A-a gradient, a/A ratio, and P: F ratio - are all different views of the same item. These formulas do not have a complete view of oxygenation. Oxygenation is a huge topic, and there are more common values that we use: How well is the patient saturating? Is there dyspnea? How do the lungs sound (auscultation) or look (POCUS)? Where is the patient on the OHCD? What does the ABG show?