Predicted Forced Expiratory Volume in 1 second (FEV1)
FEV1 is the maximal amount of air you can forcefully exhale in one second. It is then converted to a percentage of normal. For example, your FEV1 may be 80% of predicted based on your height, weight, and race.
FEV1 is a marker for the degree of obstruction in your lungs:
FEV1 greater 80% of predicted = Normal
FEV1 60% to 79% of predicted = Mild obstruction
FEV1 40% to 59% of predicted = Moderate obstruction
FEV1 less than 40% of predicted = Severe obstruction
You can calculate your predicted FEV1 using this website or you can ask your doctor. Remember that you have to enter the value in units of Litres in 1 second. The number should be in the range of: 1 – 6.
If in doubt about your peak flow readings, contact your medical team for advice.