Questions and Answers with NAM and Michael Carter, editor, Aids Treatment Update
Late last year I heard about a man who appeared to be HIV positive
but then had blood tests showing he
wasn’t. Could my HIV positive test result be wrong?
The short answer is almost certainly no. At this stage we don’t really
understand what occurred with the man that appeared to be ‘cured’
of HIV, although it has been reported he had a so-called false-positive test.
Hopefully, further tests will provide more details.
How can you be completely sure my HIV test was accurate?
You’ll have had a number of tests to check if you have HIV and how it
is affecting you. First, there’s the test that looks for antibodies
that the body produces in response to infection with HIV. You’ll have
had other tests to detect and measure HIV’s activity in your body, such
as viral load and CD4 cell count tests.
Tell me more about the antibody test.
Antibodies to HIV normally appear a few weeks after you are infected with
the virus. Two tests are used to look for antibodies: the ELISA (enzyme-linked
immunosorbent assay) and the Western Blot test.
Can you explain the difference?
Your blood will normally have been tested first using the ELISA test. It’s
quite cheap, and also has the advantage of being very sensitive. Provided
your test is performed long enough after your exposure to HIV for antibodies
to appear (for almost everybody, this is three months), the ELISA test will
almost certainly detect them. Two terms are used to describe the accuracy
of tests: ‘sensitivity’ and ‘specificity’. The ELISA
test is very sensitive (99.5 per cent) meaning it will detect even very small
quantities of HIV antibodies. But, because the ELISA test is so sensitive,
it has less specificity which means there’s a chance that it could produce
a small number of false-positive results. Because of this a second type of
HIV test, the Western Blot, is performed on all samples that test positive
for HIV antibodies using the ELISA test. The Western Blot test confirms if
a sample is positive or negative. Very rarely, a result comes back ‘indeterminate’,
meaning a person has just started to develop antibodies to HIV because it
is so close to the time when they were infected. If this happens, the person
will need to be re-tested outside the three-month window period it can take
for HIV antibodies to develop. False-positive tests are exceedingly rare after
a Western Blot test.
But false-positive results can occur?
Yes. Very, very rarely. And the reasons usually have nothing to do with the
accuracy of HIV tests themselves. Rather, they are normally because of errors
when labelling the blood, a mistake at the lab where the samples were tested,
or contamination of the testing equipment.
What about other tests to check if I have HIV?
You should have your viral load and CD4 cell count checked regularly. Although
these aren’t routinely used to diagnose HIV, they can measure how HIV
is affecting your body.
The man whose HIV antibody status changed also seems to have had a change
in his viral load. This could have been due to natural fluctuations and if
he was infected with HIV, it may simply have dipped below the limit of detection
of the tests in
routine use.
So how did this man go from being HIV positive to negative, if that
is actually what happened?
It’s not yet known. Further tests are necessary to show what really
happened. It’s worth pointing out that the news reports were based on
a letter written by an NHS legal department. Before the true
significance of this case can be determined, the initial tests as well as
further tests will have to be looked at by doctors and researchers. These
will then be scrutinised by other doctors and will almost certainly be published
in a medical journal. It is only then that the full significance, if any,
of this man’s case will be understood.
• NAM provides information to help you think about your use of HIV treatments,
but this is not intended to replace discussion with your doctor.
