Introduction
Peak flow and spirometry OSCEs test your ability to demonstrate correct technique, interpret the results, and explain findings to a patient. Distinguishing obstructive from restrictive patterns and performing reversibility testing are the highest-yield interpretive skills.
💎 Clinical Pearl
Always demonstrate the technique yourself before asking the patient to do it. "I'm going to show you what I'd like you to do — take the biggest breath in you possibly can, then blow out as hard and fast as you can, like you're blowing out birthday candles."
Part 1: Peak Expiratory Flow Rate (PEFR)
What It Measures
The maximum flow rate achieved during forced expiration from full inspiration. Measured in L/min. Reflects large airway calibre.
Technique
- 1Stand the patient upright (or seated upright if unable to stand)
- 2Ensure the peak flow meter pointer is at zero
- 3Patient takes the deepest breath possible
- 4Seal lips tightly around the mouthpiece — no air leakage
- 5Blast out as hard and fast as possible (a short sharp burst, not a long slow blow)
- 6Record the value; repeat twice more; take the best of three readings
- 7Compare to predicted value (age, sex, and height nomogram) and the patient's personal best
Interpreting PEFR
| PEFR % of predicted (or personal best) | Severity |
|---|---|
| Over 80% | Mild |
| 50-80% | Moderate |
| 33-50% | Severe (acute attack if asthma) |
| Under 33% | Life-threatening |
Uses
- Monitoring asthma (serial measurements twice daily over 2 weeks to demonstrate diurnal variation)
- Acute asthma severity assessment
- Occupational asthma (peak flow diary at work vs away)
Part 2: Spirometry
Key Measurements
| Measurement | Definition |
|---|---|
| FVC | Forced vital capacity: total volume expelled from full inspiration to full expiration |
| FEV1 | Forced expiratory volume in 1 second: volume expelled in the first second |
| FEV1/FVC ratio | The proportion of FVC expelled in the first second; normal is above 0.7 (70%) |
| TLC | Total lung capacity (requires body plethysmography) |
Technique
- 1Calibrate the spirometer
- 2Patient sits upright; apply nose clip
- 3Patient breathes normally for a few breaths (tidal volume), then on command takes the deepest possible breath in
- 4Patient then blows out as fast and hard as possible until no more air can be expelled (at least 6 seconds)
- 5Perform at least 3 technically acceptable manoeuvres; take the best two that are within 150 mL of each other
Interpretation: Obstructive vs Restrictive
🧠 Mnemonic
Obstructive = Obstruction slows flow (low FEV1, normal or high FVC, low FEV1/FVC ratio)
Restrictive = Restricted volume (low FVC, low FEV1, normal FEV1/FVC ratio)
| Pattern | FEV1 | FVC | FEV1/FVC |
|---|---|---|---|
| Normal | Normal (over 80% predicted) | Normal | Over 0.7 |
| Obstructive | Reduced | Normal or increased | Under 0.7 |
| Restrictive | Reduced | Reduced | Normal or above 0.7 |
| Mixed | Reduced | Reduced | Under 0.7 |
Common Causes
| Pattern | Conditions |
|---|---|
| Obstructive | Asthma, COPD, bronchiectasis, obliterative bronchiolitis |
| Restrictive | Pulmonary fibrosis (IPF), pleural effusion, obesity, neuromuscular disease (motor neurone disease, myasthenia), kyphoscoliosis, sarcoidosis |
Reversibility Testing
Give 400 micrograms of salbutamol via MDI and spacer; repeat spirometry after 15 minutes.
| Reversibility result | Definition | Interpretation |
|---|---|---|
| Significant reversibility | FEV1 increases by over 200 mL AND over 12% | Suggests asthma (reversible airflow obstruction) |
| Minimal reversibility | Less than 200 mL or 12% improvement | Suggests COPD (fixed airflow obstruction) |
🧠 Mnemonic
Asthma = Reversible. COPD = Fixed.
In practice there is overlap, but a significant bronchodilator response strongly favours asthma.
COPD Severity Classification (GOLD)
| Stage | FEV1 % predicted (post-bronchodilator) |
|---|---|
| GOLD 1 (Mild) | Over 80% |
| GOLD 2 (Moderate) | 50-80% |
| GOLD 3 (Severe) | 30-50% |
| GOLD 4 (Very severe) | Under 30% |
All stages require FEV1/FVC under 0.7 to confirm obstruction.
"What is the FEV1/FVC ratio and what does a value below 0.7 indicate?"
The FEV1/FVC ratio is the proportion of the forced vital capacity that can be expelled in the first second. A normal ratio is above 0.7 (70%). A ratio below 0.7 indicates an obstructive pattern — the airways are narrowed, slowing the rate of expiration. Common causes include COPD and asthma. A reduced FVC with a normal or elevated FEV1/FVC ratio indicates a restrictive pattern.
"How do you differentiate asthma from COPD on spirometry?"
Both asthma and COPD cause an obstructive pattern with FEV1/FVC below 0.7. The key differentiator is reversibility testing: give 400 mcg salbutamol and repeat spirometry after 15 minutes. An increase in FEV1 of more than 200 mL AND more than 12% from baseline indicates significant reversibility, suggesting asthma. A minimal response (less than 200 mL or 12%) suggests fixed airflow obstruction consistent with COPD.
"What is the correct technique for performing spirometry?"
The patient sits upright with a nose clip in place. After normal tidal breathing, on command they take the maximal inspiratory breath possible, then blast the air out as hard and fast as possible until completely empty (at least 6 seconds). At least 3 technically acceptable manoeuvres are performed and the best two within 150 mL of each other are used. Common errors include insufficient inspiration, early termination, air leak around the mouthpiece, and coughing during the manoeuvre.
"What are the causes of a restrictive spirometry pattern?"
Causes of restriction (reduced FVC with normal or elevated FEV1/FVC ratio): pulmonary parenchymal disease (idiopathic pulmonary fibrosis, sarcoidosis), pleural disease (effusion, mesothelioma), chest wall disease (obesity, kyphoscoliosis, ankylosing spondylitis), and neuromuscular disease (motor neurone disease, myasthenia gravis, Guillain-Barre syndrome). TLC is reduced in true restrictive disease, which requires body plethysmography to confirm.
Related guides: Inhaler Technique OSCE | How to Take a Respiratory History OSCE | Respiratory Examination OSCE