Rapid Diagnostic Test Technologies
Agglutination

Tests based on agglutination of microscopic particles have been used as diagnostic tools for many years. They employ white or colored latex particles or gelatin beads, other dyes or colloidal particles, or preserved mammalian or avian blood cells to carry antibodies or antigens.

To perform the classic version of the test, a specimen is placed on a microscope slide or in a microtest plate well, and then the carrier particles are added. The mixture is then stirred or agitated and is incubated or allowed to settle for a few minutes. The results are determined by visually observing agglutination of the carrier particles as they bind to the specific analyte(s) in the sample.

In strong reactions, the interpretation is simple, but in weak or marginal reactions, the interpretation can be quite difficult, and accurate results are possible only after much experience and practice. In the case of microwell plate assays, the pattern of particles that have settled needs to be interpreted to determine a result.

The tests can be qualitative or semi-quantitative, since for the latter, 2- to 10-fold specimen dilutions can be made.

The classic agglutination tests have often lacked sensitivity, and results could not be stored for reference. Updated methods using channeled plastic devices and/or automated readers have significantly improved sensitivity, but at added cost and complexity.

Advantages of agglutination methods include a low individual test cost, ability to semi-quantitate results, and a relatively short time to obtain results. Disadvantages include the need to carefully interpret marginal results, problems with specificity due to interfering substances in many assays, and a relatively low sensitivity, especially for antigen detection tests.

See also:
Flow-through
Lateral-flow
Solid-phase