The current Berlin definition of acute respiratory distress syndrome (ARDS) uses the PaO2/FiO2 (P/F) ratio to classify severity. However, for the same P/F ratio, a patient on a higher positive end-expiratory pressure (PEEP) may have more severe lung injury than one on a lower PEEP.
"... the multifactorial P/FP ratio has a greater predictive validity for mortality in ARDS for patients on a PEEP of > 5 cmH2O
than the P/F ratio. Its prognostic ability progressively increases with higher levels of PEEP. Changes in severity classification when the P/FP ratio is used
instead of the P/F ratio reflect both true illness severity and the applied PEEP strategy."