A patient's spirometry shows FEV1 of 2.1 L, FVC of 2.4 L, FEV1/FVC of 87.5%. TLC measured by helium dilution is 4.2 L (predicted 6.1 L), RV is 1.8 L. What is the MOST likely diagnosis?

• A Restrictive lung disease; preserved FEV1/FVC ratio with significantly reduced TLC confirms restriction ✓
• B Obstructive lung disease; FEV1 of 2.1 L in absolute terms indicates significant airflow limitation
• C Mixed obstructive-restrictive disease; FEV1/FVC above normal indicates a restrictive pattern within an obstructive baseline
• D Normal spirometry; FEV1/FVC of 87.5% exceeds the diagnostic threshold of 70% for obstruction

Explanation

Restrictive lung disease is characterized by reduced TLC (here 4.2 L vs. predicted 6.1 L, reduced to ~69% predicted), preserved or increased FEV1/FVC ratio (here 87.5%, above the lower limit of normal of 70%), and proportionally reduced FEV1 and FVC. The elevated FEV1/FVC ratio in restriction occurs because stiff lungs with reduced compliance have rapid elastic recoil that drives high early-phase expiratory flow even though absolute volumes are reduced. Option B incorrectly labels this as obstruction — the FEV1/FVC is supranormal, not reduced. Obstruction requires FEV1/FVC below 70% (GOLD criteria) or below LLN. Option C would require evidence of true obstruction (reduced ratio). Option D is partially correct but misses the TLC-based diagnosis of restriction.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.