Pulmonary Function Tests (PFTs)

Jacob Lee

Full PFTs include Spirometry + Body Plethysmography + DLCO 

  • General indications to order PFTs: unexplained respiratory signs/symptoms, pre-operative risk assessment, diagnosis of COPD, ILD

Test

Important Measures

Indications/Uses
Spirometry FVC, FEV1, PEF (with and/or without a bronchodilator) Diagnosis of obstructive lung disease/asthma
Body Plethysmography TLC, RV Diagnosis of restrictive lung disease, evaluation of air trapping
Diffusion Capacity DLCO Differentiation between intrapulmonary and extrapulmonary restriction, unexplained hypoxemia

Tips for ordering: Inpatient PFTs are rarely indicated unless part of transplant workup or CF Exacerbation; must call PFT lab to schedule if ordering inpatient 

  • Initial diagnostic workup: Full PFTs with bronchodilator challenge 
  • Chronic Lung disease: 
    • Asthma: Patients may have a PEF meter they use at home and do not need formal reassessment routinely; can check FEV1 with spirometry 
    • COPD: Spirometry to track FEV1/FVC over time ± DLCO 
    • Cystic Fibrosis: for flares, will typically only track FEV1 while inpatient, will get repeat full set of PFTs once flare has resolved to establish new baseline 
    • ILD: Full PFTs

Interpretation 

  • First, assess validity. The PFT report will state whether the results were consistent and reproducible 
  • Severity based on Z-score (LLN = z-score of -1.645) 
    • Mild: -1.65 to -2.5 
    • Moderate: -2.51 to -4.0 
    • Severe: < -4.1

Obstructive Pattern

  • FEV1/FVC ≤ 0.7 (GOLD) or z-score < 1.645 (ATS/ERS) 
    • A low FVC with an increased TLC is also suggestive of obstructive lung disease, not diagnostic 
    • Increased RV and TLC compared to FVC is suggestive of air trapping 
    • If restriction is present, you cannot grade severity of obstruction based on FEV1

Restricted Pattern

  • TLC < 80% (GOLD) or z-score < 1.645 (GOLD/ATS/ERS) 
    • Reduced FVC is suggestive, but not diagnostic of restrictive disease 
    • FEV1 is commonly low too but FEV1/FVC ratio is normal in isolated restrictive disease

Mixed Obstructive and Restrictive Deficits: both low FEV1/FVC and TLC

Impaired Gas Exchange

  • Normal DLCO: 
    • Normal DLCO + obstruction: asthma, early COPD/CF 
    • Normal DLCO + restriction: pleural disease, chest wall disorders (obesity, scoliosis), neuromuscular disorders (ALS, MG)
  • Low DLCO: <LLN (ATS/ERS) or <80% (GOLD) is low
    • Low DLCO + obstruction: emphysema/late COPD, CF, bronchiectasis
    • Low DLCO + restriction: ILD, pneumonitis, lung resection
    • Low DLCO + normal spirometry: anemia, pulmonary hypertension, pulmonary embolism, early ILD, pulmonary edema, mixed obstruction/restriction pattern (pseudonormalization of PFTs)
  • High DLCO: >120-140% or > 95th percentile/ULN (ATS/ERS)
    • L→R shunt, alveolar hemorrhage, polycythemia, asthma

Bronchodilator response: indicates reversible airflow obstruction (common in asthma) 

  • Positive: Increase in FEV1 and/or FVC by ≥ 12% AND ≥ 200 mL from baseline after bronchodilator administration 

Can try to induce bronchospasm/bronchoconstriction with a methacholine challenge to confirm diagnosis of asthma but test is imperfect

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