`What is SF Ratio ?

SF Ratio 

SpO2/FiO2 (S/F) ratio can be used instead of PaO2/FiO2 (P/F) ratio as a measure of the degree of hypoxemia. SF ratio is less invasive and more readily available.

Study done by Rice et al (2007) on derivative data set of 2613 measurements found that S/F ration threshold values of 235 and 315 resulted in 85% sensitivity with 85% specificity and 91% sensitivity with 56% specificity, respectively for P/F ratios of 200 & 300. 

There are concerns about accuracy in dark skin and those in shock with or without poor distal perfusion. Patients may also be treated to keep SpO2 in excess of 97% resulting in uninformative SF Ratio.

Reference

• Rice TW, Wheeler AP, Bernard GR et al (2007) Comparison of the SpO2/FIO2 ratio and the PaO2/FIO2 ratio in patients with acute lung injury or ARDS. Chest 132:410417.https://doi.org/10.1378/chest.07-​0617
•  Brown SM, Grissom CK, Moss M et al (2016) Nonlinear imputation of Pao2/Fio2 from Spo2/Fio2 among patients with acute respiratory distress syndrome. Chest 150:307–313. https://doi.org/10.1016/j.chest.2016.01.003