Hepatitis C test. Hand holding a vial containing human blood serum to be tested for antibodies to hepatitis C virus. This is a Monolisa EIA (enzyme immunoassay) test, in which a patient's blood serum is added to antigens produced by the virus. Antibodies in infected blood react with the antigens, and an enzyme effects a colour change. The blood serum in the multi-well sample tray at lower left shows a positive result by turning yellow. The four yellow tubes in a row are test controls. The other yellow tube indicates that that blood contains antibodies for the virus. Hepatitis C is a serious liver infection most commonly contracted after blood transfusions.