Mixed field agglutination: Unusual causes and serological approach

Mixed field agglutination in serology usually describes the presence of two populations of red cells. Here we present images with two uncommon reasons for mixed field reaction. In both the cases, we observed mixed field reaction during pretransfusion testing using column agglutination technology [Figure 1]. Relevant patient's history was obtained and further immunohematological work-up was carried out in both the cases to look for the possible cause. In the first case, there was no history of blood transfusion, direct antiglobulin test was negative. Antibody screening cells showed pattern and the antibody identified was anti-M. However, all the panel cells with heterozygous expression of M antigen showed mixed field agglutination. Presence of anti-M antibody was confirmed by enzyme treatment and phenotyping the patient for M antigen. In second case, uniform mixed field agglutination was observed with the antibody screening cells including the autocontrol. Direct antiglobulin test was negative. However, we suspected the interference of test results by fibrin residues. To confirm and to resolve the problem, we incubated patient's clotted sample at 37°C for 30 min and used well-separated serum sample for repeat antibody screening. As shown in the image of the second case, mixed field reaction resolved completely and on cross-matching the problem did not recur.

Figure 1.

Mixed field agglutination: Unusual causes and serological approach

Knowing all the possible causes of mixed field reaction would help to resolve and interpret the results accordingly. Anti-M is generally a cold reacting antibody and its reaction strength may vary because of dosage. Antibodies to Lutheran and Sd^a antigens may show mixed field appearance but it is unusual for anti M antibodies.[1] However in the present case, mixed field reaction was noticed only with the panel cells having heterozygous expression of M antigen.

Serum separation though a very simple technique has a great importance on the outcome of the immunohematological results. As illustrated in this case, failure to adhere to proper serum separation technique would unnecessarily create confusion and delay the transfusion support to the patient.