The value of serology in diagnosis is clearly related to the expected immune status of the affected birds. HI
titres may be regarded as being positive if there is inhibition at a serum dilution of 1/16 (24 or log2 4 when
expressed as the reciprocal) or more against 4 HAU of antigen. Some laboratories prefer to use 8 HAU in HI
tests. While this is permissible, it affects the interpretation of results so that a positive titre is 1/8 (23 or log2
3) or more. Back titration of antigen should be included in all tests to verify the number of HAU used.