Category: Critical Care
Keywords: Oxygenation, ARDS, P:F Ratio, S:F Ratio, Hypoxia, Mechanical Ventilation (PubMed Search)
Posted: 4/21/2026 by Mark Sutherland, MD
(Updated: 4/22/2026)
Click here to contact Mark Sutherland, MD
PaO2 to FiO2 (P:F) ratios, are often considered the gold standard in critical care for assessing the degree of oxygen-refractory hypoxia in various pathologies, particularly ARDS. P:F does have some limitations, including not accounting for the PEEP, but probably the most limiting is that it requires collecting an ABG, which is invasive and not always feasible or a top priority when resuscitating a critically ill hypoxic patient. On the other hand, SpO2 (pulse ox saturation) is routinely available, and of course the FiO2 should be known, so many have suggested perhaps using an SpO2 to FiO2 (S:F) ratio instead. But how S:F maps to P:F and how well they correlate is not fully known. Chaudhuri et al recently conducted a meta-analysis, published in Critical Care Medicine this month, which reviewed the literature on this.
Bottom Line: Yes, S:F ratios correlate well with P:F ratios, especially when the SpO2 is less than 97%, but you can't just substitute the S:F for P:F, you have to use one of the accepted formulas. See additional info on the website for the actual formula to apply and how a given S:F translates to P:F.
The authors identified 4 particularly high performing formulas from well-done studies. One was logarithmic, and two were non-linear, making the math hard, so the best is probably the linear one (correlation coefficient was 0.89, which is quite good). It is:
SF = 64 + 0.84 x PF
Usually you have the SF and want to figure out the PF, so rearranging to solve for PF (to save you all the trouble):
PF = (SF - 64) / 0.84
Since we usually care about P:F < 300 (mild ARDS), < 200 (moderate ARDS), and < 100 (severe ARDS), here are the S:F mappings for those P:Fs to make things super simple:
If P:F is 300 then S:F is 315
If P:F is 200 then S:F is 235
If P:F is 100 then S:F is148
And 150 is another P:F that is important since we often consider proning and/or paralysis under this level. That would equate to an S:F of 190 using this formula.
Don't forget! The SpO2 is expressed as a percentage, and FiO2 as a decimal. So for example, for a patient with a sat of 97% on RA (21% FiO2):
97 / 0.21 = 461 would be their S:F.
Chaudhuri D, Lazarte J, Shah K, Pitre T, Pekkarinen PT, Sendagire C, Martin GS, Jung C, Laffey JG, Rochwerg B; Sequential Organ Failure Assessment (SOFA)-2 study group. Approaches to Converting Sp o2 /F io2 Ratio to Pa o2 /F io2 Ratio for Assessment of Respiratory Failure in Critically Ill Patients: A Systematic Review. Crit Care Med. 2026 Apr 1;54(4):950-959. doi: 10.1097/CCM.0000000000007018. Epub 2026 Jan 2. PMID: 41493393.
