You're on a long run and half way through want to check your electrolyte levels. Or you take a powerful cholesterol lowering drug and need a regular liver enzyme test. Wouldn't it be great if you had a portable lab ' like diabetics with their blood glucose meters?

Except, the latest invention is a lab on paper the size of a stamp. Dr. George Whitesides of Harvard University says paper tests aren't new because for decades women have used pregnancy tests. But unlike the stick, his "stamp-lab" can detect complex diseases instantly at a penny a piece, and like the home pregnancy test, do it reliably without a doctor to interpret the results.

Whitesides uses microfluidics, which is the science of moving fluids on a tiny scale. Surprisingly, his paper test isn't made with much more than filter paper and an office printer.

A modified printer can copy over one hundred patterns per sheet using melted black wax. The dried patterns become channels that guide fluid samples along the test strip. In these channels chemicals are applied which react in the presence of specific biological substances.

One particular test detects aspartate transaminase or AST, which is released by dying liver cells. As a drop of blood is applied to the test strip, an outer membrane allows only plasma through.

Several pieces of the test paper are actually stacked and taped, so when the plasma enters the second layer and mixes with chemicals there, a reaction takes place if AST is present. If so, the third layer containing a normally blue dye, turns colorless ' testing positive for liver damage.

This test is aimed at the millions of Africans whose drug therapies for AIDS and tuberculosis can damage the liver. But, once the tests can diagnose everything from malaria to maybe even cancer, people won't have to wait until they're too sick before seeing the doctor.