If you’re living with a lung condition like asthma, COPD, bronchiectasis, or aspergillosis, you may be asked to have a lung function test. These tests help your medical team understand how well your lungs are working — and how best to treat you.
🌬️ What Are Lung Function Tests?
Lung function tests (also called pulmonary function tests) measure:
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How fast and deeply you can breathe
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How well your lungs move oxygen into your blood
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How much air gets trapped in your lungs
🔬 Main Tests and What They Measure
| Test | What It Measures | Why It Matters |
|---|---|---|
| Spirometry | Speed and volume of air breathed out | Shows obstruction or reversibility (e.g. in asthma) |
| Lung volumes | Total size of the lungs and air left after exhaling | Detects air trapping and hyperinflation |
| Gas transfer (DLCO) | How well gases pass from lungs to blood | Can show scarring, emphysema, or CPA |
| Bronchodilator test | Measures response to inhaler | Helps diagnose asthma or ABPA |
🧠 What Do These Tests Tell Us?
Here’s how different conditions show up in lung function testing:
🟢 Asthma
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Airways are narrowed, but often improve with inhalers.
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Shows obstruction that improves significantly after a bronchodilator (reversible).
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Lung volumes usually normal; DLCO often normal or high.
🔵 COPD
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Shows irreversible obstruction — lungs are stiff and narrowed.
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Often shows air trapping and hyperinflation.
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DLCO is reduced, especially in emphysema.
🟠 Bronchiectasis
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May show mild-to-moderate obstruction or mixed patterns.
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Sometimes reversible, depending on overlapping asthma or infection.
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Lung volumes and DLCO often normal unless CPA or emphysema is also present.
🟣 Aspergillosis
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ABPA: Like asthma — obstruction, often with reversibility, air trapping, high IgE and eosinophils.
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CPA: May cause a restrictive pattern (lower lung volumes) or reduced DLCO if fibrosis or cavities are present.
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Aspergillus bronchitis: May look like bronchiectasis with some obstruction.
💨 What Is Air Trapping?
Air trapping happens when you breathe in, but can’t get all the air back out. Some air stays stuck in the lungs.
| Sign | What It Means |
|---|---|
| High residual volume (RV) | Too much air remains after breathing out |
| Expiratory CT scan shows dark areas | Parts of the lungs aren’t emptying properly |
Common in:
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Asthma
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ABPA
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Bronchiectasis
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COPD
🎈 What Is Hyperinflation?
Hyperinflation means your lungs are constantly overfilled with air — not just trapped temporarily, but stretched long-term.
| Sign | What It Means |
|---|---|
| High total lung capacity (TLC) | Lungs are too large due to long-term air trapping |
| Flattened diaphragm | Lungs are pressing downward on breathing muscles |
Common in:
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Severe asthma
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Emphysema (COPD)
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ABPA (when uncontrolled)
📊 Summary Table
| Condition | Common Lung Test Features |
|---|---|
| Asthma | Obstruction, reversible, normal DLCO |
| ABPA | Obstruction, reversible, air trapping, high IgE |
| COPD | Fixed obstruction, hyperinflation, low DLCO |
| Bronchiectasis | Variable obstruction, sometimes reversible |
| CPA | Restriction or mixed pattern, reduced DLCO |
| Aspergillus bronchitis | Similar to bronchiectasis, sputum positive for fungus |
💬 Why These Tests Matter
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They help distinguish what’s causing your symptoms.
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They track whether treatment is working (inhalers, steroids, antifungals).
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They identify complications like scarring, cavities, or fungal infection.
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They help guide eligibility for biologic drugs or oxygen therapy.
📣 Ask Your Doctor:
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Have I had a bronchodilator response test?
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Do I have air trapping or hyperinflation?
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Should we test for Aspergillus (IgG, IgE, sputum)?
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Would a CT scan add helpful detail?
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