In this case the repeated ABO grouping confirmed the original results. The technologist also contacted the transfusion service where the patient was admitted after the accident. The pretransfusion ABO and Rh results were as follows:

ABO and Rh Typing (Pretransfusion)

anti-A anti-B A1 cells B cells anti-D
w+ 0 1+ 4+ 3+

Because of these results the patient had been transfused with 4 units of group O Rh positive red cells (one month previously).

Investigating possible anti-A1

The weak reaction with anti-A on the pretransfusion sample suggest that the patient may have a weak subgroup of A with anti-A1 present as an extra antibody. Tests at room temperature with A1, A2, and group O red cells, as well as an autocontrol, could confirm the hypothesis of anti-A1 and exclude other unexpected cold antibodies such as anti-M and autoanti-I.

See results of mini-panel at room temperature.

Further ABO grouping tests

The patient's ABO group was repeated using washed red cells and prewarm technique. Anti-A,B was also included, although its usefulness is limited since current monoclonal ABO typing sera can detect most weak subgroups of A and B. An additional test that can help resolve discrepancies due to weak antigens (not done in this case) is performing the test at 4o C with a prolonged incubation and suitable controls (patient autocontrol and negative control of group O cells with ABO typing sera).

anti-A,B* anti-A* Anti-B* A1 cells B cells
2+MF w+MF 0 0 4+

*monoclonal ABO typing sera

Results indicate that the anti-A1, like almost all examples of this antibody, does not react at 37o C and appears to be clinically insignificant.

Antigen phenotyping

The patient's red cells were tested with the lectin Dolichos biflorus (anti-A1) and were negative, as expected:


A1 Phenotyping
Cells Anti-A1
Patient 0
Positive control (A1) 4+
Negative control (A2)




Mixed field agglutination

In the front group, mixed field agglutination is likely due to the transfusion with 4 group O cells one month ago. If required (which is unlikely), to differentiate transfusion-related mixed-field from that associated with a weak subgroup of A, the agglutinated cells could be removed and the non-agglutinated cells again tested with anti-A. If due to a subgroup such as A3, mixed-field agglutination should again occur in the previously unagglutinated cells.>

Complex testing

To resolve an ABO discrepancy involving weak subgroups with the degree of certainty needed to classify the subgroup may require complex testing beyond the scope and expertise of many transfusion services. Investigations may involve adsorption and elution of anti-A or anti-B, testing for salivary ABH antigens, and more. Since designating the exact subgroup is of academic and not clinical relevance, most laboratories do not do such tests.

Remember: When in doubt, transfusing group O red cells and group AB plasma is the safest.