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. 2022 Apr 30;162(4):804–814. doi: 10.1016/j.chest.2022.04.145

Table 2.

Comparison Between OFDs and the 8-Level WHO COVID-19 Clinical Progress Ordinal Scale34

WHO Scale Category Category Description Notes for OFD Calculation
1 Not hospitalized without limitation in daily activity Classified as OFD
2 Not hospitalized with limitation in daily activity or home oxygen use Days with home oxygen use are classified as days with supplemental oxygen use. Days at home with limitations in daily activity, but with no home oxygen, use are classified as OFDs.
3 Hospitalized not with supplemental oxygen Classified as OFD
4 Hospitalized with standard supplemental oxygen via nasal cannula or mask Classified as day with supplemental oxygen use
5 Hospitalized with high-flow nasal cannula or noninvasive ventilation Classified as day with supplemental oxygen use
6 Hospitalized with invasive mechanical ventilation without other organ support Classified as day with supplemental oxygen use
7 Hospitalized with invasive mechanical ventilation and other organ support (including vasopressors, RRT, or ECMO) Classified as day with supplemental oxygen use
8 Death Death at any time before day 28 is coded as –1 OFDs

ECMO = extracorporeal membrane oxygenation; OFD = oxygen-free day; WHO = World Health Organization.