My peak flow is normal but the doctor says i have asthma. how?

Smallairways blocked: It is good your peak flow is normal as it means your large airways are clear and you have good respiratory muscle strength. Asthma causes inflammation/spasm/secretions especially in the smaller airways of 2 mm in diameter or less. Lung testing can pick this up by checking flow rates after blowing out about 1/2 of the air in your lungs. Using medications clears small airways and reopens lungs.

Reason....: When you are not having an exacerbation of asthma, your peak flow will be entirely normal because asthma consists of episodic obstruction. During an acute exacerbation, your peak flow will drop but between episodes, it will be normal.