HIV Detection: The Proper Tests

Overview

HIV diagnosis relies on demonstrating antibodies to HIV and/or directly detecting the virus or its components. Antibodies generally appear in circulation 3–12 weeks after infection. The CDC recommends HIV screening as a routine part of health care for all individuals aged 13–64 at least once, and annually (or every 3–6 months) for high-risk groups.

The Three Categories of HIV Tests

1. 4th-Generation Antigen/Antibody Combination Test ✅ (Recommended First-Line)

This is the current standard recommended by the CDC for initial HIV screening.

Detects both HIV-1 p24 antigen (a core viral protein) AND HIV-1/HIV-2 antibodies (IgM and IgG) simultaneously
Can detect infection as early as 2–6 weeks after exposure (window period ~16–18 days for lab-based; ~19–20 days for rapid version)
Uses ELISA-based technology
Sensitivity: >99.5%
Available in lab-based and rapid point-of-care formats (results in ≤30 minutes)

The Western blot, which was previously used for confirmation, is no longer recommended for this purpose.

2. Nucleic Acid Test (NAT) — HIV RNA PCR

Used when:

The 4th-generation screen is positive but the differentiation immunoassay is negative/indeterminate (to detect acute infection)
Screening blood products for transfusion
Diagnosing HIV in infants born to HIV-positive mothers (since maternal IgG crosses the placenta, making antibody tests unreliable in newborns)

Method	Sensitivity	Window Period
Quantitative HIV-1 RNA (viral load)	Detects ≥20–40 copies/mL	6–10 days
Qualitative HIV-1 RNA	Detects ≥100 copies/mL	10–12 days
HIV RNA by RT-PCR	Reliable to 20 copies/mL	7–28 days
HIV RNA by bDNA	Reliable to 50 copies/mL	—

NAT is also critical for monitoring response to antiretroviral therapy (ART) — a rising or non-suppressed viral load signals treatment failure or resistance.

3. Antibody-Only Tests (3rd-Generation)

Detect IgM and IgG antibodies only (no antigen detection)
Window period: 22–24 days (lab); 26–32 days (rapid)
Less preferred than 4th-generation due to longer window period
Includes rapid tests using oral fluid (saliva) or blood — results in ~20–30 minutes
FDA-approved home test: OraQuick® In-Home HIV Test (oral fluid, results in ~20 minutes)

The CDC Recommended Testing Algorithm

Step 1: Perform a 4th-generation HIV-1/2 antigen/antibody immunoassay

Negative → HIV ruled out (retest if recent exposure within 3 months)
Positive/indeterminate → Proceed to Step 2

Step 2: HIV-1/HIV-2 antibody differentiation immunoassay (e.g., Bio-Rad Geenius)

HIV-1 positive → Confirms HIV-1 infection
HIV-2 positive → Confirms HIV-2 infection
Both positive → Confirms HIV infection
Negative/indeterminate → Proceed to Step 3

Step 3: HIV-1 NAT (nucleic acid test)

NAT positive → Acute HIV-1 infection confirmed
NAT negative → HIV-1 negative (Step 1 was a false positive)

Special Situation: Infants Born to HIV-Positive Mothers

Maternal IgG crosses the placenta, so antibody tests cannot be used to diagnose HIV in infants. Instead, use:

HIV-1 DNA/RNA PCR (or HIV-1 culture)
Confirm with repeat testing
HIV DNA PCR positivity rates: 38% at 48 hours of life → 93% at 14 days → 98% at 4 weeks

Summary: Window Periods at a Glance

Test	What It Detects	Window Period
Qualitative NAT (RNA PCR)	HIV RNA	~10–12 days
4th-gen Ag/Ab combo (lab)	p24 antigen + IgM/IgG	~16–18 days
4th-gen Ag/Ab combo (rapid)	p24 antigen + IgM/IgG	~19–20 days
3rd-gen antibody EIA (lab)	IgM + IgG	~22–24 days
3rd-gen antibody EIA (rapid)	IgM + IgG	~26–32 days
Western blot (no longer recommended)	IgG	~35–40 days

Key Points

A negative test during the window period does not rule out infection — repeat testing 3 months after exposure is advised
During the window period, an infected person can still transmit the virus via sexual contact or blood donation
All positive rapid/home tests must be confirmed by a healthcare professional with laboratory testing
HIV RNA quantification (viral load) is essential for monitoring ART efficacy — goal is suppression to <50 copies/mL

— Harrison's Principles of Internal Medicine 22E, pp. 1632–1633; Sherris & Ryan's Medical Microbiology, 8th Ed., pp. 733–735; Dermatology 2-Volume Set, 5th Ed., p. 4100

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