Interpretation
The results of the back titration of the diluted antigen and the HI titre of the standard positive are both used to confirm the antigen has been diluted to a concentration equivalent to the standard 4 HA units.
If the HI titre of the positive control serum is less than the standard titre, the antigen is too concentrated. Prepare a new dilution and test again.
Conversely if the HI titre of the positive control serum is too high the antigen is too dilute. Prepare a new dilution and test again.