The HI test is primarily used to determine if antibodies indicating influenza A virus infections are
subtyped as H5 and H7. 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. The meaning of a minimum positive titre
should not be misinterpreted; it does not imply, for example, that immunised birds with that titre will be
protected against challenge or that birds with lower titres will be susceptible to challenge. Appropriate
virus/antigen control, positive control serum and RBC control well should be included with each batch
of HI tests.