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how conclusive is a negative result

Question: 

what kind of checks are done to see that the accuracy of the rapid and EIA lab tests remain at about 99.6% sensitivity for true negatives? Outside the window period. And for how many people has this sensitivity ever been checked, how often?

Of course the more regularly one tests, the more certain one is of one's negative status, just by probability. But 99.6% on one test from the last encounter is still a chance of 4/1000 false negatives right? Or not?

No diagnostic screening test is clinically always 100% I have studied, so what is meant by the term "conclusive" after the window period.

Answer: 

Hello,

Thank you for contacting AIDS Vancouver. I see you have some concerns about the accuracy of some common HIV tests, I can provide you with some more information about testing and how results are confirmed to help reassure you. It may be helpful to think about why you are concerned about a false negative test result given the very high accuracy of these tests. Is there a reason why a negative test result is hard to accept? Perhaps this is something you could discuss with a friend, or your healthcare practitioner.

The term window period is used because this is when although tests are still very accurate there is a higher chance of not detecting new infections. Three months is said to be conclusive since at this time the number of false negative results is so small that a negative test result is considered reflective of your true HIV status. You are right, no test is 100% able to detect infections but the likelihood of an un-diagnosed case of HIV is extremely unlikely. If you are concerned, we recommend regular testing, every couple of months, for all sexually active individuals.

Sensitivity measures how well the test can correctly identify that someone with HIV has HIV. Taking the test more times is not necessarily the best way to be certain of your negative status. Since the third generation tests (rapid tests and the ELISA) measure the amount of antibodies in your blood they work better after 4-6 weeks. This is because your body produces antibodies in response to the virus, so at first there are not very many copies of the virus, so not very many antibodies specific to HIV in your blood. So waiting longer then taking a test is more informative than taking several tests shortly after your exposure.

You've asked how the tests' specificity and sensitivity are checked, and in how many people. These two measurements are characteristics of the test, they do not depend on the population that is being tested. So once they have been measured they do not vary among different groups, and they would not change over time for detecting the strains of HIV they were designed to detect.

Here is a link to a site with detailed information on the different types of HIV tests available and more explanations about test accuracy.

I hope this information has clarified things for you, and that you can feel confident in your test results.

Best regards,

Alex

AIDS Vancouver Online Helpline
helpline.aidsvancouver.org
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