A–a Oxygen Gradient

What This Calculator Measures

Calculates the alveolar-arterial oxygen (A–a) gradient from an arterial blood gas to evaluate whether hypoxemia is explained by impaired gas exchange.

How to Use

1. Enter age, FiO₂ as a fraction, PaO₂, and PaCO₂ from the same arterial blood gas.
2. The calculator assumes sea level (barometric pressure 760 mmHg) and a respiratory quotient of 0.8.
3. Compare the result with the age-expected gradient (roughly age/4 + 4 mmHg).

Interpretation Guide

The A–a gradient normally increases with age (roughly age/4 + 4 mmHg). A gradient within the expected range is more consistent with hypoventilation, while a gradient above the age-expected value suggests a pulmonary cause of hypoxemia such as ventilation/perfusion mismatch, diffusion impairment, or right-to-left shunt. The severity bands used by this calculator (mild, moderate, severe elevation relative to the age-expected value) are pragmatic thresholds for this tool, not standardized guideline cut-offs.

Clinical Significance

The A–a gradient is a core ABG-derived measure that helps clinicians separate hypoxemia due to pulmonary gas-exchange abnormalities from hypoxemia due to hypoventilation alone.

When to Use

• Evaluating the cause of hypoxemia on an arterial blood gas
• Distinguishing pulmonary gas-exchange abnormalities from pure hypoventilation
• Assessing gas exchange in the appropriate clinical context

When Not to Use

• When the assumed sea-level conditions do not apply (e.g., at altitude)
• When FiO₂ and PaCO₂ are not from the same blood gas
• As a standalone test of lung function or to quantify shunt severity without further investigation

Limitations

• Assumes sea level (760 mmHg), an FiO₂-derived inspired oxygen tension, and a respiratory quotient of 0.8; at altitude or with a different RQ the result will differ.
• The age-expected value and the calculator's severity bands are convenient clinical guides, not universally established guideline thresholds.
• Requires arterial sampling, and a normal gradient does not exclude pulmonary disease.
• The gradient alone cannot quantify the relative contributions of shunt, V/Q mismatch, or diffusion impairment.

Worked Example

A 55-year-old man with dyspnea has a room-air arterial blood gas: FiO₂ 0.21, PaO₂ 65 mmHg, PaCO₂ 30 mmHg.

A–a gradient = [0.21 × (760 − 47) − 30/0.8] − 65 ≈ 47.2 mmHg — moderately elevated for age (expected ≈ 17.8 mmHg).

Evidence

Source

Classic Respiratory Physiology

Reference

Fenn WO, Rahn H, Otis AB. A theoretical study of the composition of the alveolar air at altitude. Am J Physiol. 1946;146(5):637-653. West JB. Respiratory Physiology: The Essentials. 11th ed. Wolters Kluwer; 2022.

Reviewed By

MedCalcHub Clinical Team

Version

1946

Updated

2026-08

1. Fenn WO, Rahn H, Otis AB. Am J Physiol. 1946;146(5):637-653.
2. West JB. Respiratory Physiology: The Essentials. 11th ed. Wolters Kluwer; 2022.

References

1. West JB. Respiratory Physiology. (Textbook)