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Calculates the alveolar-arterial oxygen gradient to assess gas exchange abnormalities.
Clinical decision-support tool for qualified healthcare professionals. Results are not a diagnosis and should not replace professional clinical judgment.
An increased A–a gradient suggests V/Q mismatch, diffusion impairment, or right-to-left shunt.
Specialty
PulmonologyCategory
PulmonologyDifficulty
Basic
Estimated Time
30 seconds
Version
1.0
Last Updated
2026-07
Formula
Keywords
| Classification | Range |
|---|---|
Normal | < Age/4 + 4 mmHg |
Elevated | > Expected for age |
Calculates the alveolar-arterial oxygen (A–a) gradient from an arterial blood gas to evaluate whether hypoxemia is explained by impaired gas exchange.
A–a gradient = PAO₂ − PaO₂, where PAO₂ = (FiO₂ × (PB − PH₂O)) − (PaCO₂/RQ). Assumes PB=760 mmHg, PH₂O=47 mmHg, RQ=0.8.
Used as a reference to interpret age-expected gradient (age/4 + 4 mmHg).
Fraction of inspired oxygen used to calculate alveolar oxygen tension.
Arterial oxygen tension (mmHg).
Arterial carbon dioxide tension (mmHg) used in alveolar gas equation.
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.
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.
A 55-year-old man with dyspnea has a room-air arterial blood gas: FiO₂ 0.21, PaO₂ 65 mmHg, PaCO₂ 30 mmHg.
Inputs:
Expected Result: A–a gradient = [0.21 × (760 − 47) − 30/0.8] − 65 ≈ 47.2 mmHg — moderately elevated for age (expected ≈ 17.8 mmHg).
A gradient above the age-expected value indicates that hypoxemia is not explained by simple hypoventilation alone; it points to impaired gas transfer within the lung. The calculator compares your result with an age-expected value of roughly age divided by 4 plus 4 mmHg. The gradient cannot quantify how much each underlying mechanism contributes, so the finding must be interpreted with the rest of the blood gas and the clinical context.
The alveolar gas equation pairs the inspired oxygen fraction with the measured PaCO2 to estimate alveolar oxygen, so both inputs must describe the patient at the same moment. If the FiO2 comes from a ventilator setting recorded after the arterial sample, or from a different time entirely, the calculated PAO2 will not match the measured PaO2 and the gradient loses meaning. The calculation also assumes sea-level conditions with a barometric pressure of 760 mmHg and a respiratory quotient of 0.8.
No. A gradient within the age-expected range supports hypoventilation as the explanation for hypoxemia, but the calculator notes that a normal gradient does not exclude pulmonary disease and that the gradient alone cannot separate the contributions of shunt, ventilation/perfusion mismatch, and diffusion impairment. It is one component of arterial blood gas interpretation and should be read alongside the clinical picture, the measured PaO2, and assumptions such as sea-level barometric pressure.
Disclaimer: This calculator is intended for educational and clinical decision support purposes only. The A–a gradient is one component of ABG interpretation and must be considered together with the clinical picture.