Abstract : Acute hypoxemic respiratory failure (AHRF) is very common presentation with high risk of mortality and morbidity. Acute respiratory distress is one of the main causes of AHRF, it is often difficult to differentiate between ARDS and non-ARDS hypoxemia cases. The definition of ARDS requires the partial pressure of arterial oxygen (PaO2) from the invasive arterial sample to calculate the PaO2/FiO2 according to Berlin criteria which may be difficult to apply in limited resource settings and lead to underestimation of the patients with ARDS. The SpO2/FiO2 ratio has been recently used as a measure for oxygenation status. We proposed a correlation between the non-invasive SPO2/FiO2 (S/F) ratio and the invasive PaO2/FiO2 (P/F) ratio in both non-invasive and invasive ventilation and the SpO2/FiO2 can be used instead of PaO2/FiO2. All attempts globally to change the definition of ARDS according to berlin criteria by suing the non-invasive SpO2/FiO2 as well as the new trend of using ultrasound of the lung and the use of non-invasive high flow nasal canula. To study the correlation between the SpO2/FiO2 and the PaO2/FiO2 and determine the cutoff values of SpO2/FiO2 that correlate with PaO2/FiO2 in patient with acute hypoxemic respiratory failure. Patients with hypoxemia and acute respiratory failure and treated in the ICU by invasive and non-invasive measures enrolled in this single center observational cross-sectional study which was conducted from August 2023 to March 2024 in the ICU department of Ghazi Alhariri hospital. Fractional inspired oxygen (FiO2), partial pressure of arterial oxygen (PaO2) and oxygen saturation of hemoglobin (SpO2) were recorded from blood gas analysis reports and monitoring of the patients simultaneously, the S/F and P/F ratios calculated, the correlation between the ratios were noted by scatter plot with line of fit, The degree of correlation was determined using Spearman Pearson equation and the cutoff values of S/F ratio determined by linear regression equation. A total of 100 sample data were collected. The mean age of the patients in the study was57.38 year, the PaO2 mean was 100.81, the SpO2 mean was 96.32%, the mean of calculated P/F ratio was 258.77 while the mean of calculated SpO2/FiO2 ratio was 244.75. A scatter plot diagram plotted, Pearson correlation was used to quantify the correlation between the variables: SpO2/FiO2 and PaO2/FiO2 ratios, the study shows strong positive linear correlation (r= 0.746) (p <0.05) between the 2 variables, the S/F cutoff values of the S/F ratios was determined using linear regression equation as 229, 312 and 146 for the corresponding P/F values of 300, 200 and 100 respectively. In patients with acute hypoxemic respiratory failure the non-invasive S/F ratio is a simple and promising tool can be used as a surrogate to the invasive P/F ratio in invasive and non-invasive intervention interchangeably specially in limited resources settings. The S/F ratio can be used to predict ratio in ARDS and non-ARDS patients.