HIV 1 & 2 (4th Generation p24 Antigen & Antibody Combo)
Also known as: HIV 4th Gen Test, HIV Duo Screen, HIV p24 Antigen Test, HIV 1/2 Ab/Ag EIA / CLIA
Parameters Measured in the Report
A breakdown of individual biomarkers and biological reference intervals typically found in your report:
| Biomarker | Standard Range | Unit | What It Means |
|---|---|---|---|
HIV 1/2 Antigen/Antibody Combo (4th Gen HIV Screen) | Non-Reactive / Negative (< 1.0 index COI) | Cutoff Index (COI) | Combined assay testing for both circulating HIV-1 p24 viral antigen and antibodies to HIV-1 (Groups M, N, O) and HIV-2. |
* Note: Reference ranges may vary slightly between testing instruments and reagent kits. Always refer to the reference interval printed on your specific pathology report.
Clinical Focus
Simultaneous detection of HIV-1 p24 viral capsid antigen and host IgG/IgM antibodies against HIV-1 and HIV-2.
Sample & Specimen
Venous blood (Serum tube)
Fasting Protocol
No fasting required. Confidential diagnostic testing.
Turnaround Time
2–4 hours (rapid / same day)
What Is This Test?
Why Did Your Doctor Order This Test?
Physicians routinely prescribe the HIV 1 & 2 Combo in the following clinical scenarios:
How to Prepare Before Your Test
- No fasting or dietary restriction is needed.
- If testing following a specific potential exposure, testing at 2–3 weeks (p24 peak) and re-confirming at 6 weeks provides definitive diagnostic certainty.
Understanding Your Results
What higher or lower readings generally mean in clinical practice:
If Values Are Higher Than Normal
A Reactive / Positive 4th-generation screen indicates initial reactivity. In clinical protocol, a reactive result is NEVER reported as a final diagnosis until verified through supplemental reflex testing: an HIV-1/HIV-2 antibody differentiation immunoassay followed by an HIV-1 RNA PCR viral load test.
If Values Are Lower Than Normal
A Non-Reactive / Negative result indicates the absence of detectable HIV antigen or antibodies. If tested at or beyond 45 days post-exposure, a non-reactive 4th-gen laboratory blood test is considered definitive evidence that HIV was not transmitted.
Frequently Asked Questions
What is the window period of a 4th-generation HIV blood test?
A 4th-generation laboratory blood draw has a window period of 14 to 21 days for the p24 antigen, and provides conclusive, >99% reliable results at 45 days post-exposure.
What happens if a screening test is initially reactive?
Laboratories follow strict CDC/WHO algorithms: every reactive screen automatically undergoes confirmatory testing (differentiation assay and viral load PCR) before a definitive clinical diagnosis is established.
Related Laboratory Tests
Tests frequently prescribed together or used as follow-up investigations:
Hepatitis B Surface Antigen (HBsAg)
Qualitative and quantitative detection of Hepatitis B virus outer surface envelope protein in blood.
Hepatitis C Antibody (Anti-HCV)
Qualitative and semi-quantitative detection of host IgG antibodies directed against Hepatitis C viral structural proteins.
VDRL / RPR (Syphilis Serology)
Circulating non-treponemal reaginic antibodies (IgM and IgG) directed against cardiolipin-lecithin antigens released from damaged host cells.
Complete Blood Count (CBC / Hemogram)
Counts and evaluates red blood cells, white blood cells, hemoglobin, hematocrit, and platelets.
Medical & Diagnostic Disclaimer
This guide is published for educational and patient informational purposes only. It is not a substitute for professional medical evaluation, diagnosis, or clinical advice.
A laboratory result that falls slightly outside standard statistical reference intervals does not automatically indicate disease; conversely, a normal result does not guarantee the absence of medical conditions. Only your attending physician can interpret lab findings in the context of your medical history, physical exam, and symptoms.