P/F Ratio Calculator

From arterial blood gas (ABG)

Enter as decimal (0.21 to 1.0). Room air = 0.21

P/F Ratio Interpretation:

SF Ratio to P/F Ratio Conversion:

Key Points:

What is the P/F ratio and why is it important?

The P/F ratio (PaO₂/FiO₂) is the ratio of arterial oxygen partial pressure to the fraction of inspired oxygen. It's a measure of oxygenation efficiency that accounts for the amount of oxygen being delivered. A low P/F ratio indicates impaired gas exchange, which is the hallmark of ARDS. It's more accurate than PaO₂ alone because it accounts for supplemental oxygen.

When should I use SF ratio instead of P/F ratio?

Use SF ratio (SpO₂/FiO₂) when ABG is unavailable or for continuous non-invasive monitoring. It's particularly useful for screening and trending. However, SF ratio is less accurate at extremes (SpO₂ >97% or <88%) due to the flat portion of the oxyhemoglobin dissociation curve. Always obtain ABG for formal ARDS classification and management decisions.

Why must PEEP be ≥ 5 cm H₂O for ARDS diagnosis?

The Berlin Definition requires minimum PEEP of 5 cm H₂O to standardize P/F ratio measurements. ARDS involves alveolar collapse, and PEEP recruits collapsed alveoli. Without adequate PEEP, the P/F ratio reflects both lung injury severity and degree of alveolar recruitment, making comparisons unreliable. The 5 cm H₂O threshold ensures basic alveolar recruitment.

Can the P/F ratio improve quickly?

Yes, P/F ratio can improve rapidly with interventions like increased PEEP, prone positioning, recruitment maneuvers, or diuresis. It can also worsen quickly with progression of lung injury, infection, or pulmonary edema. This is why serial measurements are important for monitoring disease trajectory and response to therapy. Single measurements should not drive major management decisions.

What factors besides lung disease affect the P/F ratio?

Many factors influence P/F ratio: PEEP level (higher PEEP increases P/F), patient position (prone improves P/F), altitude (lower at high altitude), hemoglobin level, cardiac output, ventilator settings, and even timing of measurement relative to position changes or suctioning. Always interpret in the full clinical context.

Is a low P/F ratio always ARDS?

No. Many conditions cause low P/F ratio: pneumonia, atelectasis, pulmonary embolism, cardiogenic pulmonary edema, pneumothorax, pleural effusion, and ventilation-perfusion mismatch. ARDS is a specific syndrome requiring acute onset (within 1 week), bilateral opacities on imaging, exclusion of cardiac causes, and known risk factor. Low P/F ratio alone is insufficient for diagnosis.

How do I estimate FiO₂ on nasal cannula or face mask?

Estimation formulas exist (nasal cannula: FiO₂ ≈ 0.20 + 0.04 × L/min), but accuracy is limited by mouth breathing, respiratory rate, and tidal volume. For nasal cannula at 1-6 L/min, FiO₂ ranges from 0.24-0.44. Simple masks deliver 0.35-0.50, and non-rebreathers 0.60-0.95. For accurate P/F ratios, use precise FiO₂ from mechanical ventilator or high-flow oxygen systems.

What P/F ratio indicates need for mechanical ventilation?

There's no absolute P/F ratio threshold for intubation. Decision depends on trajectory (improving vs worsening), work of breathing, mental status, ability to protect airway, and response to non-invasive support. Generally, P/F < 200 despite high-flow oxygen, with increased work of breathing or deteriorating mental status, warrants consideration of mechanical ventilation. Some patients with chronic lung disease tolerate lower P/F ratios.