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A-a Gradient

ExpectedAaG = 2.5 + (0.21 * Age)
MeasuredAaG = (FIO2 * (pAtm - pH2O)) - (pCO2 / RespQuot) + (pCO2 * FIO2 * (1 - RespQuot) / RespQuot) - paO2

Patient Temp  
Percent Inspired O2  
p CO2  
Resp Quot  
p aO2  
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 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 Temp - 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.

Related Results

  1. Kanber GJ, King FW, Eshchar YR, et. al. The alveolar-arterial oxygen gradient in young and elderly men during air and oxygen breathing. Am Rev Respir Dis. 1968 Mar;97(3):376-81. PubMed Logo
  2. Mellemgaard K. The alveolar-arterial oxygen difference: its size and components in normal man. Acta Physiol Scand. 1966 May;67(1):10-20. PubMed Logo

Associated Medical Subject Headings [MeSH] Terms
         Adolescent, Adult, Aged, Aging, Arteries, Blood Pressure Determination, Carbon Dioxide, Female, Humans, Hydrogen-Ion Concentration, Male, Middle Aged, Oximetry, Oxygen, Oxygen Consumption, Partial Pressure, Pulmonary Alveoli, Respiration, Respiratory Function Tests

Associated Logical Observation Identifiers Names and Codes [LOINC]
Parameter LOINC's and Units
Age 21612-7 : Years old      21612-7 : yr      21612-7 : years      30525-0 : Years old      30525-0 : yr      30525-0 : years      
Patient Temp 8310-5 : degC      
Percent Inspired O2 19993-5 : %O2      19992-7 : %O2      19994-3 : %O2      19996-8 : %O2      19995-0 : %O2      
p CO2 2019-8 : mmHg      34705-4 : mmHg      
p aO2 11556-8 : mmHg      2703-7 : mmHg      19254-2 : mmHg      
Measured AaG 19991-9 : mmHg      53809-0 : mmHg      

This material contains content from LOINC® ( The LOINC table, LOINC codes, and LOINC panels and forms file are copyright © 1995-2014, Regenstrief Institute, Inc. and the Logical Observation Identifiers Names and Codes (LOINC) Committee and available at no cost under the license at

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