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A–a Oxygen Gradient

PulmonologyCritical CareEmergency Medicine

Medical context

Calculate the alveolar–arterial oxygen gradient from ABG values and compare with age-expected normals.

Pulmonary teams interpret results with ABG context, imaging, and ventilator parameters.

This page explains the A–a Gradient formula, how to interpret the output, known limitations, and peer-reviewed references so you can verify results against primary sources and local protocols.

It is grouped under Pulmonology because respiratory severity and pneumonia scores.

How to use this calculator

  1. Enter the values requested by the A–a Gradient calculator using the units shown beside each field.
  2. Review the calculated result and read the interpretation section before acting on the output.
  3. Check limitations and references if the result will change management, dosing, or disposition.
  4. Use the related calculators section when you need supporting values (for example electrolytes, weight, or complementary risk scores).

Calculator

Result

9.7 mmHg

PAO₂ 99.7 · expected ≈ 14

Educational use only. Confirm results with clinical judgment, primary sources, and local protocols. Not a substitute for professional medical advice. Full disclaimer.

Formula

PAO₂ = FiO₂×(Patm−47) − PaCO₂/RQ; A–a = PAO₂ − PaO₂. Expected ≈ age/4 + 4.

Interpretation

Elevated A–a suggests V/Q mismatch, shunt, or diffusion limitation; normal A–a with hypoxemia suggests hypoventilation or low FiO₂.

Limitations

Assumes steady state and accurate FiO₂/Patm. Expected formulas are approximate.

Frequently asked questions

What FiO₂ is room air?
0.21 (21%).
Is the A–a Gradient result enough to make a treatment decision?
No. Calculators support medical decision-making but do not replace examination, shared decision-making, institutional protocols, or consultant input when stakes are high.
Does Calcmedical store the values I type in?
Inputs are processed in your browser for this session. We do not ask for patient names or medical record numbers. See our privacy policy for hosting and analytics details.

References

  • Classic respiratory physiology A–a gradient