A-a Gradient Calculator
Calculate the Alveolar-arterial gradient to evaluate lung function.
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Formula
Determines the difference between alveolar and arterial oxygen concentrations to help diagnose causes of hypoxia.
Worked Example
The A-a Gradient Explained
- PAO2 (Alveolar) = FiO2 * (Patm - PH2O) - (PaCO2 / R)
- Normal A-a gradient: (Age / 4) + 4 (estimated)
- A normal gradient with hypoxia suggests hypoventilation or low inspired oxygen (e.g., altitude)
- An elevated gradient suggests shunting, V/Q mismatch, or diffusion impairment (e.g., PE, pneumonia, fibrosis)
A-a gradient calculation requires an arterial blood gas (ABG) sample. Results must be interpreted alongside the patient's clinical status and imaging.
You can also calculate changes using our Anion Gap Calculator.
Frequently Asked Questions
What does the A-a gradient tell you?
It helps doctors decide if a patient's low oxygen level is caused by a problem within the lungs themselves or by something outside the lungs, like shallow breathing.
What is FiO2?
FiO2 is the Fraction of Inspired Oxygen. Room air is 21% (0.21). Patients on supplemental oxygen will have a higher FiO2.
Why does age matter?
The A-a gradient naturally increases as people get older, so the "normal" range shifts higher with age.
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