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Calculator: A-a gradient (alveolar-arterial gradient; AaG)

Calculator: A-a gradient (alveolar-arterial gradient; AaG)

Patient temperature  
Percent inspired O2  
Respiratory quotient  

Important: Inputs must be complete to perform calculation.

Expected AaG  
Measured AaG  
Decimal precision  


  • High A-a gradients are associated with oxygen transfer/gas exchange problems. These are usually associated with alveolar membrane diseases, interstitial diseases, or ventilation/perfusion (V/Q) mismatch.
  • Hypoxemia in the face of a normal A-a gradient implies hypoventilation with displacement of alveolar O2 by CO2 or other substance.
  • The FiO2 (fraction of inspired oxygen) is equal to the Percent inspired O2/100.
  • In this calculator, pAtm is estimated by elevation above sea level using this formula: pAtm = 760 * eTo(Elevation / -7000).
  • pH2O is dependent on body temperature and calculated with the following formula: pH2O = 47 * eTo((Patient temperature - 37) / 18.4).
  • The equation for expected A-a gradient assumes the patient is breathing room air; therefore, it is less accurate at higher percentages of inspired oxygen.

Equations used
pAtm = 760 * e(Elevation/-7000)
pH2O = 47 * e((Patient_temperature-37)/18.4)
FiO2 = Percent_inspired_O2 / 100
Expected_AaG = 2.5 + (0.21 * Age)
Measured_AaG = (FiO2 * (pAtm - pH2O)) - (pCO2 / Respiratory_quotient) + (pCO2 * FiO2 * (1 - Respiratory_quotient) / Respiratory_quotient) - PaO2


Only digits 0 to 9 and a single decimal point (".") are acceptable as numeric inputs. Attempted input of other characters into a numeric field may lead to an incorrect result.

Information on this page may not appear correctly if printed.

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All information contained in and produced by the EBMcalc system is provided for educational purposes only. This information should not be used for the diagnosis or treatment of any health problem or disease. THIS INFORMATION IS NOT INTENDED TO REPLACE CLINICAL JUDGMENT OR GUIDE INDIVIDUAL PATIENT CARE IN ANY MANNER. Click here for full notice and disclaimer.

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