HIV Testing: Types, Window Periods, and Diagnostic Algorithms

HIV Diagnostic Testing Overview

Modern HIV testing uses a combination approach to detect infection early and accurately.

Test Categories

Test Type	Target	Window Period	Use Case
Nucleic Acid Test (NAT)	HIV-1 RNA	10-33 days	Early detection, blood supply, infant diagnosis, PrEP monitoring
Ag/Ab Combination (4th gen)	p24 antigen + HIV-1/2 Ab	18-45 days	Standard lab screening (recommended first-line)
Ab Only (3rd gen)	HIV-1/2 antibodies	23-90 days	Rapid POC, self-test; confirmatory
Rapid POC	Ab or Ag/Ab	Same as above	Clinic, community, emergency; results in 20 min
Self-Test (Home)	Antibodies (oral fluid/blood)	90 days	Private screening; confirm positive with lab test
Western Blot / IFA	Specific Ab bands	N/A	Confirmatory (being replaced by differentiation assays)
HIV-1/HIV-2 Differentiation	Type-specific Abs	N/A	Confirmatory after reactive screen
Viral Load (RNA PCR)	HIV-1 RNA copies/mL	N/A	Treatment monitoring, not diagnosis
Proviral DNA PCR	Integrated HIV DNA	N/A	Research, elite controllers, infant diagnosis adjunct

Recommended Algorithm (CDC/WHO)

1. Screen with 4th-gen Ag/Ab combination assay.
2. If reactive → HIV-1/HIV-2 differentiation assay.
3. If differentiation negative/indeterminate → NAT for acute infection.
4. If NAT positive → acute HIV; if negative → false positive screen.

Special Populations

• Infants: NAT at birth, 2-4 weeks, 2 months, 4 months (maternal Ab cross placenta).
• PrEP users: q3mo HIV Ag/Ab + renal function.
• Occupational exposure: Baseline, 6 weeks, 3 months (or 4th gen at baseline, 4-6 weeks).

Key Points

• No test detects immediately post-exposure; window period varies.
• Positive screening test requires confirmatory testing.
• Counseling and linkage to care are integral.