In recent years, attempts were made to improve the sensitivity of the HI test. More sensitive detection of anti-body to avian HA was seen when horse erythrocytes were used in place of turkey erythrocytes in the HI test because infl uenza virus was better able to bind to a2,3Gal-specifi c receptor sites on these erythrocytes (35) . The presence of asparagines at aa223 (H5 numbering) in H5 HA leads to improved sensitivity of the HI test (22) .