A new test may work as a kind of early warning system for new and dangerous mutations in the avian flu virus, US researchers believe.
The test could alert scientists to when the virus starts to change into a form that easily infects people, the researchers report in the Journal of Molecular Biology.
The test, called a glycan array, shows it would take very little change for the H5N1 avian influenza virus to cause a human pandemic, said Ian Wilson of the Scripps Research Institute in La Jolla, California.
"It would appear that two mutations could change the specificity dramatically going from avian to human," Mr Wilson said.
The H5N1 strain of bird flu has killed more than 70 people since late 2003 and is spreading from Asia into Europe. It is endemic in birds in many countries and has started affecting people in a sixth country, Turkey, where it has killed at least two people.
But the H5N1 virus still primarily infects birds and only rarely passes into people. Experts fear this could change, and that a form easily transmitted from person to person could cause a pandemic, a global epidemic, that would kill millions.
Mr Wilson's team says the new test can spot this happening. They used their glycan array to survey samples of the proteins that make up the coats of strains of human and avian viruses, including from the 1918 influenza pandemic.
The key protein is called hemagglutinin - the "H" in a flu virus designation. How deadly an individual influenza infection is depends on how well a person's immune system recognises the hemagglutinin.
The virus uses hemagglutinin to attach to lung cells. It uses a lung cell receptor, a molecular doorway, called sialic acid. The Scripps Institute test can tell the difference between a bird virus that prefers bird sialic acids and a virus that prefers the human version, the researchers said.