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Oxygen Saturation Index Calculator

Assess oxygenation severity non-invasively with the OSI.

How it is calculated

The tool computes OSI = (FiO2 × MAP × 100) / SpO2. FiO2 is the inspired oxygen fraction entered as a decimal between 0 and 1 (0.6 for 60%), MAP is the mean airway pressure in cmH2O, and SpO2 is the pulse-oximeter saturation in percent. The result is shown to two decimals and labelled with a band: below 4 is no or mild impairment, 4 up to 7.5 is mild ARDS range, 7.5 up to 12.3 is moderate ARDS range, and 12.3 or more is severe ARDS range. Those cut-offs are the ones this tool applies; units and thresholds in other protocols can differ. FiO2 above 1 or SpO2 above 100 gives no result. This is an educational calculation, not a diagnosis.

Worked example

A ventilated patient has FiO2 0.6, mean airway pressure 12 cmH2O and SpO2 94%. Numerator = 0.6 × 12 × 100 = 720. OSI = 720 / 94 = 7.66. That is at or above 7.5 and below 12.3, so the tool labels it Moderate ARDS range. Result: OSI 7.66.

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Common mistakes

  • Entering FiO2 as a percentage (60) instead of a fraction (0.6). The tool rejects anything above 1, so use the decimal form.
  • Using peak inspiratory pressure or PEEP in place of mean airway pressure. OSI needs the mean airway pressure from the ventilator display.
  • Entering SpO2 as a decimal (0.94). It must be in percent, 94.
  • Comparing an OSI with an oxygenation index (OI) from an arterial blood gas as if they were the same scale. OSI uses SpO2 and its numbers differ from OI.
  • Taking one reading as the answer. A single value says little; the trend over time and a clinician's judgement matter more than the band label.

About this calculator

The Oxygen Saturation Index (OSI) is calculated as (FiO2 × mean airway pressure × 100) ÷ SpO2, giving clinicians a way to assess how severely impaired a patient's oxygenation is using only non-invasive measurements — pulse oximetry and ventilator settings — instead of requiring an arterial blood gas draw to get PaO2 for the more traditional oxygenation index (OI).

Neonatal and pediatric intensivists use OSI most often, since arterial lines are harder to place and maintain in small patients, tracking OSI over time to gauge the severity of conditions like acute respiratory distress syndrome (ARDS) and to guide decisions on escalating respiratory support such as high-frequency oscillatory ventilation or extracorporeal membrane oxygenation (ECMO), with rising OSI trends signaling worsening lung function.

This calculator takes your patient's FiO2, mean airway pressure and SpO2 values, and returns the calculated Oxygen Saturation Index.

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