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HIV Combo (Ag+Ab)

STDs Biomarker

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Collection Type: Blood

Body System

Related System: STDs

Overview

The HIV Combo (Ag+Ab) test is a fourth‑generation screening assay that detects both HIV p24 antigen (an early viral protein) and antibodies to HIV‑1 and HIV‑2. By measuring antigen and antibody it shortens the diagnostic window and can identify acute (very recent) and established infections. It is used when HIV infection is suspected—symptoms like fever, sore throat, rash, lymphadenopathy, unexplained weight loss or opportunistic infections—or after a recent high‑risk exposure. Test sensitivity and timing vary with time since exposure, age, pregnancy, and immune status (immunosuppressed people may have delayed antibody responses). It is routinely used in screening programs, prenatal care and blood donation.

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Test Preparation

No special preparation is required

Why Do I Need This Test

Profile: STDs (HIV screening) - Symptoms: acute retroviral symptoms (fever, malaise, rash, sore throat), unexplained infections, or after high‑risk exposure - Diagnoses/monitoring: screens for HIV‑1/2 infection (not for treatment viral load monitoring) - Reasons for abnormal/reactive: true infection, recent seroconversion, cross‑reactivity or technical error - Biological meaning: reactive suggests presence of p24 antigen and/or HIV antibodies indicating infection - Behaviors: unprotected sex, needle sharing, occupational exposures - Family history: known maternal or partner HIV positivity warrants testing

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Understanding Test Results

A non‑reactive result (S/CO 1.0) means no p24 antigen or HIV antibodies were detected; it generally excludes infection unless testing occurred during the early window period.
A reactive result (S/CO 1.0) suggests probable HIV infection p24 antigen may indicate very recent (acute) infection, while antibody positivity indicates established infection.
Reactive screening requires confirmatory testing with an HIV‑1/2 differentiation immunoassay and, if needed, an HIV RNA (NAAT) to detect very early infection.
False positives can occur (autoimmune disease, recent vaccines, technical factors); false negatives can occur if tested within ~7–14 days after exposure (antigen/antibody may not yet be detectable) or in severe immunosuppression.
If high‑risk exposure occurred but result is non‑reactive, repeat testing at 4–6 weeks and again at 3 months (or per local guidelines) is recommended; reactive results need urgent linkage to care, counseling, and confirmatory testing.

Normal Range

Non‑reactive (Signal‑to‑cutoff ratio 1.0, unitless)

FAQs

Q: What does HIV Ag AB combo mean?

A: HIV Ag/Ab combo is a blood test that detects both HIV p24 antigen and antibodies to HIV‑1 and HIV‑2. Detecting p24 shortens the window period, allowing earlier infection detection (often from about 2–4 weeks after exposure). It’s used for screening; reactive results need confirmatory testing. A negative result during the window period may require repeat testing per medical advice.

Q: What is the Ag AB test for HIV?

A: The HIV Ag/Ab test (fourth‑generation) detects both the p24 antigen and antibodies to HIV‑1/2 in blood. By identifying p24 it shortens the window period, allowing earlier detection of acute infection (often 2–6 weeks after exposure). It is highly sensitive, but reactive results require confirmatory testing (HIV differentiation assay and/or NAT) to establish diagnosis and guide care.

Q: How accurate is the HIV Ag AB combo test?

A: HIV Ag/Ab combo (fourth‑generation) tests are highly accurate—sensitivity and specificity exceed 99% after the window period. By detecting p24 antigen plus antibodies, they identify most infections 2–4 weeks after exposure and nearly all by 4–6 weeks. Reactive results need confirmatory testing, and very early infections (first 1–2 weeks) can still be missed, so repeat testing may be recommended.

Q: When is HIV Ag AB conclusive?

A: Fourth-generation HIV Ag/Ab tests detect p24 antigen and antibodies. They can reliably detect most infections by 4 weeks after exposure, but a negative result is considered fully conclusive only at 12 weeks (3 months). Reactive/positive results require confirmatory testing; high-risk exposures or symptoms warrant repeat testing and/or earlier nucleic-acid (PCR) testing.

Q: Can the HIV Ag AB combo detect early infection?

A: Yes. The HIV Ag/Ab combo (fourth‑generation) tests both p24 antigen and HIV antibodies, so it can detect infection earlier than antibody‑only tests—often as early as 2–3 weeks after exposure, with most infections detectable by 4–6 weeks. Very early infections can still be missed, so reactive results need confirmation and negative tests during the window should be repeated (commonly at 4–6 weeks and 3 months).

Q: What is the window period for the HIV Ag AB combo test?

A: The HIV Ag/Ab (4th‑generation) test can detect infection as early as 2 weeks after exposure (p24 antigen). Most infections are detectable by 3–4 weeks, and sensitivity exceeds 99% by about 6 weeks. Because antibody maturation varies, some guidelines advise a confirmatory negative at 12 weeks. If exposure was recent, repeat testing or consult a clinician.