Alveolar-Arterial (A-a) Oxygen Gradient

Calculate A-a gradient = PAO2 - PaO2. PAO2 = (FiO2 x (Patm - 47)) - (PaCO2 / 0.8). Normal A-a gradient on room air: age/4 + 4 mmHg (up to approximately 25 mmHg). Elevated gradient indicates V/Q mismatch, shunt, or diffusion impairment.

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Height
Clinical Guidance

BMI is a screening measure of body size, not a direct measure of body fat. The standard WHO categories are underweight (<18.5), normal (18.5–24.9), overweight (25–29.9), and obese (≥30).

BMI can overestimate body fat in athletes with high muscle mass and underestimate it in older adults with low muscle mass. Interpret it alongside waist circumference and clinical context rather than as a standalone diagnosis.

⚕ Clinical Disclaimer: These calculators are for educational and reference purposes only. Always verify results with qualified clinical judgment and current clinical guidelines. Do not use as a sole basis for medical decisions.
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How to Use

Enter the values for the scenario you are assessing. Calculate A-a gradient = PAO2 - PaO2. PAO2 = (FiO2 x (Patm - 47)) - (PaCO2 / 0.8). Normal A-a gradient on room air: age/4 + 4 mmHg (up to approximately 25 mmHg). Elevated gradient indicates V/Q mismatch, shunt, or diffusion impairment. Use the a-a gradient result to inform your calculation.

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