I thank Dr. Weiner for his thoughtful letter1 in response to our CMAJ article.2
We excluded patients who died during their stay in hospital because they did not have a full diagnosis period, as defined, to obtain pulmonary function testing (PFT).
One of the limitations of our study was that, as a result of its observational nature, an association between PFT and patient outcomes could not prove causation. It is possible that testing was a marker of overall quality of care rather than a direct source of improved outcomes.
Trying to tease apart the effects of testing and other components of good-quality care of chronic obstructive pulmonary disease (COPD) — such as appropriate medication use — is challenging, because the effects are likely to be highly correlated. Indeed, we hypothesized that appropriate medication use — which arguably can be determined only if PFT is done — is likely on the causal pathway between PFT and better patient outcomes. This is why we included a table showing medication distribution between people who did and did not receive PFT. Nonetheless, a positive association between PFT and death and COPD hospitalizations was found, even after clustering by primary care physician and adjusting for many markers of good-quality COPD and overall care, suggesting that PFT did affect these outcomes.
Footnotes
Competing interests: None declared.