How to use a peak flow meter: posture, best of three blows, and how to record results

What can a peak flow meter show you?

A peak flow meter is a small handheld device. You blow into it hard, and it gives a number that shows how easily air is moving through your airway at that moment. It cannot be used to tell whether you have asthma, and it does not replace lung function tests done by a doctor. Its value is in helping you see trends over time.

Airway narrowing in asthma is usually reversible and comes and goes. A single reading is affected by many things, but if you record it over days and weeks, you can see whether your airway is narrowing. For example, before a cold or when the seasons change, or after contact with a trigger, the number may drop first and symptoms may appear later. This early warning can help with day-to-day management and with conversations at follow-up visits.

How should you stand and hold the mouthpiece before blowing?

Before measuring, sit or stand up straight. Do not slouch. Loosen your collar and belt so your chest can open fully. Breathe calmly a few times, then take a deep breath in as far as you can.

Seal your lips tightly around the mouthpiece so no air leaks out. Blow fast and hard, as if blowing out a candle far away, and keep blowing until you have emptied your lungs. Do not take another breath during the blow, and do not pinch your nose unless the instructions say to. After blowing, relax and breathe normally.

Why blow three times and take the best value?

You blow three times for each measurement because a single blow is easily affected by how hard you blow, your posture, or whether air leaks, so the number can vary. Taking the best of the three values gets closer to how open your airway really is at that time.

You can rest for ten seconds or so between blows. Do not blow hard over and over, as tiredness can affect the later blows. If the three values are very different, or if you feel dizzy or have chest tightness after blowing, stop and rest. Do not push through it. When recording, write down only the best value, not the average of the three.

What should you write in your record, and how do you review it with your doctor?

At a minimum, record the date, time and best value. You can also note briefly whether you had a cough, wheeze or chest tightness that day, whether you had a cold, were exposed to pollen or cold air, or missed any medicine. These notes help your doctor understand why the numbers changed.

Take your record to follow-up visits and look at the trend with your doctor, rather than looking at one day's number alone. Your doctor will consider your symptoms, your medicines and your record to judge whether your airway is stable. Do not change your medicines on your own based on the numbers. Any change should be decided by your doctor.

When should you see a doctor?

If you notice any of the following signs, do not rely on the peak flow meter alone. See a doctor as soon as possible:

• Wheeze, chest tightness or cough clearly getting worse and not easing with rest.
• Peak flow values that keep falling, or that are much lower than your usual best.
• Needing to use reliever medicine often, or it working less well than before.
• Being woken at night by coughing or wheezing, which affects your sleep.
• Bluish lips or nails, difficulty speaking, or difficulty breathing.

These signs suggest your airway may be clearly narrowed and needs a doctor's assessment. If you have difficulty breathing or difficulty speaking, seek emergency medical help as soon as possible.

This article is for general information only and is not medical advice. Please follow your doctor's instructions.