In my allergy report histamine reading is given 5 stars. What does this mean?

Test Control. Histamine is used as a control on allergy tests. Meaning we expect you to have a reaction to histamine because that's the chemical that causes the symptoms seen on the skin in an allergic reaction and the presence of a reaction to it means your immune system is working properly by reacting to this chemical in the way we would expect it to. It makes your allergy test results valid.