Latex agglutination tests

Principle

• Polystyrene latex particles (about 0.8 µm) are coated with antigen or antibody. Reaction with the corresponding antibody or antigen in the sample cross-links particles into visible clumps.
• Antibody-coated latex detects antigen (e.g. CRP, cryptococcal antigen, bacterial antigens, rotavirus).
• Antigen-coated latex detects antibody (e.g. ASO with streptolysin O-coated latex; RF with IgG-coated latex).

Specimen

• Fresh serum (or other fluids such as CSF as per kit). Avoid grossly haemolysed, lipaemic or contaminated samples.

Reagents and equipment

• Latex reagent, positive and negative controls, glass or disposable card with test circles, stirrers, rotator, timer.
• Store reagents at 2–8 °C; do not freeze latex.

Procedure

• Bring reagents and samples to room temperature; mix latex gently.
• Place one drop of sample and one drop of latex in a circle; mix and spread.
• Rotate for the time stated (often 2 minutes at about 100 rpm).
• Read immediately under good light against a dark background.
• Semi-quantitative: test doubling dilutions of positive samples; titre × kit sensitivity gives approximate concentration.

Results

• Positive: clear visible agglutination. Negative: smooth, milky, uniform suspension.

Quality control and pitfalls

• Run positive and negative controls with each batch or new lot.
• Prozone (antigen or antibody excess) can give false negatives; test a diluted sample if clinically suspected.
• Reading after the stated time causes drying and false positive granularity.
• Rheumatoid factor and other interfering substances can cause false positives in some tests.

Clinical use

• Rapid screening: CRP, RF, ASO, cryptococcal antigen in CSF, Staphylococcus aureus identification (clumping factor/protein A), Lancefield grouping of streptococci.