Why is it recommended to record the highest peak flow result and not an arithmetic mean average?

Asthmatics are encouraged to keep a peak flow dairy, which records the scores of their peak expiratory flow measured twice daily; once in the morning and once in the evening. They are instructed to take the measurement three times and record the highest reading.

Why is it preferential to keep the highest reading instead of an an arithmetic mean (which I would find more accurate in this use case)?

Well, PEF (peak expiratory flow) varies thorough the day and it's at its lowest in the morning (which is when patients are usually instructed to take it x3, before their meds), so taking the highest value balances that out a little.

Also, we are measuring PEF i.e. maximum speed of expiration, not average speed of expiration ;) And a patient's personal best (the highest PEF reading they've recorded for a span of 2-3 weeks) is useful for determining asthma zones (green for when a patient's PEF measures 80%-100% of their best value, yellow for 50%-80% & red for <50%), which in turn are needed to make an asthma action plan so that the patient knows when they can treat their symptoms without going to the hospital and when they should. Using the highest reading also gives a little leeway in such cases as ≤80% PEF is not as bad as it could be!

Personal best PEF is also used to determine if a patient's meds need to be changed or the dose increased/reduced, etc.

References:

• I tend to find my three readings vary quite I bit, often one of them is ~20-30 higher than the other two. It made me ask the question whether recording the highest one is correct because although I achieved it once, it doesn't seem to be representative of my "actual" condition at that point in time, more like an outlier. Commented Dec 13, 2018 at 15:48
• @Aesir I think that peak is indeed your "actual" condition though, it's the most you were able to achieve. x3 is probably a balance between tedium and accurate measure (i.e., if you used just 1, your estimated max would often be way lower than your actual max, as you are describing). Commented Dec 13, 2018 at 17:00