Various invasive and non-invasive techniques are used to understand the oxygenation. This study aimed to assess the correlation between Spo2/Fio2ratio to Pao2/Fio2and Sao2/  Fio2 in different invasive modes of oxygen supplementation. We hypothesized that the non-invasive index SpO2/FiO2 (SF) ratio can be used as a surrogate for to invasive index PaO2/FiO2 (PF) ratio and SaO2/FiO2 ratio in all invasive modes of ventilation. Methods and methodology: A total of 313 patients admitted to the surgical and medical intensive care unit were enrolled in this retrospective cross-sectional study. Fraction of inspired oxygen (FiO2) from the monitor, while ABG sampling, Partial pressure of oxygen (PaO2) and Arterial oxygen saturation (SaO2) were noted from the ABG, reports in the medical records. The corresponding saturation of peripheral oxygen (SpO2) was noted from the monitor while ABG sampling. Calculated SF, PF and SaO2/FiO2 ratios were recorded and correlation between the same was noted in different invasive modes of ventilation. Results: Pearson’s correlation was used to quantify the relationship between the variables. The study showed a positive correlation, r=0.6003 (P< 0.001), between SF and PF ratio and also a positive correlation, r=0.9137 (P<0.001), between SF and SaO2/ FiO2 ratio. In addition, the median values of SF ratio (P-value=0) and SaO2/FiO2 ratio (P-value=0) indicated the observed difference is unlikely to be due to chance. Conclusion: In patients who are on an invasive mode of ventilation, the non-invasive SF can be used as a surrogate for invasive PF and SaO2/FiO2 ratio.