An important method for controlling the spread of the h6n2 influenza (bird flu) virus in chickens is having a procedure to determine whether chickens are infected with the virus. it is common to apply a procedure, called an elisa test, to measure the concentration of anti–bird flu antibodies in a blood sample taken from a chicken. if the elisa test reveals a high-enough concentration of antibodies, the chicken is said to test positive, and it is classified as infected with the virus. otherwise, the chicken is said to test negative, and it is classified as not infected. however, the elisa test is a complex procedure that is not always accurate. one type of mistake, a false positive result, occurs when the elisa test gives a positive result for a chicken that is not infected with the virus. a second type of mistake, a false negative result, occurs when the elisa test gives a negative result for an infected chicken.