How to Track Peak Flow Against Your Personal Best

Peak expiratory flow (PEF), or peak flow, is the fastest speed at which you can blow air out of your lungs, in litres per minute (L/min). A cheap handheld meter measures it. For people with asthma, it’s most useful compared with their own personal best: a fall from your best can show your airways narrowing before you feel it, and asthma action plans set their zones as a share of that best.

This article is general information, not medical advice. Your own action plan, from your doctor or nurse, comes first.

Finding Your Personal Best

Your personal best is the highest peak flow you reach when your asthma is well controlled. To find it:

1. Measure twice a day, morning and evening, for two to three weeks while you feel well.
2. Each time, take three blows and record the highest.
3. Your personal best is the highest reading over that period.

Predicted values from charts by age, height, and sex exist, but people vary so much that your own best is the better reference. Recheck it every year or so, and when you start using a new meter, since meters differ.

How to Blow

• Stand or sit up straight, and reset the marker to zero.
• Take the deepest breath you can, seal your lips around the mouthpiece, and blow out as hard and fast as you can, a short blast rather than a long breath.
• Repeat three times and record the highest, not the average.

The Zones

The US National Heart, Lung, and Blood Institute’s (NHLBI) asthma action plan uses three zones, by your peak flow as a share of your personal best:

Zone	Peak flow	Meaning
Green	80% to 100%	Doing well: take your usual controller medicine
Yellow	50% to 79%	Asthma is getting worse: follow your plan’s steps, often extra reliever
Red	Under 50%	Medical alert: take your reliever and get help now

Your own plan may use different numbers. Danger signs, such as trouble walking or talking because of breathlessness, or blue lips or fingernails, mean emergency care whatever the meter shows.

Reliever Use Is a Signal Too

How often you need your reliever inhaler is one of the measures the Global Initiative for Asthma (GINA) uses to judge asthma control. Needing it more than usual, or peak flows that swing widely between morning and evening, are signs your asthma isn’t well controlled and are worth discussing with your doctor.

Following It in Kaizora

Peak Expiratory Flow and Inhaler Puffs are on the Vitals page. Log readings in L/min, or bring them in from Apple Health, which also carries inhaler use from apps and smart inhalers that save it. Add your own reference ranges at 80% and 50% of your personal best, as mild and severe, so yellow- and red-zone readings are flagged and notify you. On Correlations, peak flow beside inhaler puffs by day shows whether lower readings and more reliever use go together. See Reference Ranges.

The Short Version

• Peak flow is the fastest you can blow out, in L/min.
• Compare it with your own personal best, found when you’re well.
• 80% or more of your best is green, 50% to 79% yellow, under 50% red, in NHLBI’s plan.
• Rising reliever use is a warning sign alongside peak flow.

Sources

• National Heart, Lung, and Blood Institute, Asthma Action Plan.
• Global Initiative for Asthma, GINA Reports.