Asthma - spirometry features are

In asthma,FEV will be decreased with decreased FEV1/FVC ratio.RV will be increased due to exaggerated airway closure at low lung volumes,and this elevation of RV reduces FVC.Peak flow meter shows reduced PEF.A diurnal variation in PEF of more than 20% is considered diagnostic of asthma. Reference:Harrison's medicine-18th edition,page no:2094;Davidson's medicine-22nd edition,page no:668.