Antigen Detection Assays
One-Sentence Definition
Antigen detection assays identify pathogen or toxin proteins directly in clinical specimens — enabling rapid diagnosis via lateral flow, EIA/ELISA, or immunochromatographic cards without culture.
Simple Explanation
Find pieces of the bug (or its toxin) in the sample — fast yes/no tests like strep swabs or malaria cards.
Detailed Scientific Explanation
| Format | Examples | Pros / cons |
|---|---|---|
| Lateral flow | Strep A, influenza, SARS-CoV-2, crypto, malaria RDT | Minutes; variable sensitivity |
| EIA/ELISA | C. diff toxins, HBsAg, galactomannan | Batchable; needs lab |
| Agglutination | Legionella urine Ag, pneumococcal urine Ag | Single analyte; useful in pneumonia |
| DIF | HSV/VZV lesion scraping (historical/contextual) | Expertise-dependent |
Specimen pairing
- Respiratory antigens — nasopharyngeal swab quality matters
- Urine Ag — pneumococcus, legionella (pneumonia)
- Stool — rotavirus, adeno, Giardia (parasitology overlap)
Interpretation
- Positive in high-prevalence setting → likely true
- Negative during low viral load — repeat or molecular test
- Does not provide AST (Antimicrobial Susceptibility Testing)
Mechanism
Immobilized antibody captures target antigen → visible signal (colored line/enzyme color change).
Clinical Importance
- ED and clinic triage speed
- Outbreak screening (influenza, COVID)
Diagnostic Relevance
- Bridge before PCR / culture; syndromic panels combine multiple antigens/nucleic acids
Related Methods
Related MOCs
Active Recall Questions
- Urine antigen tests for which two pneumonia pathogens?
- Lateral flow vs PCR — typical tradeoff?
- Strep antigen negative but high suspicion — next step?