The diffusing capacity of the lung for carbon monoxide (DLCO) is commonly used as a measure of gas exchange. The DLCO is usually reported as both an absolute number (DLCO) and corrected for alveolar volume (DLCO/VA). However, it remains unclear which one to use in interpretation. To resolve this dilemma we determined whether abnormal DLCO or DLCO/VA was more closely associated with abnormal gas exchange as measured by exercise oximetry.
With Institutional Review Board approval, we retrospectively reviewed the charts of all patients who had both DLCO and 6-minute walk/oximetry testing at our University Pulmonary Clinic over a 2-year period. We analyzed the association between abnormal DLCO or DLCO/VA (defined as <75% predicted) and abnormal gas exchange (defined as oxygen desaturation >/= 4%) using Chi-square or Fisher’s exact tests, and calculated sensitivity, specificity, positive and negative predictive values (PPV, NPV) and likelihood ratios (LR). We also measured the association between DLCO, DLCO/VA and oxygen desaturation using Spearman rank correlations.
A total of 93 patients had valid and complete data. Their average age was 66 years, 54% were women and 60% had interstitial lung disease. The prevalence of oxygen desaturation was 44%. The % predicted values of DLCO and DLCO/VA were highly correlated (rho=0.91, p<0.0001), but both correlated only weakly with oxygen desaturation (rho = -0.37, p=0.0003; rho= -0.42, p <0.0001, respectively). Low DLCO predicted desaturation with a sensitivity of 0.95, specificity of 0.29, PPV of 0.51, NPV of 0.88 and LR of 1.34 (p=0.003). Low DLCO/VA predicted desaturation with a sensitivity of 0.90, specificity of 0.38, PPV of 0.54, NPV of 0.83 and LR=1.45 (p=0.002). The highest LR (1.91) was seen for DLCO/VA of patients with restriction (p=0.04).
There were no differences in the ability of DLCO or DLCO/VA to predict abnormal gas exchange, but patients with abnormal gas exchange were slightly more likely to have an abnormal DLCO/VA than DLCO.
Either the DLCO or DLCO/VA may be used to assess gas exchange.
David Kaminsky, None.