A-a Gradient Calculator
Calculate the Alveolar-arterial (A-a) oxygen gradient to evaluate lung gas exchange. The A-a gradient equals PAO2 minus PaO2, with a normal value estimated as (Age/4) + 4 mmHg. An elevated gradient suggests V/Q mismatch, shunt, or diffusion impairment.
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Formula
How it works: Determines the difference between alveolar and arterial oxygen concentrations to help diagnose causes of hypoxia.
Worked Example
A-a Gradient Calculator - Alveolar-Arterial Gradient Formula
The Alveolar-arterial (A-a) gradient measures the difference between oxygen in the alveoli (the lungs' air sacs) and oxygen in the blood. It is used to identify the source of low blood oxygen.
- 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 or Corrected Calcium Calculator.
A-a Gradient: Normal Values by Age and Causes of Elevation
Age-adjusted normal A-a gradient values and common causes of an elevated gradient.
| Age | Normal A-a Gradient (mmHg) | Common Causes of Elevation |
|---|---|---|
| 20 years | 9 | - |
| 30 years | 12 | - |
| 40 years | 14 | - |
| 50 years | 17 | PE, pneumonia |
| 60 years | 19 | ILD, ARDS |
| 70 years | 22 | CHF, pulmonary fibrosis |
| 80 years | 24 | Multiple causes common |
| Any age (elevated) | Above age-adjusted normal | V/Q mismatch, shunt, diffusion impairment |
Note: Normal gradient formula: (Age / 4) + 4. Standard values: Patm = 760 mmHg, PH2O = 47 mmHg, RQ = 0.8. An elevated gradient with normal PaCO2 suggests V/Q mismatch. An elevated gradient with low PaCO2 suggests PE or shunt. For educational reference only.
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. The estimated normal is (Age / 4) + 4 mmHg.
What is a normal A-a gradient for a 40-year-old?
Using the formula (Age / 4) + 4, a 40-year-old would have an expected normal A-a gradient of about 14 mmHg. Values significantly above this suggest lung parenchymal disease.
What conditions cause an elevated A-a gradient?
Common causes include pulmonary embolism, pneumonia, pulmonary fibrosis, ARDS, and intracardiac shunts. These all impair the lung's ability to transfer oxygen from the alveoli into the blood.
Can the A-a gradient be used on supplemental oxygen?
Yes, but accuracy decreases at higher FiO2 levels. The gradient is most reliable when calculated on room air (FiO2 21%). At higher FiO2, other factors like absorption atelectasis can widen the gradient.
How do I add this A-a Gradient Calculator to my site?
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Medical Disclaimer
This calculator is provided for informational and educational purposes only. It is not intended as a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making any decisions based on these results. Do not disregard or delay seeking medical advice because of information obtained from this tool.
AI Assistant
Ask about this calculator
I can help you understand the a-a gradient calculator formula, interpret your results, and answer follow-up questions.
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Our AI assistant provides general estimates and advice only. Please consult a qualified healthcare professional before making any health-related decisions.
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