It consists of a small tube with a gauge that measures the maximum force with which one can blow air through the tube. The patient performs the peak flow meter test twice a day for about 2 weeks and records the results for review in a follow up appointment. The first test should be performed after waking in the morning, before taking bronchodilator medications. The patient should perform the peak expiratory flow maneuver 3 times and record the highest measurement. The second test should be done in the afternoon or early evening after taking a bronchodilator.
Peak flows vary during the day and the early morning peak is lower than the evening peak.
Occasionally, a patient with a suspected asthma-related airway obstruction does not demonstrate obstruction in spirometry or peak flow monitoring. In this circumstance, the diagnosis of airway obstruction may be provided by bronchial provocation . Bronchial provocation, also known as bronchoprovocation and bronchial challenge, identifies and characterizes hyperresponsive airways by having the patient inhale an aerosolized chemical, called a broncho-spastic agonist, that triggers a hyperresponsive reaction. The chemicals most often used are histamine and methacholine. Patients perform spirometry without inhaling the agent and then inhale increasingly higher doses of the agent.