ABO mixed field must be explained; find out patient transfusion history. If it is not clear what their blood type is, or if the mixed field cannot be explained (patient intubated, confused etc.) document and give type O.
Interpretation of mixed field in gel is easy, harder in tube but I would expect it to be there. I would therefore suggest checking very carefully for mixed field by tube (this may be an occasion to use a microscope to confirm mixed field if needed). Sounds like this is a good sample to use for mixed field training in your lab.