Natural disasters and other emergencies often result in the need for blood transfusions. But they also can result in a loss of electricity, which can make it impossible to perform the tests required to determine a patient’s blood type. Now researchers in China may have an alternative: a cheap, rapid blood type test made of color-changing paper. They reported their progress in the journal Science Translational Medicine.

Each of the eight blood types has its own antigens and antibodies (either type A or type B), which help the immune system defend against unwelcome interlopers. Injecting a patient with incompatible antigens and antibodies causes the immune system to attack, making a person much sicker.

The researchers’ new blood type test works by identifying which antibodies and antigens trigger this immune attack. They mixed dye into two solutions, one containing antibody A and one containing antibody B, and printed small squares of the dye mixture onto either end of a long strip of paper.

Zhang et al. 2017. Science Translational Medicine.

To test a patient’s blood, they squeezed a few drops into the center reservoir. That blood then seeped through the paper, spreading toward the antigens at either end. Different blood types react differently to the antigens, causing the dye to turn either teal or brown. The entire process takes less than two minutes.

The researchers used both their new paper test and the current time- and electricity-intensive method to test 3550 different samples. The little piece of paper was astonishingly on-point, reaching the same conclusions as the electronic test 99.9 percent of the time.

More experiments are needed before the paper test will be ready for prime time, but it’s a very promising start. Along with recently developed paper microscopes and paper centrifuges, this cheap test could do a world of good in the places that need it the most.

[h/t Popular Science]