What is a normal A-a gradient?

For a healthy young adult breathing room air the A-a gradient is typically less than 10 mmHg. A commonly used room-air age-adjusted upper limit is (Age / 4) + 4 mmHg. Supplemental oxygen widens the expected gradient, so this cutoff should not be used to classify measurements made above FiO2 0.21.

What causes a high A-a gradient?

A high A-a gradient indicates impaired oxygen transfer within the lungs and is seen in V/Q mismatch (e.g. pulmonary embolism, COPD), intrapulmonary shunting (e.g. pneumonia, pulmonary edema, ARDS), and diffusion impairment (e.g. interstitial lung disease).

What does a normal A-a gradient with low PaO2 mean?

A low PaO2 with a normal A-a gradient points to a non-pulmonary cause of hypoxemia such as hypoventilation, high altitude, or neuromuscular disease — the lungs are working correctly but are not receiving enough oxygen-rich air.

What FiO2 should I use for room air?

On room air use FiO2 = 0.21 (21%). For supplemental oxygen: simple nasal cannula at 2 L/min is approximately 0.28, 4 L/min approximately 0.36, a non-rebreather mask at 15 L/min approximately 0.70–0.90. Your respiratory therapist or ventilator display can give you the exact delivered FiO2.

What respiratory quotient (RQ) should I enter?

The standard RQ is 0.8, which reflects a mixed diet. It can range from 0.7 (pure fat metabolism) to 1.0 (pure carbohydrate metabolism). In most clinical situations 0.8 is the correct default.

How does altitude affect the A-a gradient calculation?

At higher altitudes atmospheric pressure is lower, which reduces the alveolar PO2. Enter the local atmospheric pressure in mmHg — at 1524 m (5000 ft) it is approximately 632 mmHg, at 2438 m (8000 ft) approximately 565 mmHg.

A-a Gradient Calculator

Enter arterial blood gas values and FiO2 to calculate the A-a oxygen gradient and interpret pulmonary gas exchange.

Arterial Blood Gas Inputs

FiO₂ — Fraction of Inspired Oxygen

(0.21–1.00)

Use 0.21 for room air.

PaCO₂ — Arterial CO₂ (mmHg)

mmHg

Normal: 35–45 mmHg.

PaO₂ — Arterial O₂ (mmHg)

mmHg

Normal on room air: 80–100 mmHg.

Patient Age (years)

years

Used for the room-air age-adjusted normal limit.

Enter values and click Calculate to see results.

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Summary

The alveolar-arterial (A-a) oxygen gradient measures the difference between the oxygen concentration in the alveoli and the oxygen measured in arterial blood. A normal A-a gradient indicates the lungs are effectively transferring oxygen, while a high gradient suggests a pulmonary cause of hypoxemia such as V/Q mismatch, diffusion impairment, or intrapulmonary shunting. This calculator uses the alveolar gas equation (PAO2 = FiO2 × (Patm − PH2O) − PaCO2 / RQ) to compute the A-a gradient from standard arterial blood gas values.

How it works

Enter the fraction of inspired oxygen (FiO2) — use 0.21 for room air.
Enter the arterial partial pressure of carbon dioxide (PaCO2) from the blood gas report.
Enter the arterial partial pressure of oxygen (PaO2) from the blood gas report.
Optionally adjust atmospheric pressure (default 760 mmHg at sea level) and respiratory quotient (default 0.8).
The calculator computes the alveolar PAO2 using the alveolar gas equation and subtracts PaO2 to get the gradient.
Results include the A-a gradient value and, on room air, an age-adjusted normal limit and interpretation.

Use cases

Differentiating pulmonary from non-pulmonary causes of hypoxemia.
Evaluating patients with low SpO2 or respiratory distress in the ICU or emergency department.
Assessing severity of V/Q mismatch in pulmonary embolism workup.
Monitoring gas exchange in mechanically ventilated patients.
Teaching arterial blood gas interpretation to medical students and residents.
Quick bedside calculation during arterial blood gas review.
Tracking changes in pulmonary gas exchange during treatment.
Identifying diffusion impairment in interstitial lung disease.

Frequently Asked Questions

Last updated: 2026-05-29 · Reviewed by Nham Vu

A-a Gradient Calculator

Arterial Blood Gas Inputs

Calculation Breakdown

Alveolar Gas Equation

Clinical Interpretation

Summary

How it works

Use cases

Frequently Asked Questions

A-a Gradient Calculator

Use the A-a Gradient Calculator

Arterial Blood Gas Inputs

Calculation Breakdown

Alveolar Gas Equation

Clinical Interpretation

Summary

How it works

Use cases

Frequently Asked Questions