How this instrument works
The P/F ratio divides the partial pressure of oxygen dissolved in arterial blood, PaO2, by the fraction of oxygen in the air being delivered, FiO2. Room air is roughly 0.21; a patient on a mask or ventilator can be given a known, higher fraction. Dividing the measured blood value by that fraction produces a figure that stays meaningful across different levels of oxygen support — a PaO2 of 90 on room air and a PaO2 of 300 on 70% oxygen would look wildly different as raw numbers, but the ratio reveals which lung is actually working harder to oxygenate blood.
The ratio's clinical weight comes from the 2012 Berlin Definition of acute respiratory distress syndrome, a consensus statement that replaced an older, looser 1994 definition. Berlin sorts lung injury into three severity tiers by this single ratio — provided the patient is also on at least 5 cmH2O of PEEP or CPAP and has bilateral opacities on imaging not fully explained by fluid overload or heart failure: mild sits between 200 and 300, moderate between 100 and 200, and severe at or below 100.
The number is only trustworthy when FiO2 is actually known and controlled, which is why it belongs to patients on a defined oxygen device or a ventilator rather than casual pulse-oximetry readings on room air. It is also just one of several Berlin criteria working together — timing, chest imaging, and the exclusion of fluid overload all matter alongside the ratio — so a low P/F reading by itself never confirms ARDS without the rest of the clinical picture agreeing.
- Enter PaO2 in mmHg, taken from an arterial blood gas.
- Enter FiO2 as a fraction between 0.21 and 1.0 — the known, delivered oxygen concentration, not an estimate from a nasal cannula flow rate.
- Read P/F ratio: PaO2 divided by FiO2, recalculated as soon as either value changes.
- Compare the result against the Berlin bands — above 300, 200-300, 100-200, or 100 and below — alongside PEEP level and chest imaging, not alone.
Worked example — 90 mmHg on 30% oxygen
A PaO2 of 90 mmHg on an FiO2 of 0.3 (30% oxygen): 90 / 0.3 = 300 exactly — the upper boundary the Berlin Definition uses for 'mild' ARDS, assuming the PEEP and imaging criteria are also satisfied.
Drop the oxygen fraction's effectiveness and the picture changes fast: a PaO2 of 80 mmHg on FiO2 0.4 gives 80 / 0.4 = 200, the boundary between 'mild' and 'moderate.' A PaO2 of 60 mmHg even on 100% oxygen (FiO2 1.0) gives just 60 / 1.0 = 60 — deep in the 'severe' band, a lung barely transferring oxygen despite maximal support.
Questions
What is considered a normal P/F ratio?
Above roughly 400-500 in a healthy person breathing room air or supplemental oxygen without lung injury. The Berlin Definition's severity bands only apply once someone already has a clinical picture consistent with ARDS — bilateral opacities, an acute timeline, and adequate PEEP — a healthy person's ratio isn't being graded against these thresholds at all.
Can I calculate P/F ratio from a pulse oximeter instead of an arterial blood gas?
Not reliably. The ratio's clinical validity depends on PaO2 from an actual arterial sample; oxygen saturation from a fingertip sensor doesn't map cleanly onto PaO2, especially at higher saturations where the oxyhemoglobin curve flattens. Some bedside estimates substitute SpO2/FiO2 as a rough non-invasive stand-in, but it's a different, looser measurement than the one Berlin defines.
Does a low P/F ratio mean I have ARDS?
Not by itself. The Berlin Definition requires the ratio alongside PEEP or CPAP of at least 5 cmH2O, bilateral opacities on chest imaging, an onset within about a week of a known trigger, and exclusion of fluid overload or heart failure as the sole cause. A low ratio without those other findings points to some other cause of impaired oxygenation, not necessarily ARDS.
Why does FiO2 have to be known precisely for this ratio to mean anything?
Because the ratio's whole purpose is comparing oxygen delivered against oxygen achieved. A nasal cannula's actual delivered fraction varies with breathing pattern and flow rate and is only ever an estimate, which is why the ratio is most meaningful in patients on a sealed device or ventilator where FiO2 is a set, known number.
How is the P/F ratio used to grade ARDS severity?
The Berlin Definition splits it into three bands, assuming other criteria are met: mild is 200 to 300, moderate is 100 to 200, and severe is 100 or below. Lower ratios generally track with worse outcomes on average, though the bands describe a population pattern, not a certainty for any one patient.
References
- ARDS Definition Task Force — Berlin Definition, JAMA 2012 (PubMed)
- NHLBI — Acute Respiratory Distress Syndrome (patient information)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.