Often, it is difficult to determine what is triggering asthma attacks. Allergy skin testing may be performed, especially if the doctor suspects the child's symptoms are persistent. An allergic skin response does not always mean that the allergen being tested is causing the asthma. Also, the body's immune system produces an antibody to fight off the allergen. The amount of antibody can be measured by a blood test that will show how sensitive the patient is to a particular allergen. If the diagnosis is still in doubt, the patient can inhale a suspect allergen while using a spirometer to detect airway narrowing. Spirometry can also be repeated after a bout of exercise if exercise-induced asthma is a possibility. A chest x ray will help rule out other disorders.