A 45-year-old woman has DLCO reduced to 45% of predicted with normal spirometry and lung volumes. The most likely explanation for isolated DLCO reduction with normal spirometry is:

• A Severe obstructive disease with air trapping and reduced alveolar surface area
• B Pulmonary vascular disease (pulmonary arterial hypertension) or early interstitial lung disease causing loss of functional alveolar-capillary surface area without obstructive or restrictive defect on spirometry ✓
• C Anemia, which reduces CO-carrying capacity in blood, reflecting reduced hemoglobin rather than lung pathology
• D Respiratory muscle weakness reducing diffusion by reduced inspiratory effort

Explanation

DLCO (diffusing capacity for carbon monoxide) reflects the total alveolar-capillary surface area available for gas exchange, the membrane thickness, and pulmonary capillary blood volume/hemoglobin. Isolated DLCO reduction with preserved FEV1, FVC, and normal lung volumes (no obstruction or restriction on spirometry) is characteristic of: (1) Pulmonary arterial hypertension—reduced capillary bed reduces the vascular component; (2) Early/mild ILD before restriction develops. Anemia does reduce DLCO (CO binds Hb; hemoglobin-corrected DLCO should be used), but the question asks about pathological reduction in the context of the clinical scenario—pulmonary vascular disease is the key diagnosis here. Emphysema causes both obstructive pattern and DLCO reduction (not isolated). Respiratory muscle weakness reduces total lung capacity (volume), not specifically DLCO.

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

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Written and medically reviewed by the StethoPrep medical team.