P/F Ratio

Aka: P/F Ratio, PF Ratio, Horowitz Index, Carrico Index

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II. Indications

  1. Acute Respiratory Failure
    1. Allows for interpretation of PaO2 on Arterial Blood Gas when a patient is on Supplemental Oxygen
    2. PaO2 interpretation would otherwise require the patient to be on room air for ABG
    3. Used in SOFA Score and SMART-COP

III. Precautions

  1. P/F Ratio may be unreliable when PaO2 >100 mmHg and FiO2 < 0.5
  2. P/F Ratio cannot distinguish between Acute Respiratory Failure and acute on chronic Respiratory Failure

IV. Labs: Calculation

  1. P/F Ratio = PaO2 / FIO2
    1. Where PaO2 = arterial PO2 (from Arterial Blood Gas)
    2. Where FIO2 = fraction of inspired oxygen (ranging from 0.3 for 30% to 1.0 for 100%)

V. Interpretation: Rough Approximations on Room Air

  1. PF Ratio 400 to 500 mmHg: >90 mmHg on room air (normal)
  2. PF Ratio 300 mmHg: PaO2 <60 mmHg on room air (mild Respiratory Failure)
  3. PF Ratio 250 mmHg: PaO2 <50 mmHg on room air
  4. PF Ratio 200 mmHg: PaO2 <40 mmHg on room air
  5. PF Ratio 150 mmHg: PaO2 <30 mmHg on room air
  6. PF Ratio 100 mmHg: PaO2 <20 mmHg on room air

VI. Resources

  1. P/F Ratio (Nickson, LITFL)
    1. https://litfl.com/pao2-fio2-ratio/

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