Hmmm... Never have used the prewarm technique to resolve a forward type. Just "thinking out loud": In the scenario of a cold interfering autoantibody, prewarming is typically used to prevent "free" cold autoantibody from binding onto red cells causing unwanted reactivity. In testing a patient's red cells, there should be no "free" autoantibody present if the sample was adequately washed. The issue with the forward type is the antibody that is already bound to the patient's cells. If warm washing did not remove it, then I'm not sure how prewarming the forward type would be more effective. If it works though, great! Whatever it takes to resolve ABO discrepancies due to cold autoantibody interference, I'm for it!
We warm wash the patient's cells with 37C saline. If needed, move to a form of heat elution (incubate or even wash with 40-45C saline) and if that's not effective, treat the cells with 0.01M DTT (treated controls must be included). Reverse type resolution: I- cells, prewarm at 37C spin, and just recently, resolved a back type issue with testing at 37C/AHG with heavy chain specific anti-IgG.
I'm sure I don't need to say this, but I'm going to anyway: In a situation where you have an unresolved ABO discrepancy and you have to transfuse, don't guess. Give group O red cells.