The assessment is made at 36 weeks or discharge for babies born at < 32 weeks. If they've spent at least 28 days on > 21% oxygen then they have BPD and the amount of support they're receiving at the time of assessment determines the severity. If they're on room air at 36 weeks they're mild . If they're on < 30% oxygen they're moderate (and the conversion from liter flow via nasal cannula to FiO2 depends on the size of the baby). If they're on > 30% oxygen or PPV they're severe. Classifying based on FiO2 can be a little tricky because most BPD babies go home on nasal cannula and the conversion from liter flow to FiO2 depends on the size of the baby. A 2kg baby would get an effective FiO2 > 30% with just 1/4 LPM of 100% oxygen by nasal cannula. A bigger baby would need to be on a higher flow rate to hit that 30% threshold.
So to answer your question, no, strictly speaking being on CPAP alone wouldn't make them BPD if they spent less than 28 days on > 21% oxygen. But as you've seen from your observations, different practices between institutions certainly would affect how often babies are diagnosed with BPD.
