Testing for p24 antigen can help detect HIV earlier than tests that only test for HIV antibodies which shortens the window period. Third-generation HIV tests are in an ELISA format (enzyme-linked immune-absorbent assay) and screen for HIV antibodies and not the virus itself.įourth-generation tests, introduced in the late 1990s, are ELISAs which test for both HIV antibodies and p24 antigen. Most HIV antibody tests are in an ELISA or EIA format (enzyme-linked immunosorbent assay). What is the difference between generations of HIV tests? Like SH:24’s HIV test, POCTs are also screening tests and give a ‘reactive’ result which needs to be confirmed with a blood test sent to the laboratory. The main difference is that with a POCT you get the result immediately (within 20 minutes depending on which test is used). Different clinics will use different tests. Some of them just test for HIV 1 & 2 antibody and one tests for both HIV antibody and p24 antigen. There are different types of point of care test (POCT).
The sample has to be sent off to the laboratory for testing and you receive your result by text in a few days The HIV test that SH:24 uses tests for HIV 1 & 2 and looks for both p24 antigen (part of the virus) and antibody (what the body’s immune system makes to fight the virus). What is the difference between a POCT and an HIV test on SH:24?
Of these, 55 (less than 1% of all HIV tests processed) had a reactive result. In the first six months of 2016 – SH:24 processed 7,297 HIV tests.
How common are reactive results and how many of these are false positives?įalse positives are rare. This is because the test is very sensitive and we don’t expect any false negatives as long as the test has been done after the window period of the test (7 weeks). 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.Īn HIV-negative result can be confidently given on the small blood sample we send to the lab. So a ‘reactive result’ may be a true positive – but it may also be a ‘false positive’. 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). This means it looks like HIV is present but it has not been confirmed. If it is positive a second time the test is called ‘reactive’. If this is positive, they repeat the screening test. When the sample reaches the laboratory they do a screening test to look for the presence of HIV antibodies.
The micro-container is the name of the little bottle you collect your blood sample in. In order to make testing at home as easy as possible, SH:24 uses a finger prick test and collects a small blood sample in the micro-container.
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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?