How should spirometry results showing an FEV1/FVC ratio of 91, FVC of 81, FEV1 of 75, and DLCO of 108 percent be interpreted and managed?

Interpretation of Spirometry Results

These spirometry results show a mild restrictive pattern that requires total lung capacity (TLC) measurement by body plethysmography to confirm true restriction, as the proportionally reduced FEV1 (75%) and FVC (81%) with a preserved FEV1/FVC ratio (91%) cannot distinguish between true restriction, poor effort, or peripheral airway dysfunction without lung volume confirmation. 1, 2

Primary Pattern Analysis

Normal Airway Caliber

• The FEV1/FVC ratio of 91% is well above the 5th percentile threshold for obstruction, definitively ruling out obstructive airway disease 1, 2
• This elevated ratio (>85-90%) combined with reduced volumes suggests a restrictive pattern rather than obstruction 1

Proportionally Reduced Volumes

• Both FEV1 (75%) and FVC (81%) are mildly reduced below 80% of predicted, falling proportionally together 2, 3
• This pattern of concomitant FEV1 and FVC reduction with normal FEV1/FVC ratio most frequently reflects either incomplete inhalation/exhalation effort OR true restriction, which cannot be differentiated without TLC measurement 1, 3

Elevated DLCO

• The DLCO of 108% is supranormal, which is unusual and argues strongly against intrinsic parenchymal lung disease 4, 5
• A normal or elevated DLCO makes interstitial lung disease highly unlikely 5
• This pattern suggests the reduced volumes are more likely due to extrapulmonary factors (chest wall, obesity, poor effort) rather than lung parenchymal disease 4

Critical Diagnostic Limitation

Spirometry alone has poor positive predictive value for restriction—only 41-58% of patients with reduced FVC actually have confirmed restriction on lung volume measurement 6, 7

Why TLC Measurement is Mandatory

• A reduced FVC with normal FEV1/FVC is associated with true low TLC only about half the time 1
• True restrictive defect requires TLC below the 5th percentile of predicted by body plethysmography 1, 2
• Do not diagnose restriction based on spirometry alone—this leads to frequent misdiagnosis 2, 3, 6

Alternative Explanations Without TLC

• Poor effort or submaximal inspiration/expiration is the most common cause of this pattern 1, 2, 3
• Patchy peripheral airway collapse can produce this pattern with normal TLC but elevated RV 1, 3
• Early small airway disease may present this way 1

Management Algorithm

Immediate Next Step

1. Order full pulmonary function testing with body plethysmography to measure TLC, RV, and RV/TLC ratio 1, 2, 3

Interpretation Based on TLC Results

If TLC < 5th percentile (true restriction confirmed):

• The supranormal DLCO (108%) indicates extrapulmonary restriction (obesity, chest wall, neuromuscular) rather than parenchymal disease 4, 5
• No pulmonary medications are indicated 4
• Focus on treating underlying extrapulmonary cause 4

If TLC normal with elevated RV/TLC:

• Suggests peripheral airway dysfunction or early small airway disease 1, 3
• Consider bronchodilator trial to assess for reversible component 1

If TLC and RV both normal:

• Confirms poor effort or submaximal testing 1, 3
• Repeat spirometry with better coaching and technique 3

Key Clinical Certainties

What Can Be Ruled Out Now

• Obstructive airway disease is definitively excluded by the FEV1/FVC ratio of 91% 1, 2
• Interstitial lung disease is highly unlikely given the supranormal DLCO of 108% 4, 5
• Emphysema phenotype COPD is excluded by the normal FEV1/FVC ratio and elevated DLCO 5

Common Pitfalls to Avoid

• Do not diagnose restriction without TLC confirmation—this is the single most common error in spirometry interpretation 2, 3, 6
• Do not use single-breath VA from DLCO testing to confirm restriction, as it systematically underestimates TLC 1
• Do not assume parenchymal lung disease when DLCO is normal or elevated 4, 5