Current electrocardiographic algorithms lack sensitivity to diagnose acute myocardial infarction () in the presence of left bundle branch block. A multicenter retrospective cohort study including consecutive patients with suspected and left bundle branch block, referred for primary percutaneous coronary intervention between 2009 and 2018. Pre-2015 patients formed the derivation cohort (n=163, 61 with ); patients between 2015 and 2018 formed the validation cohort (n=107, 40 with ). A control group of patients without suspected was also studied (n=214). Different electrocardiographic criteria were tested. A total of 484 patients were studied. A new electrocardiographic algorithm ( algorithm) was derived and validated. The algorithm is positive in the presence of deviation ≥1 mm (0.1 mV) concordant with polarity, in any lead, or deviation ≥1 mm (0.1 mV) discordant with the , in leads with max (R