A 6-year-old boy has a 1-year history of moderate persistent asthma that is normally well controlled with budesonide via dry powder inhaler (DPI) twice a day and the use of albuterol once or twice a week as needed for wheezing. Three days ago, he developed a sore throat, clear nasal discharge, and a dry cough. In the past 24 hours, he has had intermittent wheezing, necessitating the use of albuterol two puffs with use of an age-appropriate spacer every 3 hours) with partial relief. Your next most appropriate action is to obtain:
A. a chest radiograph.
B. an oxygen saturation measurement.
C. a peak expiratory flow (PEF) measurement.
D. a sputum smear for WBCs.

The peak expiratory flow (PEF), or peak expiratory flow rate (PEFR), is a person's maximum speed of expiration, as measured with a peak flow meter, a small, hand-held device used to monitor a person's ability to breath out air. It measures the airflow through the bronchi and thus the degree of obstruction in the airways. Peak expiratory flow is typically measured in units of liters per minute (L/min).

Measurements may be based on 1 second or less but are usually reported as a volume per minute. Electronic devices will sample the flow and multiply the same volume (Liters) by 60, divided by the sample (seconds) for a result measured in L/minute: (sample volume*60)/sample time.

Peak flow readings are often classified into 3 zones of measurement according to the American Lung Association, green, yellow, and red.

Green zone: 80 to 100 percent of the usual or normal peak flow readings are clear. A peak flow reading in the green zone indicates that the asthma is under good control.

Yellow zone: 50 to 59 percent of the usual or normal peak flow readings. Indicates caution. It may mean respiratory airways are narrowing and additional medication may be required.

Red zone: less than 50 percent of the usual or normal peak flow readings. Indicates medical emergency. Severe airway narrowing may be occurring and immediate action needs to be taken. This would usually involve contacting a doctor or hospital.