What is the typical appearance of mixed-field agglutination?
The appearance of mixed-field agglutination (MFA) varies according to the relative percentages of red cells positive for the antigen being typed.
If the red cells positive for the antigen are in the minority, the MFA may be apparent only microscopically. In such cases, the few agglutinates appear in a sea of negative cells. If visible macroscopically, the reaction may appear to be very weak or grainy.
If the red cells positive for the antigen are in the majority, the MFA will be apparent macroscopically. The reaction (with anti-A or anti-B, for example) will be 3+ or 4+ but the supernatant will be cloudy instead of clear. Such MFA reactions are easy to miss in ABO grouping if technologists do not take care to observe the supernatants of cell buttons for cloudiness.