PFT Interpretation
Pulmonary function tests -spirometry, lung volumes, and DLCO. A systematic approach: (1) FEV₁/FVC ratio, (2) lung volumes, (3) DLCO. This tells you obstructive vs restrictive vs mixed, and narrows the differential.
🔍 Systematic Interpretation
Step 1 -FEV₁/FVC Ratio
| FEV₁/FVC | Pattern | Examples |
|---|---|---|
| < 0.70 (or < LLN) | Obstructive GOLD Criteria, 2024 | COPD, asthma, bronchiectasis, bronchiolitis obliterans |
| Normal or ↑ | Restrictive (if TLC reduced) | ILD (IPF, sarcoidosis), chest wall disease, neuromuscular (ALS, myasthenia), obesity |
| ↓ with ↓ TLC | Mixed | Combined COPD + ILD, sarcoidosis |
Low FVC with normal FEV₁/FVC does NOT confirm restriction. Air trapping in obstruction can reduce FVC, making the ratio look normal. You need lung volumes (TLC) to confirm restriction. TLC < 80% predicted = true restriction.
Step 2 -Bronchodilator Response
- Positive response: FEV₁ or FVC improves ≥ 12% AND ≥ 200 mL after albuterol ATS/ERS Spirometry Standardization, 2005
- Positive → suggests asthma (reversible obstruction)
- Negative → suggests COPD (fixed obstruction), but some COPD patients have partial reversibility
Step 3 -DLCO (Diffusion Capacity)
| DLCO | Pattern | Think |
|---|---|---|
| Low DLCO + Obstruction | Emphysema (parenchymal destruction) | COPD (emphysema phenotype). Asthma has NORMAL DLCO -key differentiator. |
| Low DLCO + Restriction | Interstitial lung disease | IPF, sarcoidosis, hypersensitivity pneumonitis, drug-induced ILD |
| Low DLCO + Normal spirometry | Pulmonary vascular disease | Pulmonary hypertension, PE, early ILD |
| Normal/↑ DLCO + Restriction | Extrapulmonary restriction | Obesity, neuromuscular disease, chest wall deformity (lungs are normal) |
| ↑ DLCO | Increased pulmonary blood flow | Diffuse alveolar hemorrhage, polycythemia, L-to-R shunt, asthma, obesity |
Lung Volumes Summary
| Volume | Obstructive | Restrictive |
|---|---|---|
| TLC | ↑ or normal (hyperinflation, air trapping) | ↓ (< 80% predicted) |
| RV | ↑↑ (air trapping) | ↓ or normal |
| RV/TLC ratio | ↑ | Normal or ↓ |
| FRC | ↑ | ↓ |
🧪 Workup
Workup
- Spirometry -FEV₁, FVC, ratio
- Lung volumes -TLC (confirm restriction)
- DLCO -low=emphysema/ILD; normal=asthma
- Bronchodilator response
- Methacholine challenge
- Flow-volume loop
💊 Medications
Medications
| Drug | Dose | Route | Notes |
|---|---|---|---|
| Albuterol neb | 2.5mg | Neb | Reversibility testing |
| PFTs are diagnostic | not therapeutic | - | - |
📋 On Rounds
How do you distinguish asthma from COPD on PFTs?
Both are obstructive (FEV₁/FVC < 0.70). Key differences: (1) Bronchodilator response: asthma shows significant reversibility (≥ 12% + 200 mL), COPD has minimal reversibility. (2) DLCO: asthma has normal or increased DLCO, emphysema has decreased DLCO (alveolar destruction). (3) Methacholine challenge: positive in asthma (bronchial hyperresponsiveness), negative in COPD
A patient has low DLCO with normal spirometry. What's your differential?
Pulmonary vascular disease is the classic answer: pulmonary hypertension, chronic PE, or pulmonary arteriovenous malformations -the vascular bed is compromised but the airways and parenchyma are intact, so spirometry is normal.
How do you differentiate asthma from COPD on PFTs?
Both show obstruction (FEV₁/FVC < 0.70). The key differentiator: bronchodilator reversibility. Asthma: FEV₁ improves ≥ 12% AND ≥ 200 mL after bronchodilator (significant reversibility -airway obstruction is dynamic). COPD: FEV₁ improves < 12% or < 200 mL (fixed obstruction -structural airway damage). Other clues: DLCO: normal in asthma (parenchyma is fine), LOW in COPD/emphysema (alveolar destruction).
What does a low DLCO tell you, and what conditions have normal DLCO?
Low DLCO (reduced gas transfer across the alveolar membrane): (1) Emphysema -destroyed alveoli → reduced surface area for gas exchange. (2) ILD/pulmonary fibrosis -thickened interstitium → diffusion barrier. (3) Pulmonary hypertension -vascular bed destruction. (4) Anemia -less hemoglobin to bind CO (correct DLCO for Hgb). (5) Pulmonary embolism -dead space.
Clinical Examples
📋 Case 1, Obstructive Pattern (COPD)
Patient: 64M, 45-pack-year smoker. Progressive dyspnea × 3 years. PFTs: FEV₁ 42% predicted, FVC 78%, FEV₁/FVC 0.54, DLCO 48%. Post-bronchodilator: FEV₁ improves 6% and 90 mL.
Key findings: Obstructive pattern: FEV₁/FVC < 0.70 (the defining ratio). Low DLCO = emphysema (alveolar destruction). Minimal bronchodilator response (< 12% AND < 200 mL) = COPD, not asthma. GOLD Stage III (severe: FEV₁ 30-49%).
Management:
- LABA/LAMA combination inhaler (first-line for symptomatic COPD GOLD B-E)
- Add ICS only if eosinophils > 300 or frequent exacerbations despite dual bronchodilators
- Smoking cessation (the ONLY intervention proven to slow FEV₁ decline)
- Pulmonary rehab referral (improves exercise capacity, dyspnea, and QOL)
- Annual PFTs to track FEV₁ decline (normal: ~30 mL/yr; COPD smoker: 60-100 mL/yr)
Teaching point: FEV₁/FVC < 0.70 = obstruction. The FEV₁ alone determines severity (GOLD staging). Bronchodilator response distinguishes COPD (minimal) from asthma (significant: ≥ 12% AND ≥ 200 mL). But there is overlap, some patients have both.
📋 Case 2, Restrictive Pattern (IPF)
Patient: 70F with progressive dyspnea and dry cough × 18 months. Bibasilar velcro crackles. PFTs: FEV₁ 62%, FVC 58%, FEV₁/FVC 0.85 (normal ratio), TLC 65%, DLCO 38%.
Key findings: Restrictive pattern: reduced TLC (< 80%) with preserved or elevated FEV₁/FVC ratio. Very low DLCO = impaired gas exchange (fibrosis thickens alveolar-capillary membrane). HRCT needed to determine cause.
Management:
- HRCT chest, look for UIP pattern (honeycombing, traction bronchiectasis = IPF) vs NSIP pattern (GGO predominant = potentially treatable)
- If UIP pattern: antifibrotic therapy (pirfenidone or nintedanib)
- Serial PFTs q3-6 months, FVC decline > 10% absolute or DLCO decline > 15% = disease progression
- 6-minute walk test (6MWT), desaturation > 4% = significant; supplemental O₂ if SpO₂ < 88%
- Early lung transplant referral if FVC < 80% or DLCO < 40% at diagnosis
Teaching point: FVC is the most important PFT for monitoring ILD progression, a 10% absolute decline in FVC over 6-12 months doubles mortality risk. DLCO is the most sensitive early marker but also declines with anemia and pulmonary HTN.
📋 Case 3, Mixed Obstructive-Restrictive Pattern
Patient: 58M with COPD and morbid obesity (BMI 48). PFTs: FEV₁ 45%, FVC 52%, FEV₁/FVC 0.62, TLC 72%, DLCO 55%. Lung volumes show decreased TLC but elevated RV/TLC ratio.
Key findings: Mixed pattern: FEV₁/FVC < 0.70 (obstruction) + TLC < 80% (restriction). Common in COPD + obesity, COPD + ILD overlap, or sarcoidosis. The reduced TLC from obesity masks hyperinflation from COPD.
Management:
- Separate the contributions: body plethysmography (most accurate TLC) + chest imaging to assess parenchyma
- Treat both: bronchodilators for COPD component + weight loss for restrictive component
- Weight loss target: 10% body weight → expect ~5-10% improvement in FVC
- If DLCO disproportionately low for the degree of obstruction: consider concurrent ILD or pulmonary HTN
- ABG if concern for OHS (BMI > 40 + daytime hypercapnia)
Teaching point: A "normal" FEV₁/FVC ratio in a patient with low FVC does NOT rule out obstruction, both FEV₁ and FVC may be proportionally reduced ("pseudo-normalization"). Always check lung volumes (TLC) to confirm true restriction.
📣 Sample Presentation
One-Liner
"Mr. Johnson is a 62-year-old with 40-pack-year smoking history referred for PFTs. Results: FEV₁ 48% predicted, FVC 82%, FEV₁/FVC 0.58, DLCO 55%. Post-bronchodilator FEV₁ improvement: 8% and 120 mL."
Key Points to Cover on Rounds
Obstructive pattern (FEV₁/FVC 0.58, <0.70). GOLD classification: FEV₁ 48% = GOLD stage III (severe). Bronchodilator response: 8% and 120 mL (negative -doesn't meet 12% AND 200 mL threshold for significant reversibility → supports COPD over asthma). DLCO 55% (low -suggests emphysema component). Correlation: CT chest shows upper lobe centrilobular emphysema. Current treatment: LAMA only. Recommended: add ICS/LABA (triple therapy if exacerbation history). Pulmonary rehab referral. Reassess for LTOT if SpO₂ <88% at rest or exertion. Annual PFTs to track decline.
Monitoring
- PFTs q6-12mo
- FVC decline >5-10%/yr in ILD → escalate
- DLCO <40% → O₂+transplant eval
- 6MWT
⚡ Summary
Summary
Read Them in Three Steps
FEV1/FVC ratio first, then lung volumes, then DLCO. That sequence separates obstructive from restrictive from mixed disease and then narrows the differential within each. Reading DLCO before establishing the pattern leads to misinterpretation, because the same DLCO means different things in obstruction and restriction.
Step 1: The Ratio Defines Obstruction
FEV1/FVC below 0.70 (or below the lower limit of normal) means obstruction, and then FEV1 percent predicted grades its severity. A normal ratio does not mean normal lungs: it just means the problem is not obstruction.
Low FVC Alone Does Not Prove Restriction
Air trapping in obstruction reduces FVC and can make the ratio look normal. You need lung volumes to confirm restriction: TLC below 80% predicted. Calling a low FVC 'restriction' without a TLC is the commonest PFT interpretation error, and it sends patients down an interstitial lung disease workup they do not need.
Step 2: Bronchodilator Response
A significant response is a 12% and 200 mL improvement in FEV1 or FVC. It supports asthma but does not exclude COPD, since a proportion of COPD patients respond, and asthmatics can have a normal response on a good day. Persistent obstruction after bronchodilator is what defines COPD.
Step 3: DLCO Splits Each Pattern in Two
Obstruction with a low DLCO: emphysema, where alveolar surface is destroyed. Obstruction with a normal or high DLCO: asthma or chronic bronchitis. Restriction with a low DLCO: interstitial lung disease, a parenchymal problem. Restriction with a normal DLCO: extraparenchymal, so neuromuscular weakness, obesity, kyphoscoliosis or pleural disease.
Correct DLCO for Hemoglobin
Anemia lowers the measured DLCO and polycythemia raises it, because the test depends on hemoglobin binding carbon monoxide. An uncorrected value in an anemic patient looks like interstitial disease that is not there. A raised DLCO also occurs in alveolar hemorrhage, asthma, obesity and left-to-right shunt.
Distinguish Neuromuscular Restriction
Check maximal inspiratory and expiratory pressures, and a supine FVC. A fall of more than about 10% in FVC from sitting to supine indicates diaphragmatic weakness, which is what separates neuromuscular restriction from parenchymal disease and is the measure that matters in ALS and myasthenia.
Check Effort and the Flow-Volume Loop Before Believing Anything
Poor effort or poor technique invalidates the study, so look at the reproducibility criteria before interpreting a number. The loop shape diagnoses upper airway obstruction: flattened inspiratory limb means a variable extrathoracic lesion (vocal cord dysfunction), flattened expiratory limb means variable intrathoracic, and both flattened means a fixed lesion such as tracheal stenosis. These are entirely missed if only the numbers are read.
📄 One Pager
Text version
Pulmonology · One Pager
Pulmonary Function Tests
FEV₁/FVC < 0.70 = obstruction. Reversibility (≥ 12% + 200 mL) = asthma. Low DLCO = emphysema/ILD/pulm HTN (normal in asthma). TLC needed to confirm restriction.
Obstructive Pattern
FEV₁/FVC < 0.70. COPD: no significant reversibility, low DLCO. Asthma: significant reversibility (≥ 12% AND ≥ 200 mL), normal DLCO. COPD staging: GOLD I-IV by FEV₁ %.
Restrictive Pattern
FEV₁/FVC normal or elevated + reduced TLC (need lung volumes). Causes: ILD (low DLCO), neuromuscular (normal DLCO), obesity (normal DLCO), chest wall deformity.
DLCO Interpretation
Low: emphysema, ILD, pulmonary HTN, anemia, PE. Normal with obstruction: asthma. Elevated: pulmonary hemorrhage, polycythemia, left-to-right shunt. Best test to differentiate asthma from emphysema.
Key Drugs
Bronchodilator testAlbuterol → repeat spirometry
Methacholine challengeIf normal PFTs + suspected asthma
Lung volumesBody plethysmography for TLC
DLCOGas transfer assessment
Pitfalls
- Diagnosing restriction without lung volumes (need TLC)
- Missing asthma vs COPD distinction (reversibility + DLCO)
- Not correcting DLCO for anemia
- Ignoring the flow-volume loop (upper airway obstruction)
Related Topics
AsthmaCOPD -Chronic ManagementHemoptysisInterstitial Lung DiseaseObstructive Sleep ApneaPleural Effusion