Spirometry Interpretation Basics: Obstructive vs Restrictive Patterns

Spirometry reports can look complicated, but a structured approach makes interpretation straightforward.

Step 1: Check quality

Good interpretation depends on good tests. Look for:

• A rapid, maximal start with no hesitation.
• No cough in the first second.
• Exhalation continued to a plateau.
• At least three acceptable manoeuvres, with the best FVC and FEV1 values within about 150 mL of each other (repeatability).

Software such as SPIROEXCELL includes encouragement screens and quality prompts.

Step 2: Look at FEV1/FVC

• Reduced ratio → obstruction (asthma, COPD, bronchiectasis).
• Interpret against the lower limit of normal (LLN) for age, sex and height. GOLD uses a fixed post-bronchodilator ratio below 0.70 to diagnose COPD.

Step 3: Look at FVC

• Reduced FVC with normal or high ratio → possible restriction (interstitial lung disease, chest wall or neuromuscular disease, obesity). Confirm with lung volumes.
• Both ratio and FVC reduced → mixed pattern or air trapping.

Step 4: Grade severity

Use FEV1 % predicted or z-scores according to the guideline your lab follows.

Step 5: Bronchodilator response

Repeat after a bronchodilator. A significant improvement supports asthma. Current ERS/ATS interpretation guidance expresses the change relative to predicted values, while older practice used ≥12% and ≥200 mL in FEV1 or FVC.

Flow-volume loop clues

• Concave (scooped) expiratory limb: obstruction.
• Small, narrow loop with preserved shape: restriction.
• Flattened loops: possible upper-airway obstruction.

Prediction equations matter

Choosing appropriate reference equations is crucial. See prediction equations for Indian patients.

Need a spirometer with automatic interpretation? See the PFT range.