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Diagnosing HIV – How Does It Work?

The BSI is supporting World AIDS Day – this  is an opportunity for people worldwide to unite in the fight against HIV.  Many of our members are involved in research on the HIV virus, finding out more about how it affects immune cells and trying to find new treatments to combat this virus.  To show our support, in this blog post we take a look at the tests currently used to diagnose HIV, how they work and how you can access them.

HIV budding colour

Public Health England’s (PHE) announcement in 2014 that there were around 110,000 people in the UK living with HIV meant that the prevalence of the virus had reached an all-time high within the country. This included an estimated 26,100 people who were undiagnosed and therefore at risk of unknowingly passing the HIV virus on to others.

In the year since these statistics were published, there have been growing calls from individuals and organisations for increases in both the number and frequency of HIV diagnostic tests. So what has changed? And to what extent are these essential resources accessible to everyone?

Diagnosing HIV – how does it work?

Amongst the striking statistics on HIV in the UK, PHE also highlighted that fact that people who are diagnosed early with HIV have significantly better survival rates in the first year after diagnosis compared to people with a late diagnosis. This reminder of the importance of early HIV testing was a response to the lower numbers of people receiving an HIV test in 2014 (compared to 2013), and highlighted one of the major obstacles in lowering HIV incidence and prevalence.

The most commonly used HIV testing method in the UK is known as fourth-generation testing, although third-generation testing and rapid (point-of-care) testing are still in use by some places. The principle of each test revolves around the ability to identify HIV indirectly through looking at whether a person has any antibodies present against the HIV virus.

The reason for this is that, once the body has been exposed to the HIV virus, the immune system reacts by producing antibodies against a variety of specific antigens (i.e. proteins) present on the surface of the virus. Usually by one to two weeks after initial exposure to the virus, the antibodies produced against the various HIV antigens begin to increase in concentration within the blood and become more easily detectable.

Third-generation testing for HIV

For third-generation testing, the laboratory examines a person’s blood sample and uses a technique called Enzyme Linked Immunosorbent Assay (ELISA) to detect whether these HIV-specific antibodies are present. Whilst these tests are extremely accurate, they are unable to detect the early period of HIV infection due to the low concentrations of anti-HIV antibodies during this initial stage. Therefore it is now recommended that fourth-generation testing is used when testing for HIV.

Fourth-generation testing for HIV

The difference for fourth-generation testing is that it not only tests a person’s blood sample for HIV antibodies, but it also test for a specific protein. This protein, known as p24, is present on the outer shell of the HIV virus and can be detected by using another type of ELISA that is specific for p24. This type of test is the recommended routine test in the UK due to its ability to accurately detect the HIV virus even during the early stages of its infection.

Rapid (point-of-care) testing

World Aids Day red ribbon

There may be certain cases where the results of an HIV test are needed within a short time period, and it is in these circumstances that a rapid (point-of-care) test is a viable option. These tests are designed to be conducted away from a specialised laboratory and provide results in under 30 minutes. While these tests can be performed in a doctor’s office, they are also designed to be used as a method of self-testing at home.

These tests require a small droplet of blood for the device to detect the presence of HIV antibodies. However this rapid test cannot detect an HIV infection in its early period of infection. This means it is important that any positive results obtained with a rapid test should be followed up with confirmative laboratory tests.

How to get tested

Over recent years HIV testing has become easier and more accessible in the UK than ever before. HIV testing services are provided by the NHS and a number of other healthcare facilities including:

  • Sexual health clinics or genitourinary medicine (GUM) clinics at hospitals (locations of these clinics can be found here)
  • Your GP surgery – ask a doctor or practice nurse about the availability of HIV testing there
  • Fastest clinics – these are rapid testing clinics run by the HIV charity The Terrence Higgins Trust
  • Private clinics
  • Local pharmacies (if you are an injecting drug user)
  • Antenatal clinics (if you are pregnant)

HIV tests in a clinical setting usually consist of obtaining a small blood sample that is then sent to a specialist laboratory for testing. The time it takes to obtain the results of the test will vary, with the outcome either being ‘negative’ meaning that there are no signs of infection, or ‘positive’ in which the HIV virus has been found in the blood. All positive results from an HIV test should be confirmed with a follow-up blood test conducted by a medical professional.

Self-testing kits for HIV provide the ability for a person to test themselves at home. For National HIV Testing Week this year, PHE have made such self-test kits free and deliverable across the UK.

Mark Roberts

Mark is currently working as an intern for the British Society for Immunology having recently received his MSc in Immunology of Infectious Diseases from the London School of Hygiene & Tropical Medicine.

Image credits: C. Goldsmith/CDC; “Red Ribbon” by Gary van der Merwe, graphics by Niki K Aids Awareness Pin Lapels. Licensed under CC BY-SA 3.0