Drs. Parkman, Meyer, and George L. Stewart, and Hope Hopps, Barbara Meyer, Robert D. Douglas, and Judith P. Hamilton were part of the team who developed a better blood test (rubella hemagglutination-inhibition immunity test) to screen people for rubella antibodies in 1967. By 1970 it was available for use in laboratories and hospitals throughout the United States. The rubella hemagglutination-inhibition immunity test was patented in 1971. The test employs the biological principle of hemagglutination, or red blood cell clumping. Special preparations of rubella virus cause the red blood cells of newly hatched chicks to clump. When a sample of blood from a person who is immune to rubella is added, the antibodies inhibit clumping. Thus, the inhibition of agglutination demonstrates the presence of antibody and immunity. The rubella hemagglutination-inhibition immunity test was used to ascertain whether expectant mothers who have been exposed to rubella have cause for concern or are immune and at no risk. It was also used in testing for rubella susceptibility in women of child-bearing age.