Recent Uses of Microspheres in Diagnostic Tests and Assays

Uniform latex particles or microspheres were first used in medical diagnostic applications as “latex” agglutination tests (LAT). Sensitive particle-enhanced turbidimetric assays are still in common use and are read with clinical chemistry analyzers via spectrophotometric or nephelometric methods. Dyed agglutinated particles caught on filters form the basis of another class of tests. Particle capture ELISA tests and assays are in common use (e.g., Abbott’s IMx and AxSym). The popular “strip tests” for pregnancy, ovulation, drugs of abuse in urine, etc. use dyed microspheres, and quantitative strip assays are beginning to appear. Solid phase assays and tests use particles for positive or negative capture of a wide variety of analytes. Solid-liquid separations can be made by centrifugal density separation, or filtration, or via magnetic separation of superparamagnetic particles. Proximity assays, like scintillation proximity assay (SPA), luminescent oxygen channeling immunoassay (LOCI), and fluorescence resonance energy transfer (FRET), all use microspheres. Single microsphere assays are now possible in flow cytometers and on the newer flow-based analysers. Dyed microspheres can be much more sensitive as stains or markers since a single 100 nm microsphere can carry 1000 dye molecules. Molecular biology applications include the Human Genome Project where superparamagnetic and silica microspheres are used to separate DNA from cell debris. Microspheres have been used in immunosensors based on piezoelectrics and evanescent-wave optical fiber-based immunosensors. One new assay system currently being developed uses single beads caught in wells etched in the ends of optical fibers.The hottest new immunological use for microspheres is in homogeneous multiplexed pharmaceutical high-throughput screening assays.