclusive, I wonder then WHY the French authority has set this 50 pg/ml condition on the detection sensitivity?

Thank you very much, Doctors, for your answer.

Best regards.
300980_tn?1194933000
Welcome to our Forum.  Your results are conclusive and prove that you did not get HIV from your exposure of concern.  You can be sure of that.  All tests for either antibodies or for p24 antigen have what scientists call "limits of detection" meaning that at very, low concentrations the virus, antigen or antibodies (depending on what is being tested for) will not be detected.  This is what is called "analytical sensitivity" and sometimes is a source of anxiety for non-scientists.  HOWEVER, the fact is that when persons have HIV the levels of virus/antigen/antibody are so high, particularly in early infection but also throughout the course of infection, that they do detect all infections.  Your test results are accurate and should be believed.

As far as French vs. Canadian standards, again, you are OK.  Tests must be used according to the standards under which they are approved and manufactured, no matter where they are used.  Please do not worry that your test results are not accurate- they are.  yo do not have HIV.

I hope my comment is helpful. EWH
8 Comments
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Avatar_m_tn
Thank you for your answer doctor.
Minutes ago, I sent an email to the lab where I performed my Elisa combo p24 test, and they answered me that they do respect the limit of detection requirement of 50 pg/ml.
One last question doctor, do you approve the decision of the French health authority to reduce the window period to 6 weeks with the Elisa combo tests? Do you think other countries should do as well?
I am not aware of any other country that have done this so far ...
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300980_tn?1194933000
Actually, on this  Forum we tell clients that combo tests are accurate at 4 weeks.  Again, believe your test results.  EWH
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Avatar_m_tn
I have been disconnected few days, Just a final question doctor:

- Does it ever take more than 8 weeks for a laboratory antibody blood test to be positive (I mean the existing reported cases of such an event)?

I took a final blood test today (9 weeks after exposure), still waiting for my results, and then I promised myself to move on ...

Thank you very much Doctor
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300980_tn?1194933000
No, I know of no specialist who has seen it take more than 8 weeks for an infected persons to develop a positve antibody test for several years now.  Believe your test results. EWH
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Avatar_m_tn
Dear Doctor, My very last question

Thank you a lot for your answer, it provided me a lot of reassurance,
But as you know people in my case, I couldn't eliminate worries completely and they were emerging again from time to time.

To definitively turn this page of my life, I had a blood test yesterday 13weeks after my exposure, and received the result this morning, it was negative.

In many posts, you stress that we can still read the 6 months testing advice in the FDA and the CDC guidelines because they rely on outdated data obtained from older tests and because of conservatism.

Please doctor, my question is simple.

Do you agree that, even today, the people that have immune system disorder or that take drugs for cancer or HCV etc... may have delayed response with the current HIV assays (beyond 3 months)? (excluding the case of PEP)

Formulated differently, if an anonymous patient tells you today that he has a 3 months HIV negative result, would you ask him further questions about whether or not he takes some medicines, if he has cancer or HCV etc..., before deciding on the conclusiveness?

As far as my very modest knowledge goes in the subject, such people do not even develop antibodies within 3 months, so I don't understand how a more precise assay would now be capable of detecting their infection.

(i hope your answer will be also useful for the other worried readers in this forum)
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Avatar_m_tn
(for this question, I assume it is a standard antibody test not a combo one that looks also for the virus itself)
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300980_tn?1194933000
You are having trouble accepting that you did not get HIV despite the evidence that you did not get HIV.  As I've said before, there is no evidence that you have acquired HIV and no reason to doubt the results of the test.  The fact is that even most people who could be immunosppressed, either form another disease or from medications, would still ahve a positive result by now.  Only persons who are exceedingly sick would even have a possiblity of a negative result.  You need to beleive you test results.

I'll have nothing mroe to say.  EWH  
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