In a 56-year-old woman with normal pulmonary function tests and spirometry but a mildly reduced diffusing capacity of the lung for carbon monoxide (DLCO) at 74% of predicted (Z score -1.83) and a body mass index of 33.7 kg/m², is the reduced DLCO attributable to obesity?

Is Reduced DLCO at 74% Predicted Attributable to Obesity in This Patient?

The mildly reduced DLCO at 74% predicted in this 56-year-old woman with BMI 33.7 kg/m² is unlikely to be fully explained by obesity alone, and warrants further evaluation for other causes of impaired gas exchange. 1

Understanding DLCO in Obesity

What the Guidelines Tell Us

The European Respiratory Society guidelines specifically state that a high DLCO is associated with obesity 1, not a reduced DLCO. This is a critical distinction that fundamentally changes the interpretation of this patient's results.

• Obesity typically increases DLCO due to elevated pulmonary capillary blood volume from circulatory congestion and high cardiac output states 2
• The severity grading shows this patient has mild impairment (DLCO >60% and <LLN) 1
• Normal spirometry and lung volumes with decreased DLCO suggests: anemia, pulmonary vascular disorders, early interstitial lung disease, or early emphysema 1

What Recent Research Shows

A 2023 study of 380 service members free of cardiopulmonary disease found that increased BMI was NOT significantly associated with measured DLCO after controlling for age, sex, hemoglobin, and height 3. However, obesity did affect the percent predicted values:

• For every 5 kg/m² increase in BMI, percent predicted DLCO increased by 4.2-6.5% 3
• This means obesity actually makes DLCO appear more normal on paper, not reduced 3
• The presence of obesity (BMI ≥30) reduced the prevalence of DLCO abnormalities by 4.1-12.1% across different reference equations 3

The Physiologic Mechanism

In severe obesity without manifest cardiopulmonary disease, the typical pattern is 2:

• Increased capillary blood volume (Vc/VA) - mean 118% predicted
• Decreased membrane diffusion (Dm/VA) - mean 77% predicted
• Normal overall DLCO - mean 98% predicted (these effects cancel out)

This patient's reduced DLCO at 74% predicted falls below what obesity alone would produce.

Clinical Approach

Essential Next Steps

1. Check hemoglobin level - Anemia is a common, correctable cause of reduced DLCO 1

   • DLCO should be adjusted for hemoglobin values 1
   • This is particularly important when monitoring for drug toxicity or in patients with fluctuating hemoglobin 1

2. Evaluate for pulmonary vascular disease 1

   • Consider echocardiography to assess for pulmonary hypertension
   • Obesity can cause circulatory congestion, but this typically elevates DLCO 2

3. Consider early parenchymal lung disease 1

   • Early interstitial lung disease
   • Early emphysema (even without obstruction on spirometry)
   • High-resolution CT may be warranted if clinical suspicion exists

4. Review the DLCO/VA (KCO) ratio 1

   • If DLCO is low but KCO is high or normal, this suggests extraparenchymal restriction (which obesity can cause)
   • If both DLCO and KCO are low, this suggests parenchymal abnormalities 1

Important Caveats

• Reference equations matter: The 2023 study found that Global Lung Function Initiative (GLI) values showed the smallest effect from BMI and obesity 3
• Ensure the predicted values used appropriate reference equations accounting for age, sex, height, and ethnicity 1
• The Z-score of -1.83 places this patient below the 5th percentile, confirming true abnormality 1

Bottom Line

Do not attribute this reduced DLCO to obesity. The physiologic effects of obesity on gas exchange typically produce normal or elevated DLCO values, not reduced ones. This patient requires systematic evaluation for anemia, pulmonary vascular disease, and early parenchymal lung disease. 1, 3, 2