Definition of Respiratory Failure using PaO2 alone is confounded when patients are commenced on oxygen therapy prior to arterial blood gas (ABG) measurement. Furthermore, classification of Respiratory Failure as Type 1 or Type 2 using PaCO2 alone can give an inaccurate account of events as both types can co-exist. 100 consecutive presentations of acute respiratory distress were assessed initially using PaO2, and subsequently PaO2/FiO2 ratio, to diagnose Respiratory Failure. Respiratory Failure cases were classified as Type 1 or Type 2 initially using PaCO2, and subsequently alveolar-arterial (A-a) gradient. Any resultant change in management was documented. Of 100 presentations, an additional 16 cases were diagnosed as Respiratory Failure using PaO2/FiO2 ratio in place of PaO2 alone (p = 0.0338). Of 57 cases of Respiratory Failure, 22 cases classified as Type 2 using PaCO2 alone were reclassified as Type 1 using A-a gradient (p < 0.001). Of these 22 cases, management changed in 18.