How is a pe diagnosed definitively, would a blood test give a definite result to determine this?

In short, no: A blood test called a d-dimer will tell you if there is increased likelihood of having a pe or a clot anywhere else if it is elevated. To diagnose, an imaging study is required, either a cat scan of the chest with angiogram or a v/q scan of the lungs to determine if any of the vessels are occluded.

CT scan of the Chest: The most sensitive test to evaluate for pulmonary embolism is a ct angiogram of the chest. Lab work such as a d dimer can help rule out the disease but is faslely positive in many patients that do not have pulmonary embolism or dvt. Since most pulmonary embolisms come from the legs an ultrasound of the veins there is almost always done.