A method of determining a potential of a diabetic patient to benefit from anti oxidant therapy for treatment of a vascular complication, the method comprising determining a haptoglobin phenotype of the diabetic patient and thereby determining the potential of the diabetic patient to benefit from said anti oxidant therapy, whereby a patient having a haptoglobin 2-2 phenotype benefits from anti oxidant therapy more than a patient having a haptoglobin 1-2 phenotype or a patient having a haptoglobin 1-1 phenotype