SpO2FiO2

SpO2/FiO2, commonly abbreviated as the SF ratio, is a bedside index used to assess the efficiency of lung oxygenation. It combines peripheral oxygen saturation (SpO2) measured by pulse oximetry and the fraction of inspired oxygen (FiO2) delivered to the patient. The ratio provides a noninvasive surrogate for evaluating how well the lungs transfer oxygen into the blood, particularly when arterial blood gas measurements are unavailable or to monitor trends over time.

Calculation and interpretation: The SF ratio is calculated by dividing SpO2 (expressed as a percentage) by FiO2 (expressed as a decimal). For example, SpO2 92% on FiO2 0.50 yields an SF ratio of 184 (92 / 0.50 = 184). In general, lower SF ratios indicate worse oxygenation, while higher ratios indicate better oxygenation. Although SF ratio correlates with the PaO2/FiO2 ratio used in ARDS definitions, it is not a perfect substitute and exact thresholds can vary by population and measurement context. Clinicians use SF ratio mainly to gauge oxygenation status, track changes, and complement, not replace, arterial blood gas analysis when precise PaO2 values are needed.

Clinical use: The SF ratio is commonly employed in critical care and respiratory settings, including assessment of patients with suspected or established ARDS, monitoring during noninvasive or invasive ventilation, and situations where frequent ABG sampling is impractical. It is particularly useful for noninvasive assessment and for trend analysis.

Limitations: SpO2 readings can be affected by factors such as poor perfusion, motion, nail polish, skin pigmentation, anemia, or dyshemoglobinemias, which can distort the SF ratio. FiO2 estimation can also be uncertain outside controlled settings. Therefore, SF ratio should be interpreted in the clinical context and confirmed with arterial oxygen measurements when precise oxygenation assessment is required.