spirometry cut offs
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spirometry cutoff values FEV1 FVC ratio interpretation
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spirometry obstructive restrictive pattern grading severity
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Here is a comprehensive summary of spirometry cutoffs and interpretation:
Spirometry Cutoffs & Interpretation
Key Parameters
| Parameter | Normal | Abnormal |
|---|---|---|
| FEV₁/FVC ratio | ≥ LLN (lower limit of normal, ~0.70 in adults) | < LLN → Obstruction |
| FVC | ≥ LLN | Low → possible restriction |
| FEV₁ | ≥ 80% predicted | < 80% → reduced |
| TLC | ≥ LLN | < LLN → confirms restriction |
LLN = 5th percentile of the reference population. Fixed ratio (FEV₁/FVC < 0.70) is widely used but can over-diagnose obstruction in elderly and under-diagnose in young adults. GLI 2012 guidelines (and ATS/ERS 2022 standards) prefer LLN-based interpretation.
Step-by-Step Interpretation Algorithm
Step 1 — Check FEV₁/FVC ratio
- Low (< LLN / < 0.70) → Obstructive pattern → go to Step 2
- Normal → go to Step 3
Step 2 — Obstruction confirmed → Check FVC
- FVC normal → Pure obstruction
- FVC low → Possible mixed disorder → measure TLC
- TLC low → Mixed obstructive + restrictive
- TLC normal → Pure obstruction (FVC low due to air trapping)
Step 3 — No obstruction → Check FVC
- FVC normal → Normal spirometry
- FVC low → Possible restriction → measure TLC
- TLC low → Restrictive pattern confirmed
- TLC normal → Non-specific (possible restriction) pattern — may be chest wall or neuromuscular disorder
(Technical Standard on Interpretive Strategies for Routine Lung Function Tests, p. 17)
Severity Grading (ATS/ERS) — Based on FEV₁ % Predicted
| Severity | FEV₁ % Predicted |
|---|---|
| Mild | ≥ 70% |
| Moderate | 60–69% |
| Moderately severe | 50–59% |
| Severe | 35–49% |
| Very severe | < 35% |
(GOLD also uses similar thresholds for COPD staging)
GOLD Staging for COPD (post-bronchodilator FEV₁/FVC < 0.70)
| GOLD Grade | FEV₁ % Predicted | Severity |
|---|---|---|
| 1 | ≥ 80% | Mild |
| 2 | 50–79% | Moderate |
| 3 | 30–49% | Severe |
| 4 | < 30% | Very severe |
Restrictive Pattern
- Confirmed by TLC < LLN (5th percentile)
- Spirometry alone can suggest restriction: low FVC + normal/elevated FEV₁/FVC + convex flow-volume curve (high elastic recoil)
- Reduced FVC alone predicts low TLC less than half the time — TLC measurement is essential
- Normal FVC + normal FEV₁/FVC reliably rules out restriction (p. 21)
Quick Reference
| Pattern | FEV₁/FVC | FVC | TLC |
|---|---|---|---|
| Normal | Normal | Normal | Normal |
| Obstruction | ↓ | Normal or ↓ | Normal or ↑ |
| Restriction | Normal or ↑ | ↓ | ↓ |
| Mixed | ↓ | ↓ | ↓ |
| Non-specific | Normal | ↓ | Normal |
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