To detect the AIDS-causing virus using the new method, researchers add serum from a patient's blood sample to a solution of gold nanoparticles. If the nanoparticles come into contact with an HIV biomarker called p24, they clump together into an irregular pattern that turns the mixture blue--indicating a positive test result. If p24 is absent, the gold nanoparticles separate into ball shapes, and the mixture turns red, signaling a negative result.