[Question #7927] Testing Window
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51 months ago
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Dear Doctor,
I had anal sex (protected receptive) 5 weeks ago with a TS CSW. I did not check to see if the condom remained intact although intercourse lasted less than 2 min. Due to my anxiety, I got tested twice. The first test at 23.75 days after the encounter, and the second test on 27.75 days after encounter. Both tests were performed on the Abbott Architect ci 41000 module. I wanted to ask the following questions please:
1. How reliable are both results given that CDC says testing window is 18-45 days for these tests.
2. Is it possible that the lab only performed the antibody test on the Abbott Architect and did not conduct the antigen test?
3. Do I need to re test at 6 weeks or should I consider this conclusive?
I would like to add that I have not had any symptoms suggesting ARS. Although I did develop some pain in my knees but it is most probably because I was went for a jog on a flat surface.
Thanks in advance.
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51 months ago
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Just want to add that both results were negative with index values of 0.07 on the first test and 0.08 on the second test. (S/CO was 1 as per the lab report). Does it matter for these reference values are same or different form each other.
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H. Hunter Handsfield, MD
51 months ago
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Welcome to the forum. Thanks for your confidence in our services.
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Directly to your questions. The Abbott test you had is one of several antigen-antibody (AgAb) HIV blood tests, formerly clled "4th generation" tests.)
1,3) We on this forum use exactly the same data as CDC does on test performance of the AgAb HIV blood tests and cite the same sources. However, you might slightly misunderstand the meaning of the 18-45 day window. It means that the test may sometimes be positive as early as 18 days, but 45 days is required for 100% certainty that a negative result is valid. (On the forum, we usually cite 6 weeks, i.e. 42 days, just to round it off: the test probably is equally reliable at 42 as at 45 d.) Your negative result at 28 days is around 95% reliable. (The intervals aren't nearly precise enough to quibble about 27.75 vs 28 dats.) For a 100% conclusive result, have a final test at least 6 weeks (42-45 days) after the event.
2) The test is highly automated and includes simultaneous testing for Ag and antibody Ab; they are not separate procedures, and it isn't possible to run the test without both being measured.
Your knee pain doesn't suggest HIV. The numerical values of your test mean nothing. Any number below the cut-off (usually 0.90) is equally negative. In fact, the very same specimen tested 10 times would give 10 different numbers, which might vary much more than your two results did -- e.g. if you had two results of, say, 0.08 and 0.75, both results would be totally negative.)
The chance you have HIV is extremely low. Your current tests are 95% certain -- and with the probably intact condom, the exposure itself carried well under one chance in several thousand that you were infected. So considering both that fact and the test result, the chance you have HIV probably is under one in a million. So you definitely can expect another negative result if you have a final test at 6 weeks.
I hope these comments are helpful. Let me know if anything isn't clear.
HHH, MD
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50 months ago
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Hi Doctor, thank you for the extensive reply. I have gotten another 4th Gen Ag/Abs test from another lab using the Abbott Architect ci8200. This test was done at 36 days and results are negative. However, i consulted a doctor and he told me that these tests are useless until 6 months. He has recommended that I get a quantitative PCR test for 100% certainty. I have the following questions please:
1. How reliable is my 36 days result given that it was from another lab? (the index value was the same as previous tests so does it mean that machine is not calibrated as you have said that index numbers are always different?)
2. Should I get tested again at 42 days or is 36 days equally good? (should it be 42 full days or test on the 42nd day?)
3. Do I really need to get PCR test for 100% certainty or should I just rely on the 4th gen test?
Thanks in advance!
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H. Hunter Handsfield, MD
50 months ago
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The doctor you cite doesn't understand the current HIV tests, or you misunderstood. It is absolute nonsense that the tests are "useless" until 6 months. And we stand by the advice on this forum -- same as CDC's -- that 6 weeks is conclusive with the AgAb tests. Your doctor is stuck in information that became out of date at least 10 years ago. A PCR test would indeed confirm you don't have HIV, but is unnecessary.
1. The tests are done identically in all labs - there is little if any difference in test reliability from one lab to the next. Don't over-interpret such trivial nonsense as the index value being the same as before. A negative result at 36 days is around 99% reliable.
2. Based on the exposure plus test reliability at 36 days, there is under 1 chance in a million you have HIV. If I were in your situation, I would accept that as zero for all practical purposes and would not have another test. However, I suspect you're not going to relax until you have had a 100% conclusive test result. If so, have another test at 42+ days. It's up to you.
3. No, you do not need PCR for 100% proof you don't have HIV.
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50 months ago
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Thank you for the extensive reply doctor. Just wanted ask last few questions:
1. As I am in a 3rd world country, do you think that there is a greater chance of lab staff errors or machinery errors as compared to USA or UK or are the tests equally reliable. Would a machine report wrong results if it isn’t calibrated?
2. Have you or your colleagues ever had a negative test at 5 weeks change later on?
Thanks in advance!
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H. Hunter Handsfield, MD
50 months ago
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1. I wouldn't be at all worred about lab errors. These tests are highly automated and the technology virtually foolproof. The results are equally reliable regardless of country or region.
2. No, I've never seen or even heard rumors about any patients with negative results at 5 weeks who later had positive results.
That completes the two follow-up exchanges included with each question and so ends this thread. I hope the discussion has been helpful.
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