PF Ratio Calculator
Assess ARDS severity and oxygenation status instantly with our professional-grade PF ratio calculator. Calculate PaO2/FiO2 ratio using Berlin criteria with step-by-step analysis and visual charts.
Interactive PF Ratio Calculator
Calculate the PaO2/FiO2 ratio to assess oxygenation status and ARDS severity according to Berlin criteria. Clinical tool for ICU and respiratory assessment. Learn more about ARDS diagnosis at Medscape.
PF Ratio Formula & Berlin ARDS Criteria
PF Ratio Definition
The PF ratio (PaO2/FiO2) quantifies oxygenation efficiency by comparing arterial oxygen concentration to inspired oxygen concentration:
Berlin ARDS Criteria (2012)
| PF Ratio | ARDS Severity | Mortality Rate | Clinical Features | Ventilation Strategy |
|---|---|---|---|---|
| > 300 | No ARDS | Baseline | Normal oxygenation | Standard support |
| 200 – 300 | Mild ARDS | 27% | Bilateral infiltrates | Consider CPAP |
| 100 – 200 | Moderate ARDS | 32% | Refractory hypoxemia | Lung-protective ventilation |
| < 100 | Severe ARDS | 45% | Severe hypoxemia | ECMO consideration |
Key Facts: The Berlin criteria require: (1) Acute onset within 7 days, (2) Bilateral infiltrates on imaging, (3) Respiratory failure not fully explained by cardiac failure, (4) PF ratio ≤300 on PEEP ≥5 cmH₂O. PF ratio correlates with lung weight and shunt fraction. Each 50-point decrease increases mortality by 8-12%.
How to Use the PF Ratio Calculator
Simple steps for accurate ARDS assessment
Enter PaO₂ Value
Input arterial oxygen tension from ABG analysis in mmHg
Enter FiO₂ Value
Input fraction of inspired oxygen as percentage (21-100%)
Assess ARDS Severity
View PF ratio with Berlin criteria classification and clinical recommendations
Advantages of PF Ratio
Why clinicians rely on this vital oxygenation index
Quick and Simple
Probably the main reason for its widespread use in ICU settings worldwide.
Rough Guide to A-a Gradient
Can be used as a rough guide to whether there is a significant A-a gradient present: PaO2 should = FiO2 x 500 (e.g. 0.21 x 500 = 105 mmHg). See caveats in clinical guidelines.
More Practical than a/A Ratio
More practical than the a/A ratio, as measurement of alveolar oxygen tension (PAO2) is not required.
Used in Severity Scoring
Integral to APACHE IV, SOFA, SAPS-II and SAPS-III scoring systems for critical illness prognosis.
Clinical Applications & Mortality Correlation
The PF ratio is part of the Berlin definition of Acute Respiratory Distress Syndrome (ARDS) and correlates directly with mortality risk.
| ARDS Severity | PaO2/FiO2 | Mortality | PEEP Requirement |
|---|---|---|---|
| Mild | 200 – 300 | 27% | ≥5 cmH₂O |
| Moderate | 100 – 200 | 32% | ≥5 cmH₂O |
| Severe | < 100 | 45% | ≥5 cmH₂O |
Note: The Berlin definitions require PF ratio measurement with a PEEP setting of 5 cmH₂O or more.
Frequently Asked Questions
The PF ratio (PaO₂/FiO₂) quantifies oxygenation efficiency by comparing arterial oxygen concentration to inspired oxygen concentration. It’s critical for diagnosing and monitoring ARDS severity, guiding ventilator management, and predicting mortality. Lower ratios indicate worse lung function and higher shunt fraction.
Berlin criteria (2012) define ARDS by: (1) Acute onset within 7 days of insult, (2) Bilateral infiltrates on imaging not explained by effusions/collapse, (3) Respiratory failure not fully explained by cardiac failure/fluid overload, (4) PF ratio ≤300 on PEEP ≥5 cmH₂O. Severity: Mild (200-300), Moderate (100-200), Severe (<100).
PF Ratio = PaO₂ (mmHg) ÷ FiO₂ (decimal). If FiO₂ is given as percentage, divide by 100 first. Example: PaO₂ = 85 mmHg on 40% oxygen → 85 ÷ 0.40 = 212.5. This falls into Mild ARDS category. Always ensure PaO₂ is from arterial blood gas and FiO₂ is the actual delivered oxygen concentration.
A PF ratio <100 indicates severe ARDS with 45% mortality risk. Patients require invasive mechanical ventilation with lung-protective strategy (tidal volume 6mL/kg, plateau pressure ≤30 cmH₂O), prone positioning for >12 hours daily, and consideration of ECMO. This represents severe hypoxemia refractory to conventional ventilation.
Oxygenation Index (OI) = [FiO₂ × Mean Airway Pressure × 100] ÷ PaO₂. OI is used in pediatrics and includes ventilator pressure, making it more comprehensive for assessing ventilation support. PF ratio is simpler and used in adults for ARDS diagnosis. OI >40 indicates severe ARDS in children and often triggers ECMO consideration.
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