Độ chính xác xét nghiệm ( Test accuracy, results and further testing )
Test accuracy, results and further testing
Are HIV tests accurate?
Yes. Modern HIV tests are very accurate.
This accuracy has to be considered with the window period.
For example, 4th generation tests will pick up 95% of infections at 28 days after exposure.
A confirmatory test three months after the exposure is always recommended. This is because 5% of people take this long to show a positive result.
A positive test result is routinely confirmed using a different type of test called western blot. The western blot test looks for immune responses to specific HIV proteins and is 100% accurate.
Can anything affect the result of my HIV test?
HIV antibody tests are not affected by other circumstances.
This includes infections, medications, most vaccinations, putting on weight, eating or drinking anything before the test, use of alcohol or recreational drugs, mouthwash or time of day.
Your test result is accurate even if you had flu or a cold or are using any medication.
You do not need to fast before your test. Food and drink do not affect the results.
Do I need to take another test?
This will depend on how recent your last exposure was.
As part of good practice, if the exposure was less than three months ago then testing at three months after exposure is usually recommended.
Can it take longer than three months for a test to work?
This is so unlikely that UK guidelines consider a negative result three months after an exposure as being HIV negative.
Is a negative test 100% accurate?
HIV tests after the 3 month window are more than 99.97% accurate. They work for all types and subtypes of HIV.
Very few medical tests have 100% accuracy. There will still be rare cases where someone is HIV positive and not picked up.
However, HIV tests are one of the more accurate tests for any medical infection. Tests showing a negative result are ‘interpreted’ as negative.
If the result is negative three months after exposure your result is interpreted as negative. This assumes you have had no further risks.
At this point you can stop worrying. This is the purpose of testing. Learn from the experience you have gone through in taking a test.
Learn about how to protect yourself in the future so you don’t have to go through this stress again. This will enable you to make informed decisions and to look after your sexual health.
If the result is negative four weeks after the exposure, this tells you that you are very likely to be HIV negative. The test after three months will confirm this.
What is a ‘false negative’ test result?
A false negative test result occurs when the test shows negative and the person is really HIV positive.
This is very rare and usually occurs during the window period when people are newly infected but the test can’t quite pick up the infection.
As with other types of tests, there is always be a small margin of error. With antibody-only tests (3rd generation) only 0.3% of tests (3 tests in every 1000) will be a false negative after 3 months.
With 4th generation tests this is even lower. In practice, a negative result after three months means you do not have HIV.
You do not need to test again unless you expose yourself further in the future.
What is a ‘false positive’ test result?
A false positive test is when the test result shows positive but the person is really negative. This can happen with antibody tests when the test picks up antibodies for other infectious agents.
Approximately 1.5% (15 out of every 1000) antibody tests are a false positive. The fourth generation tests have a much lower chance of a false positive.
This means that a small percentage of people who test positive on a rapid test (where the results are given within an hour) may turn out to be HIV negative.
A second blood sample will be tested in a lab to look for this.
If your blood test was originally performed in a laboratory, a positive result would have already been confirmed before giving you this result.
All positive laboratory tests in the UK are routinely confirmed using a second type of test called western blot that is 100% accurate.
1 February 2013
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