# HIV Diagnosis and Testing

> Rapid test algorithm, window period, p24 antigen and early infant HIV diagnosis explained for NEET-PG Medicine exam preparation, with Indian programme context.

- Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/hiv-diagnosis-and-testing
- Exam / course: NEET-PG · Subject: Medicine
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-08
- Last updated: 2026-10-08
- Reference basis: NACO National Guidelines for HIV Care and Treatment; Harrison's Principles of Internal Medicine; Mandell's Principles and Practice of Infectious Diseases
- How to cite: "HIV Diagnosis and Testing", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/hiv-diagnosis-and-testing

## Direct answer

Diagnosing HIV rests on a serial testing algorithm in which a reactive screening test is confirmed by further tests of a different principle before the diagnosis is given. Fourth-generation assays detect p24 antigen as well as antibodies, shortening the window period, and nucleic acid testing detects infection earlier still. Infants of positive mothers cannot be diagnosed by antibody tests before about 18 months; early infant diagnosis uses HIV DNA PCR from around six weeks.

## What you must remember

- **Window period:** The interval between infection and a detectable marker; antibody-only assays take roughly three to twelve weeks, while p24 antigen and nucleic acid tests detect infection sooner.
- **p24 antigen:** A core viral protein detectable during acute infection, before antibodies appear; fourth-generation combined antigen and antibody assays rely on it.
- **Screening algorithm:** Serial rapid tests based on different antigens or principles; under the NACO programme the usual strategy uses up to three tests, and a positive result needs the programme algorithm to be completed, with discordant results resolved by the next step.
- **Nucleic acid test:** HIV RNA PCR detects infection within days to a few weeks; used in acute infection and infants, not routine screening.
- **Pregnancy:** Every pregnant woman is offered testing under the national prevention of parent-to-child transmission programme, and a positive result is confirmed before antiretroviral therapy is planned.
- **Infant diagnosis:** HIV DNA PCR at about six weeks; antibody tests are uninterpretable before 18 months because maternal IgG crosses the placenta.
- **Confirmation:** Older algorithms used the Western blot; indeterminate results are repeated, not assumed negative.
- **Post-exposure:** Prophylaxis should begin as early as possible, ideally within 2 hours and not later than 72 hours, and testing is repeated at fixed intervals afterwards.

## Worked scenario: a 28-year-old with a recent exposure

A 28-year-old man presents three days after a needle-stick injury with a clear history of exposure.

1. **Start prophylaxis at once:** the 72-hour window is short, so prophylaxis does not wait for results.
2. **Send baseline tests:** a negative antibody result on day three is not reassuring, since the window period has not closed.
3. **Retest at four to six weeks:** he returns for a fourth-generation assay, and a negative result at that point is reassuring, with later follow-up tests as per protocol.
4. **Escalate if symptomatic:** if fever, rash or lymphadenopathy develop in the interim, acute HIV is suspected and HIV RNA PCR is sent immediately, because antigen and nucleic acid tests catch infection that antibodies have not yet revealed.
5. **Repeat at fixed intervals:** testing is repeated at fixed intervals afterwards, in line with the post-exposure rule.

Timing governs which test is meaningful: antibody tests miss early infection, while p24 antigen and nucleic acid tests detect it sooner.

## Where the examiner tests the window

Three traps recur.

- **Trap:** declaring a patient free of HIV after one early antibody result. — **Correct:** that negative falls inside the window period, so repeat testing is required before excluding infection.
- **Trap:** treating a positive antibody test at nine months in a breastfed baby of a positive mother as confirmed infection. — **Correct:** it does not confirm infection; PCR is expected, not a repeat antibody test.
- **Trap:** accepting a single reactive rapid test in pregnancy as a diagnosis. — **Correct:** a single reactive result is not a diagnosis until the algorithm is completed.

## Frequently asked questions

### What is the window period in HIV testing?

The window period in HIV testing is the interval between infection and the appearance of detectable markers in the blood. For antibody-only assays it is roughly three to twelve weeks, while p24 antigen and nucleic acid tests shorten it considerably.

### Why is a p24 antigen test useful in acute HIV infection?

A p24 antigen test is useful in acute HIV infection because the viral core protein appears in blood before antibodies are detectable. Fourth-generation combined antigen and antibody assays use this to detect infection earlier than antibody-only tests.

### When can HIV infection be confirmed in an infant born to an HIV-positive mother?

HIV infection in an infant born to an HIV-positive mother is confirmed by HIV DNA PCR from around six weeks of age, with a repeat test to confirm a positive result. Antibody tests cannot confirm infection before 18 months because maternal antibodies persist.

### Why are different rapid tests used in series for HIV screening?

Different rapid tests are used in series because each targets different antigens, which reduces false-positive reactions and improves specificity. A positive diagnosis needs the programme algorithm to be completed, which in NACO practice means up to three tests, and discordant results are resolved by a further step.

### What test is used to diagnose HIV in a baby at six weeks of age?

HIV DNA PCR on dried blood spot is the test used to diagnose HIV in a baby at around six weeks of age, because it detects viral DNA directly. Antibody testing at this age is uninterpretable because of maternal antibodies. A positive PCR is repeated to confirm before lifelong therapy.