Antigen vs Antibody Test
Direct answer: An antigen test detects the pathogen's own protein and indicates an active infection; an antibody test detects the host's immune response and indicates exposure — which may be current, recent or past. The right choice depends on the diagnostic window.
Comparison table
| Antigen test | Antibody test | |
|---|---|---|
| Detects | Pathogen protein | Host immune response (IgM/IgG) |
| Indicates | Active infection | Exposure (current or past) |
| Typical timing | Early (acute phase) | Later (days to weeks) |
| Example | Dengue NS1, H. pylori stool antigen, Strep A | HIV, HCV, Dengue IgM/IgG, Syphilis |
What each method detects
- Antigen: a structural or secreted protein of the pathogen — present only while the pathogen is active.
- Antibody: IgM (recent) and IgG (later/past) produced by the immune system — can persist long after the infection clears.
Strengths
- Antigen: direct active-infection signal; fewer false positives from past exposure; detects early.
- Antibody: broader detection window; useful for immunity assessment (e.g., rubella IgG) and late presentation.
Limitations
- Antigen: may miss low pathogen load; narrow detection window.
- Antibody: cannot distinguish active from past infection alone (e.g., HCV antibody needs RNA confirmation); window period before antibodies appear.
False negative / positive considerations
- False negative: testing before the marker appears (antigen too early, antibody in the window period).
- False positive: antibody cross-reactivity or persistent antibodies from past infection.
Confirmatory testing
Rapid antigen and antibody tests are screening-grade. Confirmation uses molecular (PCR/RNA) testing or laboratory reference methods, depending on the pathogen.
Procurement considerations
- Combined antigen + antibody devices (e.g., dengue NS1 + IgM/IgG) cover both phases in one SKU.
- Confirm the marker and specimen match the intended use before ordering.
Related pages
Which test to choose
Choose an antigen test to detect active infection; choose an antibody test for exposure or immunity assessment. When the goal is "is the pathogen present now", antigen is the correct marker.
| Scenario | Recommended |
|---|---|
| Detect active infection | Antigen test |
| Assess immunity / past exposure | Antibody test |
Sources: Standard immunodiagnostic terminology; WHO/CDC testing guidance.
Last reviewed: 2026-09-12