Thursday, December 21, 2006


Improvements in diagnostic testing


P Hari

Testing for an infectious disease is very simple, theoretically speaking. The patient’s blood or tissue sample would contain the parasite or a molecule that will normally be absent in a healthy individual. Biologists call it an antigen. You take the tissue sample and bring it in contact with an antibody that can recognise and bind to the antigen. Then you only need to find out whether the antibody has actually bound to the antigen, which is easy if enough antibodies bind to the antigens. So, what is the problem?

The difficulty is in bringing the antigen and antibody near enough for them to bind to each other. During a normal laboratory test, this actually happens through a random process. The lesser the number of antigens in a sample, the more difficult it is to form an antigen-antibody complex. If the number of antigens is too small, the complex may not form at all — which is one reason why many tests fail. Now, physicists at the Indian Institute of Science (IISc) in Bangalore have come up with a method to solve this problem. Their method is so general that it has potential applications in any diagnostic test involving antigen-antibody interaction.

Ajay Sood at the department of physics came up with an important discovery last year. He found that a fluid passing over a carbon nanotube generates a voltage and, thus, a current inside the tube. While he was exploring the potential applications of this discovery, he was also working on several other topics, particularly on the physics of colloids (a suspension of particles in a liquid). The current invention stems directly from his work on the behaviour of colloids in electric fields.

Other scientists have tried many methods to bring the antigens close to each other. Some have used ultrasound waves, which force the particles to move and thus collide. Others have tried magnetising the colloidal particles and then applying an external field, a process that brings the antigens and antibodies together. It was known that an electric field applied parallel to the electrodes (negative and positive plates that generate the field) makes the particles form chains. But this fact is not useful for diagnostic tests. Sood and his student Ajay Negi used fields, but perpendicular to the plates instead of parallel to them.

When a perpendicular electric field of a certain strength is applied to a colloidal suspension, particles in the sample start coming together in a cluster. While clustering, they also bring the antigen and the antibody closer to each other, thus improving the chances of forming the antigen-antibody complex. Sood’s calculations show that the chances increase at least 200 times, an enormous improvement in an actual test. Sood had tried the method on two types of antigens in his lab: on specifically coated antigens and antibodies, and then on the commercially available kit for rheumatoid factor. He also filed a patent application in the Patent Cooperation Treaty countries, which includes all developed and several developing countries.

Sood’s method is general, and needs to be applied to specific kits for specific diseases. His team started with typhoid, in a joint project with the Gwalior-based Defence Research and Development Establishment, which is already developing kits for this disease. His next goal is to work on malaria diagnostic kits. IISc will then look for a commercial partner. If the IISc team can improve the efficiency of the kits for major diseases, the institute is looking at a market worth billions of dollars.

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