Biologic assay

An Ambitious New Method of Diagnosis

France's ESPCI¹ and CNRS² researchers have come up with a new diagnostic test they say is one thousand times more sensitive but just as affordable as the Latex Agglutination Immunoassay (LAI), the cheapest method currently available on the market. The new system has already attracted attention from key players in French industry, and is currently in developmental phase preceding market availability.

“Hundreds of papers come out every year describing potential detection tests. Some are extremely sensitive, but all are too complicated to realistically implement–they aren't viable at a commercial level,” says ESPCI physicist Jerome Bibette, who conceptualized the test. “This is the first time the industrial sector has embraced a diagnostic test since an improved version of ELISA came out in the 1980s.”

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© PNAS vol.103, oct. 06

Magnetic beads (grey) grafted with antibodies (red) are mixed with the antigen (green), and then submitted to a magnetic field. This induces doublet formation, which is detected by a turbidity assay.

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LAI and ELISA are the two methods available to diagnose infectious diseases such as hepatitis and AIDS. LAI, because it involves a simple one-step immunochemistry reaction and few reactants, is cheap. Its downside is that it detects only “high” levels of antigens or proteins, that is, when the sickness is already well established.

In developed countries, LAI is used for pregnancy tests and simple protein dosages. To diagnose infectious diseases at early stages, the developed world relies on ELISA–one thousand times more sensitive, but also very burdensome. “Only technicians or machines can run ELISA, so the cost is about ten times higher than that of LAI,” says Bibette.

This is why poor countries often have little choice but to turn to the generic LAI method to test for infectious diseases. “With LAI, hepatitis and HIV are diagnosed when patients are already very sick–which is akin to not detecting them at all,” observes Bibette.

The formula proposed by Bibette's team offers a hopeful alternative by combining ELISA's sensitivity with LAI's affordability. Its principle, published in the October issue of PNAS,³ is based on applying a magnetic field to the LAI strategy and replacing latex particles with magnetic ones. The field creates a “trap” for the reactants attached to the magnetic particles, a trap in which they meet and combine more rapidly.

“With this technique, infectious agents could be detected at an earlier stage among populations of the developing world, too,” says Bibette. “This would enable medical teams to act on the diseases more effectively at earlier stages.”

Clémentine Wallace