Measurement of antigen concentration by an ultrasound-enhanced latex immunoagglutination assay.

A novel ultrasonic technique that increases the rate and sensitivity of latex agglutination tests (LATs) has recently been described. The technique is based on the fact that suspended latex particles become concentrated in an ultrasonic standing wavefield, thereby increasing the rate of particle-particle collisions compared to the standard LAT procedure. The present work extends earlier qualitative assessments of agglutination and seeks to establish whether quantitative measurement of agglutinate size may be used as an indicator of antigen concentration. The agglutination of latex microparticles coated with antibody to C-reactive protein (CRP) is studied here as a model system to determine the dependence of agglutinate size on analyte (CRP) concentration. Agglutinate size is characterised by image analysis techniques. The results show that agglutinate size decreases with decreasing CRP concentration. A near linear relationship is shown between analyte concentration and the size of agglutinate formed over a 100-fold dilution range. The threshold concentration of 230 pg/mL for detection of CRP in the ultrasonic test is 2560 times lower than that required for a conventional test-card CRP latex agglutination assay.