Nanoparticles could allow doctors to detect diseases such as HIV, cancer, BSE and Alzheimer's long before they take hold.

Chad Mirkin at Northwestern University in Chicago used nanotechnology, the manipulation of matter at the molecular scale, to develop tests which are a million times more sensitive than those used now.

The tests pick up disease markers -specific proteins found in blood and body fluids when diseases occur. The markers are often too low to detect using conventional tests, or only show up when a disease is advanced.

Prof Mirkin's tests can detect just a handful of proteins in a drop of blood. The tests use two types of nanoparticles, the first magnetic and the second covered in short strands of DNA. When mixed into a blood sample, the particles, which measure just 30 billionths of a metre, latch on to different ends of any disease proteins that are there.

Using a magnetic field, the scientists then pull the magnetised nanoparticles out of the blood, bringing the disease proteins and DNA-covered particles with them.

Prof Mirkin has used the test to detect a protein that is linked to Alzheimer's disease. Currently, Alzheimer's can only be confirmed by testing people's brains once they have died.

Detecting HIV early could vastly improve patient care. Today's tests cannot pick up the HIV virus until around three months after infection.