Can you provide a step‑by‑step explanation of how to read and interpret a spirometry report?

How to Read and Interpret Spirometry: A Step-by-Step Guide

Start by looking at three key numbers: FEV1, FVC, and the FEV1/FVC ratio—these three values will tell you almost everything you need to know about whether the lungs are obstructed, restricted, or normal 1, 2.

Step 1: Check the Quality First

Before interpreting anything, verify the test was done correctly:

• Look at the forced expiratory time (FET) to ensure the patient exhaled long enough 1
• Check that multiple attempts were made and the values are reproducible
• Poor effort shows up as both FEV1 and FVC being low with a normal ratio—this usually means the patient didn't inhale or exhale completely, not actual lung disease 2

Step 2: Look at the FEV1/FVC Ratio (The Most Important Number)

This ratio tells you if there's obstruction:

• Report it as a decimal (e.g., 0.70), not a percentage, to avoid confusion 1
• Obstruction = FEV1/FVC below the lower limit of normal (LLN), typically a z-score below -1.64 or below the 5th percentile 2, 3
• If the ratio is normal or high (>0.70-0.85), there's no obstruction 2

Common Pitfall:

Don't use a fixed cutoff like 0.70 for everyone—this misses obstruction in young people and over-diagnoses it in elderly patients. Always compare to the predicted LLN for that patient's age, height, sex, and ethnicity 2.

Step 3: Determine the Pattern

Pattern A: Obstruction (Low FEV1/FVC ratio)

• FEV1/FVC is below LLN
• Flows are reduced throughout the entire exhalation
• Think: asthma, COPD, bronchitis 2
• The flow-volume curve will look concave (scooped out) 2

Severity grading (based on FEV1 % predicted):

• Mild: FEV1 ≥70%
• Moderate: FEV1 50-69%
• Moderately severe: FEV1 35-49%
• Severe: FEV1 <35% 4

Pattern B: Restriction (Normal or high FEV1/FVC ratio with low volumes)

• FEV1/FVC is normal or increased (>0.85-0.90)
• Both FEV1 and FVC are reduced proportionally
• You MUST measure total lung capacity (TLC) to confirm true restriction—a low FVC alone is NOT enough 2
• True restriction = TLC below the 5th percentile 2
• Think: pulmonary fibrosis, chest wall disease, neuromuscular weakness 2
• The flow-volume curve will look convex (pushed outward) 2

Pattern C: Mixed (Low FEV1/FVC ratio AND low TLC)

• Both obstruction and restriction present
• FEV1/FVC is low AND TLC is low 2
• Think: combined emphysema and fibrosis, or severe COPD with scarring 2

Pattern D: Normal

• FEV1, FVC, and FEV1/FVC all within normal limits (above LLN)

Step 4: Check for Reversibility (If Obstruction is Present)

Give a short-acting bronchodilator and repeat spirometry:

• Significant reversibility = improvement in FEV1 or FVC by ≥12% AND ≥200 mL in adults 3, 4
• In children: improvement by >12% alone is sufficient 4
• Reversibility suggests asthma rather than fixed obstruction like emphysema 2

Step 5: Grade the Severity

Use FEV1 % predicted to determine severity for both obstruction and restriction:

• The lower the FEV1, the worse the disease 4
• Compare the measured FEV1 to what's predicted for that specific patient

Critical Pitfalls to Avoid

1. Don't diagnose restriction from spirometry alone—you need TLC measurement. A low FVC with normal FEV1/FVC is wrong 50% of the time for restriction 2

2. Watch for "pseudo-restriction"—when both FEV1 and FVC are low but the ratio is normal, this is usually poor effort or early airway closure, NOT true restriction 2

3. Use appropriate reference values—wrong reference equations (especially for ethnicity) will give you wrong interpretations 2, 5

4. Look at the flow-volume curve, not just numbers—the shape tells you what's happening:

   • Concave = obstruction
   • Convex = restriction
   • Flat top = upper airway obstruction 2

5. Single-breath TLC measurements (like from DLCO tests) underestimate true TLC in obstructive disease—don't use these to diagnose restriction 2

The Bottom Line Algorithm

1. Is FEV1/FVC low? → YES = Obstruction → Check reversibility and grade severity
2. Is FEV1/FVC normal but FVC low? → Suspect restriction → Measure TLC to confirm
3. Are both FEV1/FVC low AND TLC low? → Mixed pattern
4. Everything normal? → Normal spirometry

Always interpret spirometry in context with the patient's symptoms, examination findings, and other tests—spirometry patterns are not diagnoses by themselves 5.