A patient is noted to have a DLCO (diffusing capacity for CO) that is markedly reduced with normal spirometry. Which condition best explains this pattern?

• A Pure obstructive lung disease (COPD without emphysema) with intact alveolar surface
• B Pulmonary vascular disease (e.g., pulmonary arterial hypertension) or emphysema — reduced alveolar capillary surface area and membrane area ✓
• C Bronchial asthma in remission with normal airway caliber
• D Obesity-related restrictive disease with elevated hemoglobin concentration

Explanation

DLCO measures transfer of CO across the alveolar-capillary membrane, dependent on membrane area and thickness, pulmonary capillary blood volume, and hemoglobin concentration. Isolated low DLCO with normal spirometry (normal FEV1/FVC and FVC) points to conditions that reduce the alveolo-capillary bed without obstructing airways: pulmonary arterial hypertension (reduces capillary perfusion surface), or emphysema (destroys alveolar walls, reducing surface area). Pure obstructive disease without alveolar destruction has normal or elevated DLCO (air trapping), asthma is typically normal, and polycythemia increases DLCO.

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

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Written and medically reviewed by the StethoPrep medical team.