If you tested in a healthcare facility, the staff there
should make sure you have the necessary follow-up tests. This sequence of
confirmatory tests is carefully planned to prevent inaccurate results from
being given. Healthcare professionals call it a testing algorithm. If you have been tested in this way, with a
sequence of confirmatory tests, and you have been told that you are HIV
positive, you can be confident of the result. The rest of the information on this
page does not apply to you.
On the other hand, you might have been tested with a rapid,
point-of-care test at a location where the staff could not do the confirmatory
tests immediately. If your initial result was reactive, the staff may have
asked you to come back on another day or to go to another healthcare facility
for the follow-up testing. Or you might have taken the test yourself, using a
device for self-testing or home testing.
If that is the case, the confirmatory testing is essential.
There is a possibility that your reactive result is in fact a false positive.
The only way to know is to go and have it checked by a qualified healthcare
professional.
The accuracy of a single HIV test depends in part on
characteristics known as sensitivity and specificity. Sensitivity is an
indication of test performance when testing people who do have HIV, whereas
specificity reflects test performance in relation to people who do not have HIV. Many modern HIV testing
devices are extremely sensitive (over 99%) and extremely specific (over 99%).
There’s more information on sensitivity and specificity on another page.
The chances of having a false positive also depend on how
common HIV is in your community. False positive results are quite a rare event,
but in a community where very few people have HIV, true positive results are
even rarer. In a setting where HIV is more common, a reactive result is less
likely to be a false positive.
For example, take a test with 99.5% sensitivity and 99.5%
specificity. If this test is used in a setting where 0.2% of people have HIV
(for example, among the general population in the UK), the probability of a
reactive result being correct is 28.5%. The number of people given an incorrect
reactive result (71.5%) is greater than the number given a correct result
(28.5%).
In contrast, if the same test is used in a setting where 8%
of people have HIV (for example, among gay and bisexual men in the UK), the
probability of a reactive result being correct is 94.5%. There are far fewer
false positives (5.5%), but they still occur.
The figures describing the probability of a reactive result
being ‘true’ are known to healthcare workers as the ‘positive predictive
value’. Some more examples, all based on a test with 99.5% sensitivity and
99.5% specificity, are given below.
Prevalence of
HIV
|
Examples of
settings with this prevalence of HIV
|
Probability
that a reactive test result reflects a true HIV infection (positive
predictive value)
|
0.1%
|
General
population in Australia
|
16.6%
|
0.2%
|
General
population in the UK, Sweden or India
|
28.5%
|
1.4%
|
African
Americans in the US
|
73.8%
|
5%
|
General
population in Kenya; female sex workers in Brazil
|
91.2%
|
8%
|
Men who have
sex with men (MSM) in the UK
|
94.5%
|
18%
|
General
population in South Africa; MSM in Malawi
|
97.7%
|
30%
|
General
population in KwaZulu-Natal, South Africa; MSM in Bangkok, Thailand; female
sex workers in Kenya
|
98.8%
|
To put this in individual terms:
- If your behaviour is unlikely to put you at risk
for HIV and you live in a community in which HIV infection is quite uncommon, and
you have received a reactive result with a single test, this result should be
interpreted with caution. In these circumstances, a reactive result is much
more likely to be a ‘false positive’ than to reflect a real HIV infection.
-
If you come from a community with a higher
prevalence of HIV or if the sex you have been having puts you at greater risk
of HIV, there is a greater possibility that the result is accurate.
For these reasons, an HIV diagnosis is never made on the
basis of a single test result. All reactive (potentially positive) results are
checked with extra confirmatory tests.
The positive predictive value of a testing algorithm, using
a sequence of two or three different tests, is almost 100%. If a positive
result has been confirmed in this way, you can be confident that it is
accurate.