HIV Test

Types of HIV tests

What is the window period?

HIV test results

PrEP and PEP

PrEP (pre-exposure prophylaxis) is a daily medication that prevents HIV in HIV-negative high-risk individuals: over 99% effective. PEP (post-exposure prophylaxis) is emergency medication started within 72 hours of a potential exposure. It must be started as soon as possible. Regular HIV testing is recommended for people on PrEP (every 3 months).

Questions to ask your doctor

• Which generation HIV test am I having?
• When was my potential exposure: am I still in the window period?
• Am I eligible for PrEP?
• If positive: what is my CD4 count and viral load?

Frequently Asked Questions

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

1. HIV Testing and Screening. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK482145

References

Sources cited on this page. PubMed links open the original abstract.

1. Branson BM, Handsfield HH, Lampe MA, et al. Revised recommendations for HIV testing of adults, adolescents, and pregnant women in health-care settings. MMWR Recomm Rep. 2006;55(RR-14):1–17. PMID 16988643

The HIV testing cascade – which test at which stage

HIV testing uses different assays at different points in the infection timeline because the immune response develops over weeks:

• 4th generation combined Ag/Ab test (the standard NHS HIV test): Detects both HIV p24 antigen (the virus's core protein – present from approximately 2–4 weeks after infection) and HIV antibodies (from approximately 4 weeks). The window period – time from infection to reliable detection – is 45 days (6.5 weeks) for 4th generation tests. A negative result at 45 days after the last possible exposure definitively excludes HIV infection. This is the test used in all NHS services and most private clinics.
• HIV RNA (viral load PCR): Detects viral RNA from approximately 10–14 days after infection – the earliest possible detection. Used in acute HIV infection (where the patient presents with seroconversion illness: fever, pharyngitis, rash, lymphadenopathy) before antibodies develop, and in "post-exposure prophylaxis" (PEP) monitoring. Not used as a routine screening test.
• Point-of-care rapid HIV tests: Finger-prick blood or oral fluid tests giving results in 20–60 minutes. Sensitivity is slightly lower than laboratory tests. UK NICE guidance (PH51) recommends offering point-of-care testing in community settings (sexual health clinics, drug services, sexual health events) to reach people who would not attend formal clinical settings.

What a positive HIV test means next

A reactive HIV test (in any clinical or community setting) must be confirmed with a laboratory 4th generation test before disclosure, unless the person is acutely unwell. Once confirmed:

• CD4 cell count: The primary immune function marker. CD4 below 200 cells/µL defines AIDS regardless of other symptoms. CD4 count guides urgency of ART initiation and determines which opportunistic infection prophylaxis is needed (PCP prophylaxis if CD4 below 200; MAC prophylaxis below 50).
• HIV viral load (RNA): Quantifies the amount of virus in blood. Used to assess treatment response – the goal of antiretroviral therapy (ART) is viral load below 50 copies/mL ("undetectable"). An undetectable viral load means the person cannot sexually transmit HIV (U=U: Undetectable = Untransmissable – the most important public health message in HIV medicine since HAART).
• Resistance genotyping: Before ART is started, resistance testing determines which drug classes will be effective. Transmitted drug resistance is present in 10–15% of new HIV diagnoses in the UK.
• Screening for co-infections: All newly diagnosed HIV-positive individuals should be screened for hepatitis B (HBsAg, anti-HBs, anti-HBc), hepatitis C (anti-HCV), syphilis, gonorrhoea, and chlamydia – STI co-infection rates are high and many co-infections are treatable.