Allergy to milk protein is usually not evident from routine blood tests. To diagnose an allergy to a substance or to find an allergen, an allergen test is required. Allergen testing requires blood sampling. Routine blood tests can check for viral or bacterial infections, hemoglobin levels or platelet counts, etc. Finding allergens requires special reagents. People who are allergic to milk protein should avoid diets containing milk protein, choose other sources of protein, and take probiotic supplements to change their allergic constitution.