What is the definitive test for asthma or c o p d ?

Flow Spirometry : Asthma and COPD (chronic bronchitis and emphysema) are constrictive lung diseases defined by a decreased ability to exhale air. A physician, typically a pulmonologist, can administer a flow spirometry test to measure the volumes of air during inspiration and expiration. The hallmark of constrictive lung disease is a decreased fev1/fvc, or a decreased volume during the first second of exhalation.

PFT: Pulmonary function testing with bronchodilators. Spirometry is only a part of this. Complete pfts give some more useful information, esp in copd. Also, asthma doesn't always show up, you may need a methacholine challenge.

15 to 20% reversible: Reversibility of airway spasm is the test. A 15 to 20% improvement in airway narrowing seen in fev1 test after administration of a bronchodilator is considered the suggestive of asthma. Lung tests (spirometry) showing airflow limitation (fev1/fvc less than 70%) plus fev1 less than 80% that is incompletely reversible after administration of bronchodilator is supportive of the diagnosis of copd.