HIV p24 Antigen
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Detect infection with Human Immunodeficiency Virus
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🧪 What is the HIV p24 Antigen Test?
The HIV p24 Antigen test detects the p24 protein, a core antigen of the HIV virus. It appears in the blood early in the course of infection, even before antibodies are detectable.
- p24 antigen becomes detectable within 2–4 weeks after HIV exposure.
- Used in 4th generation ELISA/Combo tests that detect both p24 antigen and HIV antibodies (IgM/IgG).
❓ Why is the Test Done?
To:
- Detect early HIV infection (window period)
- Screen blood donors or individuals recently exposed to HIV
- Reduce the diagnostic window and start treatment earlier
- Serve as part of routine HIV screening, especially in high-risk or pregnant individuals
Especially useful when antibody tests may still be negative in early (acute) HIV infection.
📊 Time Course After HIV Exposure
Time After Exposure | Marker Detected |
|---|---|
1–10 days | Viral RNA (via NAT) |
10–28 days | p24 Antigen (peaks ~3–4 weeks) |
>3 weeks onward | HIV antibodies (IgM → IgG) |
6+ weeks | Antibodies predominate; p24 declines |
📈 Interpretation of Results
Result | Meaning |
|---|---|
Positive p24 Antigen | Suggests early HIV infection – especially if antibodies are negative |
Negative p24 + Positive Antibodies | Likely chronic HIV infection (antibodies replace antigen over time) |
Negative both | Likely no infection or testing done too early – retest in 2–3 weeks if high risk |
Positive p24 must be confirmed by HIV RNA PCR or Western Blot.
🧠 Associated Organs & Conditions
System | Effects |
|---|---|
Immune system | CD4+ T-cell depletion, immune suppression |
Neurological | Early neuroinvasion, possible acute retroviral syndrome |
Others | Fever, rash, sore throat (flu-like illness in acute HIV) |
🔄 Related / Follow-Up Tests
- HIV Antibody Test (ELISA) – Combined in 4th Gen test
- HIV RNA PCR (Viral Load) – Confirms and quantifies infection
- Western Blot / Immunoblot – Older confirmatory test
- CD4 Count – Immune status evaluation
- HIV Drug Resistance Test – Prior to ART initiation
- STD Panel – Co-infections are common (e.g., syphilis, hepatitis B/C)
✅ Fasting Required?
Test | Fasting Requirement |
|---|---|
HIV p24 Antigen | ❌ No fasting required – Can be done at any time |
📝 Summary Table
Parameter | Description |
|---|---|
What | Detects p24 core antigen of HIV virus – an early marker of infection |
Why | To identify acute (early) HIV infection, reduce the window period |
Timeframe | Detectable 10–28 days after exposure |
Positive Result | Indicates early infection – before antibodies appear |
Follow-up Tests | HIV RNA PCR, ELISA, CD4, Western Blot |
Associated Conditions | Acute HIV syndrome, early immune suppression |
Fasting Required | ❌ No |
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