Peak flow measurement: technique and explanation
PLAB 2 & PRES 3 · practical skills / respiratory · easy · 8 minutes
Candidate brief
You are an FY2 doctor in a GP surgery. Miss Aisha Rahman, 19, has asthma and attends for review. Her GP wants her peak flow technique checked and a reading recorded today, because her reliever use has been increasing. Her predicted peak flow is about 450 L/min; her personal best, recorded last year, is 430 L/min. Please teach and check her peak flow technique, obtain and record the readings, and explain the results and plan to her. Verbalise your reasoning as you go.
What examiners look for
Peak flow is a technique test disguised as a measurement. Incomplete inhalation, a loose seal, or a slow blow will halve the number. The clinician's first job is to coach, not to record — an uncached first blow of 280 in this patient was a skills gap, not an exacerbation. Best of three, and best against HER best. Personal best is the anchor that matters for an individual patient — 420 against a best of 430 tells you far more than 420 against a textbook predicted value of 450. Series of readings (a peak flow diary) beat single readings for monitoring. A normal peak flow does not equal controlled asthma. Control is a history: reliever frequency, night waking, activity limitation, exacerbations. This patient's number was nearly normal on the same day her control history was poor — candidates who ask the questions get the diagnosis; candidates who worship the number miss it. In acute attacks, peak flow is expressed as a percentage of best or predicted: below 50% suggests a severe attack and below a third life-threatening (alongside clinical features) — which is exactly why every patient with asthma should know their personal best and the team should record it. Infection control is simple and examined: new single-patient mouthpiece or filter for every patient, hands washed. A meter passed mouth-to-mouth around a clinic is a genuine cross-infection risk. Preventer adherence is usually a routine problem, not a defiance problem: a student who forgets a twice-daily invisible medicine and reaches for the one that works in two minutes is behaving rationally. Fix the routine (link to brushing teeth, set phone reminders), check inhaler technique, and the reliever use follows.
Guideline references
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