Question
Latex agglutination test
Answer
The latex agglutination test is a passive agglutination technique in which a soluble antigen (or antibody) is artificially adsorbed/coated onto the surface of inert latex particles, which then visibly clump (agglutinate) when mixed with a sample containing the corresponding specific antibody (or antigen) — a rapid, simple slide test widely used in clinical microbiology/serology.
Principle: latex particles are too small individually to be seen, but when cross-linked by antigen-antibody interaction into larger aggregates, the clumping becomes visible to the naked eye against a dark background card, within minutes.
Formats:
- Direct latex agglutination (antigen-coated latex, detecting antibody) or reverse passive latex agglutination (antibody-coated latex, detecting antigen) — the choice depends on which reagent (antigen or antibody) is used to sensitize the particles and what the test is designed to detect.
Applications:
- Detection of rheumatoid factor (RA factor) and C-reactive protein (CRP) in serological screening for rheumatoid arthritis/inflammation.
- ASO (antistreptolysin O) titre estimation.
- Rapid detection of bacterial capsular antigens directly in CSF (e.g., Streptococcus pneumoniae, Haemophilus influenzae type b, Neisseria meningitidis, Cryptococcus neoformans — cryptococcal antigen test) — useful for rapid presumptive diagnosis of bacterial/fungal meningitis, particularly when the organism has already been killed by prior antibiotic therapy and culture may be negative.
- Serodiagnosis of syphilis (TPHA is a related passive haemagglutination test using sensitized red cells rather than latex).
- Pregnancy testing — detection of beta-hCG using anti-hCG-coated latex particles.
Advantages: rapid (results within minutes), technically simple, requires no special equipment, and can be performed at the bedside or in a basic laboratory setting. Limitations: generally qualitative/semi-quantitative rather than precisely quantitative, and can be less sensitive than ELISA or molecular methods for some applications.