Table 3. Investigations for asthma.

Investigation	Description
Demonstration of airway obstruction
Spirometry	A FEV1 (forced expiratory volume in 1 second)/FVC (forced vital capacity) ratio of <70% is a positive test for obstructive airway disease.
Demonstration of airway obstruction variability
Bronchodilator reversibility	An improvement in FEV1 of ≥12% AND ≥200 ml is a positive bronchodilator reversibility test.
Other method	An increase in FEV1 >12% and >200 ml [or peak expiratory flow (PEF) >20%] from baseline after four weeks on inhaled corticosteroids (ICS) is a positive test. Patient must not have respiratory infections.
Peak flow charting	Peak flow monitoring over 2 – 4 weeks. Calculate mean variability. Daily diurnal PEF variability is calculated from twice daily PEFs as [(day's highest — day's lowest)/mean of day's highest and lowest] and averaged over one week. Variability ≥20% or diurnal variation >15% on >3 days/week indicates a positive test.