FAQs about testing for HIV
The definitions below may be useful when reading these FAQs about HIV:
- HIV - Human Immunodeficiency Virus
- Antibody -something your body produces to fight an infection
- Antigen - a part of the virus or bacteria causing an infection that a test may detect
- Point of Care Test (POCT) - a test that is done by a health care professional with the patient present that gives an instant result
- Micro-container - the container that SH:24 uses to collect blood samples to send to the laboratory
- Screening test - a test that looks for the likelihood of an infection or disease being present but is not a definite diagnosis – other tests are needed to confirm the disease or infection is present
- Diagnostic test - a test or group of tests that are used to confirm the presence of a disease or infection (often done after a screening test)
- Reactive - a term used for a screening test that means the test has suggested the presence of infection but needs to be confirmed
- Positive - a term that is used with more confidence when a test or group of tests confirms the presence of a disease or infection
- Sensitivity - a measure of how accurately a test can confirm an infection is present (a very sensitive test would have very low or no false negatives)
- Specificity - a measure of how accurately a test can exclude that an infection is present (a very specific test would have very low or no false positives)
- Window period - the time it takes from someone catching an infection to when it can be confidently detected by a test
What does ‘reactive’ mean, and why do I need to have another test?
In order to make testing at home as easy as possible, we use a finger prick test and collect a small blood sample in the micro-container. The micro-container is the name of the little bottle you collect your blood sample in. When the sample reaches the laboratory they do a screening test to look for the presence of HIV antibodies. If this is positive, they repeat the screening test. If it is positive a second time the test is called ‘reactive’. This means it looks like HIV is present but it has not been confirmed.
It is not possible to confirm a positive result on the micro-container sample – this is because of the sensitivity and specificity of the test (see definitions above). So a ‘reactive result’ may be a true positive – but it may also be a ‘false positive’. This is why all ‘reactive’ test results need to be referred into clinic for further blood tests before an HIV positive diagnosis can be made.
An HIV-negative result can be confidently given on the small blood sample we send to the lab. This is because the test is very sensitive and we don’t expect any false negatives as long as the test has been done within the window period of the test (4 weeks).
Could I get a false positive HIV result because I have another condition?
False positive results are rare and occur in less than 0.3% of cases.
A false positive can be the result of a recent flu vaccination, current or recent viral infections and sometimes multiple pregnancies. More rarely, false positive results have been reported as a result of a small number of other medical conditions and vaccines, though such results are unusual. Most commonly there is no obvious reason for a false positive and they often happen in healthy people. This may be because other normal antibodies that are present in the blood ‘cross react’ or ‘confuse’ the test.
Why do I have to have another test to confirm an HIV reactive result?
Nobody should be told they are HIV positive based on the result of a single test – this is why we call the HIV result ‘reactive’. Following a reactive result, you will be referred into clinic where you will be offered a further screening test which is a blood test taken from your arm with a syringe – this allows the clinic to get a larger sample of blood which they can send to the laboratory to run a sequence of tests for confirmatory testing.
The principle of this multiple testing strategy is that the screening test used should have a very high sensitivity (i.e. you don’t expect any false negatives). In other words, they will detect all people who are truly HIV negative. But the test may also incorrectly report as positive a small number of people who are not infected at all i.e. a small false positive rate – this is because the test is not 100% specific. This is why we refer all reactives in for confirmatory testing.
What are the differences between HIV 1 and 2?
There are two major types of the human immunodeficiency virus. HIV-1, which was discovered first, is the most widespread type worldwide. HIV-2 is most common in people of west African origin. HIV-2 is very rare in the UK – with just a few hundred people infected with this strain (<1%). Both viruses, if untreated, will result in AIDS and the infection is contracted in the same way.
