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  A-a Gradient

Patient Temp  
Percent Inspired O2  
p CO2  
Resp Quot  
p aO2  

Important: Inputs must be complete to perform calculation.

Expected AaG  
Measured AaG  
Decimal Precision  

Include literature references: Yes   No

  • 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.

Equations used
pAtm = 760 * e(Elevation/-7000)
pH2O = 47 * e((PatientTemp-37)/18.4)
FIO2 = PercentInspiredO2 / 100
ExpectedAaG = 2.5 + (0.21 * Age)
MeasuredAaG = (FIO2 * (pAtm - pH2O)) - (pCO2 / RespQuot) + (pCO2 * FIO2 * (1 - RespQuot) / RespQuot) - paO2


  1. West JB. Respiratory physiology: the essentials. 7th edition, Philadelphia, Lippincott William & Wilkins 2005, p.170. NLMN ID: 101205971 NCBI Logo
  2. 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 ID: 5638492 PubMed Logo
  3. Mellemgaard K. The alveolar-arterial oxygen difference: its size and components in normal man. Acta Physiol Scand. 1966 May;67(1):10-20. PubMed ID: 5963295 PubMed Logo
  4. Cole RB, Bishop JM. Variation in alveolar-arterial O2 tension difference at high levels of alveolar O2 tension. J Appl Physiol. 1967 Apr;22(4):685-93. PubMed ID: 6023181 PubMed Logo
  5. Lilienthal JL, Riley RL, et al. An experimental analysis in man of the oxygen pressure gradient from alveolar air to arterial blood during rest and exercise at sea level and at altitude. Am J Physiol. 1946 Sep;147:199-216. PubMed ID: 21000737 PubMed Logo
  6. Le Huu Thien B,Huynh Quang D. Equation des gaz alveolaires et difference alveolo-arterielle en oxygene. J Fran Viet Pneu. 2014; 05(14): 4-10.

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

Associated Logical Observation Identifiers Names and Codes [LOINC]
Parameter LOINC's and Suggested Units
Age 21612-7 : yr      21612-7 : yr      21612-7 : years      30525-0 : Years old      30525-0 : yr      30525-0 : years      
Patient Temp 8329-5 : degC      8310-5 : degC      8331-1 : degC      8332-9 : degC      8333-7 : degC      8328-7 : degC      75539-7 : degC      
Percent Inspired O2 3150-0 : %      19993-5 : %      19992-7 : %      19994-3 : %      19996-8 : %      19995-0 : %      
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 (http://loinc.org). LOINC is copyright © 1995-2023, Regenstrief Institute, Inc. and the Logical Observation Identifiers Names and Codes (LOINC) Committee and is available at no cost under the license at http://loinc.org/license. LOINC® is a registered United States trademark of Regenstrief Institute, Inc.

Associated ICD10-CM Codes
   I27.82Chronic pulmonary embolism
   I50.9Heart failure, unspecified
   T79.1XXAFat embolism (traumatic), initial encounter
   J18.0Bronchopneumonia, unspecified organism

Associated ICD9-CM Codes
   482.9Bacterial pneumonia nos
   485Bronchopneumonia org nos
   481Pneumococcal pneumonia
   480.9Viral pneumonia nos
   958.1Fat embolism
   673.20Ob pulm embol nos-unspec
   428.0Chf nos


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