A positive or abnormal Coomb's test indicates antibodies in the blood that fight against the red blood cells, according to MedlinePlus. These antibodies form for several reasons, including a mismatched transfusion, hemolytic anemia, lupus and leukemia. Newborn babies sometimes test positive due to differences in the infant and mother's blood.
Before a person receives a blood transfusion in 2015, most medical facilities match the transfusion blood with the patient's blood. With a mistake in matching blood, the body's immune system sees the transfused blood as a threat. It forms antigens to attack the red blood cells from the transfusion. This condition leads to serious consequences or even death, according to WebMD.