The DLNO/DLCO ratio: Can it be clinically used?

The ratio of pulmonary diffusion capacity for nitric oxide (DLNO) to carbon monoxide (DLCO) is known to vary according to the underlying pathophysiology in patients. However, it remains to be seen whether a substantial overlap in this ratio exists among different pathologies, potentially rendering it clinically insignificant. To address this question, we systematically reviewed literature examining mean differences in the DLNO/DLCO ratio across various pathologies. Our search encompassed articles published from 1983 (the inception of DLNO abstracts) to February 1st, 2024, across multiple databases including EMBASE, Medline, Scopus, Web of Science, and the Cochrane Library. We included studies in all languages involving human subjects and assessed full-text articles. Conference abstracts were considered. Our review identified a total of 60 articles/abstracts addressing a range of cardiopulmonary diseases or acute pulmonary conditions, each featuring both diseased / non-diseased control groups. The PFT edevice type was considered. Among the 14 different cardiopulmonary diseases and acute pathologies examined, significant differences in the DLNO/DLCO ratio were observed only in cases of CO poisoning, pulmonary arterial hypertension, pulmonary embolism, and COPD, where the ratio was notably elevated. Conversely, conditions such as bronchopulmonary dysplasia, cystic fibrosis, and exposure to high altitudes demonstrated a decreased ratio. Notably, no significant differences were found in the ratio for interstitial lung diseases, cancers, diabetes, obesity, post-COVID syndrome, and blood vessel disorders. This suggests that the ratio can be used clinically for only a certain number of diseases, as a substantial overlap in the ratio does exist