Bronchial asthma is definitely diagnosed by-

Diagnosis The diagnosis of asthma is predominantly clinical and is based on the combination of the history, lung function and 'other' tests, which allows high, intermediate or low probability of asthma to emerge. The approach may vary from patient to patient and may need to be re-evaluated following the introduction of treatment. Suppoive evidence is provided by the demonstration of variable airflow obstruction, preferably by using spirometry to measure FEV1 and FVC. This identifies the obstructive defect, defines its severity, and provides a baseline for bronchodilator reversibility . If spirometry is not available, a peak flow meter may be used. Symptomatic patients should be instructed to record peak flow readings after rising in the morning and before retiring in the evening. A diurnal variation in PEF of more than 20% (the lowest values typically being recorded in the morning) is considered diagnostic, and the magnitude of variability provides some indication of disease severity . A trial of glucocoicoids (e.g. 30 mg daily for 2 weeks) may be useful in establishing the diagnosis, by demonstrating an improvement in either FEV1 or PEF. It is not uncommon for patients whose symptoms are suggestive of asthma to have normal lung function. In these circumstances, the demonstration of AHR by challenge tests may be useful to confirm the diagnosis . AHR has a high negative predictive value but positive results may be seen in other conditions, such as COPD, bronchiectasis and cystic fibrosis. The use of exercise tests is useful when symptoms are predominantly related to exercise . The diagnosis may be suppoed by the presence of atopy demonstrated by skin-prick tests or measurement of total and allergen-specific IgE, an FENO (a surrogate of eosinophilic airway inflammation) of >=40pas per billion in a glucocoicoid-naive adult, or a peripheral blood eosinophilia. Chest X-ray appearances are often normal but lobar collapse may be seen if mucus occludes a large bronchus and, if accompanied by the presence of flitting infiltrates, may suggest that asthma has been complicated by allergic bronchopulmonary aspergillosis . A high-resolution CT scan (HRCT) may be useful to detect bronchiectasis. Ref Davidson edition23rd pg 569