Knowing a patient’s blood type is critical before a transfusion, but current techniques are time consuming and require expensive equipment. Now, researchers have proposed a new paper-based method for quickly and inexpensively determining blood type, published today in Science Translational Medicine. The method relies on a special dye that shows up brown when mixed with whole blood, but turns teal when mixed with plasma that has been separated from red blood cells. To make that separation happen, scientists employed antibodies that force red blood cells to clump. Blood cells are dotted with markers that can be recognized by antibodies produced by the immune system—type A blood has A markers, type B has B markers, type AB has both, and type O has neither. To test the blood, researchers place a droplet in the center of a paper-based chip that holds anti-A antibodies to the left, and anti-B antibodies to the right. As blood absorbs and moves through the paper membrane toward the ends of the chip, it hits the regions of antibodies, clumping or not, depending on what markers it carries. If the red blood cells clump, only the plasma will make it through, causing the dye to turn teal and indicating that the corresponding marker (A on the left and B on the right) is present. If the marker isn’t there, the blood won’t clump and the dye will be brown. The process takes about 30 seconds and can be modified for rare blood types. The researchers hope the tests will be useful in regions with limited resources and during emergencies.