🔬 HIV Testing

HIV testing in the UK primarily utilises 4th generation combined antigen/antibody assays. These tests are highly accurate and can detect infection earlier than older antibody-only tests. Testing is guided by clinical indicators, high-risk group status, and the move towards routine 'opt-out' screening in many clinical settings to reduce the number of undiagnosed individuals living with HIV.

HIV testing should be offered to anyone with clinical features of immunodeficiency or 'indicator' conditions (e.g., unexplained weight loss, chronic diarrhoea, shingles, glandular fever-like illness, or TB). It is also indicated for sexual health screens, antenatal screening, needle-stick injuries, and for patients from high-prevalence areas or groups (MSM, IVDU). Opt-out testing is standard in many UK Emergency Departments and GUM clinics.

In the UK, the standard screening test is a 4th generation laboratory-based venous blood test (yellow top). This assay simultaneously detects HIV-1/2 antibodies and the p24 antigen (a protein component of the virus). Rapid Point-of-Care Tests (POCT) are also available (finger-prick or saliva) for faster results, but these only detect antibodies and have a longer window period.

A normal finding is a 'Non-reactive' or 'Negative' result. This means that neither HIV antibodies nor the p24 antigen were detected in the sample. In the absence of high-risk exposure within the window period, this indicates the patient does not have HIV.

A negative 4th generation test result 4 weeks after a potential exposure is highly reassuring, but a repeat test at 12 weeks (the 'window period') is required to definitively rule out infection. A 'reactive' result on a screening test is not a final diagnosis until confirmed by a more specific secondary test (e.g., Western Blot or specific antibody differentiation) performed by a reference lab.

A 'reactive' or 'positive' result indicates the presence of HIV antibodies or the p24 antigen. All reactive screening results must be confirmed with a second, different assay on a new blood sample. A positive result indicates HIV infection, requiring immediate referral to specialist HIV services for staging (CD4 count, viral load) and initiation of antiretroviral therapy (ART).

Early diagnosis of HIV is critical; patients diagnosed late (CD4 <350) have a significantly higher morbidity and mortality. With modern ART, HIV is a manageable chronic condition with a near-normal life expectancy. Furthermore, 'Undetectable = Untransmittable' (U=U), meaning effective treatment prevents transmission to partners. HIV testing is now a routine part of many medical admissions in high-prevalence areas in the UK.

The biggest pitfall is failing to offer the test due to 'perceived' low risk (stigma), leading to late diagnoses. Another is misinterpreting a 'reactive' screen as a definitive diagnosis before the confirmatory test is back. Lastly, failing to counsel the patient about the window period can lead to a false sense of security after a negative test following a very recent exposure.

The main limitation is the 'window period'—the time between infection and the test being able to detect the virus. Fourth-generation tests shorten this to about 4 weeks, but earlier testing may yield a false negative. False positives can occur (though rare) due to other viral infections or recent vaccinations, which is why confirmatory testing is mandatory.

Key for MLA: Always obtain 'informed consent' through a verbal discussion (no written consent required). Be able to list HIV indicator conditions (e.g. Seborrhoeic dermatitis, oral candidiasis). Understand the 'window period'. Recognise that in the UK, the aim is '95-95-95' (95% diagnosed, 95% on treatment, 95% virally suppressed).