| ICD‐9‐CM codes or ATC codes | Proxy of COPD severity |
| Refer to the preceding texts | Previous COPD hospitalization |
| 518.81–518.84 | Diagnosis of respiratory failure |
| 311, 312,967, V44.0 | Invasive respiratory procedures |
| Staying in intensive care unit during a COPD hospitalization | |
| Liquid oxygen |
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