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.
Family history is a risk factor for asthma, with many different genes being implicated.  If one identical twin is affected, the probability of the other having the disease is approximately 25%.  By the end of 2005, 25 genes had been associated with asthma in six or more separate populations, including GSTM1 , IL10 , CTLA-4 , SPINK5 , LTC4S , IL4R and ADAM33 , among others.  Many of these genes are related to the immune system or modulating inflammation. Even among this list of genes supported by highly replicated studies, results have not been consistent among all populations tested.  In 2006 over 100 genes were associated with asthma in one genetic association study alone;  more continue to be found. 
Walter Robinson, ., .
Senior Research Scientist
Center for Applied Ethics
Education Development Center, Inc.
Associate Professor of Pediatrics
Department of Pediatrics
Division of Pediatric Allergy, Immunology, and Pulmonary Medicine
Associate Professor of Medical Ethics
Center for Biomedical Ethics and Society
Vanderbilt University School of Medicine