HIV Testing and Prevention

Introduction

HIV (human immunodeficiency virus) is a retrovirus that infects and progressively depletes CD4+ T lymphocytes, leading, without treatment, to acquired immunodeficiency syndrome (AIDS) and susceptibility to opportunistic infections and malignancies.¹ With modern antiretroviral therapy (ART), HIV has become a manageable chronic condition: a person diagnosed early, who starts and adheres to treatment, has a near-normal life expectancy.

The UKMLA focus is less on managing established HIV disease - which sits with specialist services - and more on recognising who to test, testing appropriately, and knowing the tools (PEP, PrEP, U=U, antenatal pathways) that prevent transmission. Late diagnosis remains the single biggest driver of HIV-related morbidity and mortality in the UK, which is why lowering the threshold for testing matters so much.²

Transmission

HIV is transmitted through contact with infected blood, semen, vaginal fluid or breast milk, principally via condomless vaginal or anal sex, sharing injecting equipment, and mother-to-child transmission during pregnancy, delivery or breastfeeding. Receptive anal sex carries the highest per-act risk of any sexual practice.¹ Casual contact, saliva, sweat and insect bites do not transmit HIV.

Certain co-factors increase transmission risk, including the presence of another STI (particularly ulcerative disease), a high viral load in the infected partner (especially during untreated primary infection), and lack of circumcision in the HIV-negative male partner.

Natural history

Primary (acute) HIV infection

Around 2-4 weeks after acquisition, most people develop a seroconversion illness: fever, sore throat, malaise, myalgia, lymphadenopathy and a maculopapular rash, sometimes with mouth ulcers - a presentation easily mistaken for a viral upper respiratory tract infection or glandular fever.¹ Viral load is extremely high during this period, making it the point of greatest infectivity, and a time when a standard antibody test may still be falsely negative.

Clinical latency

Following seroconversion, most people enter a prolonged asymptomatic phase, lasting years without treatment, during which CD4 counts gradually decline despite the absence of symptoms - which is exactly why relying on symptoms to prompt testing misses most people who could benefit from an earlier diagnosis.

Advanced HIV / AIDS

Without treatment, progressive CD4 depletion (below 200 cells/microlitre) leads to AIDS-defining illnesses: opportunistic infections such as Pneumocystis jirovecii pneumonia, cerebral toxoplasmosis and cytomegalovirus retinitis, and malignancies such as Kaposi's sarcoma and non-Hodgkin lymphoma.¹

Who to test - indicator conditions

UK guidance promotes an expanded, low-threshold approach to testing rather than relying only on a sexual history suggesting risk, because a substantial proportion of people diagnosed late had a healthcare contact in the preceding years where testing was not offered.²

• Anyone requesting a test, or reporting a relevant sexual history or injecting drug use
• All patients newly registering in general practice or admitted to hospital in areas of high HIV prevalence
• Universal offer in emergency departments in high-prevalence areas
• Diagnosis of any other STI, tuberculosis, hepatitis B or C, or lymphoma
• Unexplained lymphadenopathy, weight loss, chronic diarrhoea, oral candidiasis, or recurrent shingles at a young age
• Any AIDS-defining illness
• Routine antenatal booking bloods (universal offer to all pregnant women)

Testing

Laboratory fourth-generation combined antigen/antibody tests are standard: they detect both the p24 viral antigen (present early, before antibodies form) and HIV antibodies, giving a reliable result from around 45 days after exposure.³ Point-of-care tests (finger-prick or oral fluid) are convenient for opportunistic testing but are generally less sensitive in early infection, with a longer window period of around 90 days.

A reactive screening test is always confirmed with a further laboratory test before a diagnosis is given, and confirmed positive results are followed by viral load and CD4 count to establish disease stage and by resistance testing before starting treatment.

Where a test is negative but exposure was within the window period, retesting at the appropriate interval should be arranged, and the patient advised to use condoms or consider PEP/PrEP in the interim as appropriate.

Post-exposure prophylaxis (PEP)

PEP is a course of antiretroviral drugs given after a potential exposure to reduce the risk of the virus establishing infection. It should be started as soon as possible after exposure, and is generally offered up to 72 hours afterwards, though efficacy falls the longer the delay - it is far more effective within the first few hours.⁴ A standard course is taken for 28 days, and the patient is tested for HIV at baseline and again after completing the course (and often again at 12 weeks).

PEP is considered after condomless anal or vaginal sex with a partner of unknown or positive HIV status without viral suppression, needlestick or sharps injury with a known or high-risk source, or sharing injecting equipment. It is accessed through emergency departments (out of hours) or sexual health clinics.

Pre-exposure prophylaxis (PrEP)

PrEP is a daily or event-based regimen of antiretroviral medication (typically tenofovir/emtricitabine) taken by an HIV-negative person before potential exposure, and is highly effective at preventing acquisition when taken correctly.⁵ It is offered to people assessed as being at ongoing substantial risk of HIV acquisition - for example, men who have sex with men with condomless sex with multiple partners, or a partner of someone with HIV who is not virally suppressed - through sexual health services.

PrEP does not protect against other STIs, so it is offered alongside, not instead of, regular STI screening and condom use advice. Renal function is monitored periodically given the nephrotoxic potential of tenofovir.

Undetectable = untransmittable (U=U)

A person living with HIV who takes ART consistently and has a sustained undetectable viral load (typically defined as below 200 copies/mL, confirmed on repeated testing over at least six months) cannot transmit HIV sexually to their partners.⁶ This principle, known as U=U, is now well established by large prospective studies and underpins modern counselling: effective treatment is itself a highly effective form of prevention, which is part of the rationale for early diagnosis and immediate treatment initiation regardless of CD4 count.

Prevention of vertical transmission

Universal antenatal HIV screening, maternal ART throughout pregnancy to achieve viral suppression, appropriately planned mode of delivery (vaginal delivery is now recommended for women with a suppressed viral load), and avoidance of breastfeeding in resource-rich settings where safe formula feeding is available, together reduce mother-to-child transmission from around 25-30% without any intervention to well below 1%.⁷ Neonates born to HIV-positive mothers receive a course of postnatal antiretroviral prophylaxis.

Red flags

Prognosis

With early diagnosis and modern ART, people living with HIV in the UK now have a life expectancy approaching that of the general population, and, once virally suppressed, cannot transmit the virus sexually. The prognosis is worst for those diagnosed late, with an already low CD4 count and established opportunistic disease, which remains the strongest argument for widening testing beyond those who present with an obvious risk history.