Guideline for Interpreting Pulmonary Function Test Graphs

All statements are based on citations from the European Respiratory Society (ERS) literature.

General Principles

Obstructive Pattern

Restrictive Pattern

Upper Airway Obstruction

Mixed Obstructive‑Restrictive Pattern

Systematic Interpretation Algorithm

Common Pitfalls

Interpreting Lung Volume Loops for Ventilatory Defects

Classification of Ventilatory Defects

• The European Respiratory Society recommends examining the flow-volume loop shape and pattern alongside key spirometric values to systematically classify the ventilatory defect as obstructive, restrictive, mixed, or normal 7
• Check if FEV1/VC is below the 5th percentile of predicted to identify obstruction, with a strength of evidence based on European Respiratory Journal guidelines 7, 8
• Use VC rather than FVC to more accurately identify obstructive patterns, as recommended by the European Respiratory Society 8

Identifying Obstructive and Restrictive Patterns

• Look for flows that are less than expected over the entire volume range with a characteristic "scooped out" or concave appearance of the expiratory curve to identify obstruction, according to the European Respiratory Society 7
• The flow-volume curve shows a convex pattern with flow higher than expected at a given lung volume to identify restriction, as stated by the European Respiratory Society 7, 9
• A reduced VC with normal FEV1/VC does not prove restriction and requires TLC measurement for confirmation, as recommended by the European Respiratory Society 9, 10

Confirming Diagnosis with Lung Volume Measurements

• Measure TLC, RV, and RV/TLC ratio to assess for hyperinflation and confirm obstruction, as recommended by the European Respiratory Society 7
• TLC measurement by body plethysmography is mandatory to confirm true restriction, according to the European Respiratory Society 9, 10

Clinical Context and Common Pitfalls

• When FEV1/VC is low and VC is normal, restriction can be ruled out, as stated by the European Respiratory Society 9, 8
• Do not diagnose restriction based on spirometry alone, always confirm with TLC measurement, as recommended by the European Respiratory Society 9, 10
• Do not overlook poor effort as the most common cause of proportionally reduced FEV1 and FVC with normal ratio, according to the European Respiratory Society 7, 10

Differentiating Simple and Complex Restrictive Patterns on Spirometry

Definitions

• Simple restrictive pattern – Spirometry shows a reduced vital capacity (VC) with a normal or elevated FEV₁/VC ratio (≥ 5th percentile). This pattern is only suggestive of restriction and cannot confirm it without lung‑volume measurement. 11, 12
• Complex restrictive pattern – True restriction is confirmed when total lung capacity (TLC) measured by body plethysmography is below the 5th percentile of predicted values. 11, 12

Diagnostic Criteria

• Spirometric suggestion of restriction – A reduced FVC together with a normal or elevated FEV₁/VC ratio (> 85–90%) is frequently misleading and should be interpreted as a possible, not definitive, restriction. 11, 12
• Critical pitfall – A reduced VC alone does not prove restriction; spirometry cannot establish a restrictive defect without TLC measurement. 11, 12
• TLC measurement requirement – TLC < 5th percentile measured by body plethysmography is mandatory to confirm true parenchymal or chest‑wall restriction and to separate it from pseudo‑restrictive patterns caused by obstruction with gas trapping. 11, 12
• Single‑breath TLC estimates – VA derived from DLCO systematically underestimates TLC by up to 3 L in severe obstruction and must not be used to diagnose restriction. 11
• Pure restriction – TLC < 5th percentile and FEV₁/VC ≥ 5th percentile. 11, 12
• Mixed obstruction‑restriction – TLC < 5th percentile and FEV₁/VC < 5th percentile, indicating coexistence of both defects. 11, 12

Algorithmic Approach

Step 1 – Assess FEV₁/VC Ratio

• FEV₁/VC < 5th percentile → Obstruction is present; proceed to lung‑volume measurement to evaluate possible coexisting restriction. 11, 12
• FEV₁/VC ≥ 5th percentile → Obstruction is ruled out; if VC is reduced, TLC measurement should be considered. 11, 12

Step 2 – Evaluate Vital Capacity

• Reduced VC with normal FEV₁/VC → Measure TLC to differentiate true restriction from pseudo‑restriction or poor effort. 11, 12

Step 3 – Measure TLC When Indicated

• TLC < 5th percentile → Confirms true restriction (complex pattern). 11, 12
• TLC normal or elevated → Excludes restriction; the reduced VC likely reflects gas trapping, hyperinflation, or submaximal effort. 11, 12

Step 4 – Identify Mixed Defects in Obstructive Patients

• Low FEV₁/VC + Reduced VC → Superimposed restriction cannot be excluded without TLC measurement. 11
• Low FEV₁/VC + Normal VC → Superimposed restriction can be confidently excluded. 11, 12

Clinical Nuances

• Common causes of pseudo‑restrictive patterns – Poor effort or submaximal exhalation is the most frequent cause; severe obstruction with gas trapping can also lower VC while TLC remains normal or high. 11
• Borderline TLC values – When TLC lies at the lower limit of normal in a disease that typically causes restriction (e.g., post‑resection, interstitial lung disease), the restrictive defect may be statistically subtle. Additional testing (DLCO, gas‑exchange studies, exercise testing) is recommended to fully characterize functional impairment. 11, 12