Can an allergy test determine how badly (the severity) allergic you are to something?

Unfortunately no: Great question! the results of allergy skin testing or ige blood testing for foods is not related to the severity of a reaction, but instead the likelihood (risk) of having a reaction. For environmental allergies, the larger the skin test the more likely that it is clinically significant (true positive) but again does not necessarily mean the reaction the patient experiences will be more severe.