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A a gradient

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The term A-a gradient, or alveolar-arterial gradient, refers to the difference between the oxygen concentration in the alveoli of the lungs and in the arteries. The A-a gradient has important clinical utility as it can help narrow the differential diagnosis for hypoxemia.


It is calculated by the following formula

A-a (O2) = (FiO2%/100) * (Patm - 47 mmHg) - (PaCO2/0.8) - PaO2

FiO2 Room Air = 21 %

Atmospheric Pressure = 760 mm Hg at sea level

Water vapor pressure pH2O (mmHg) = 47 mm Hg at 37 degrees Celsius

Respiratory quotient RQ (VCO2/VO2) = 0.8 (usual)

Another way is: PiO2 – PaCO2/0.8

  • PiO2 = FiO2 x (barometric pressure – saturated vapour pressure of H20)
  • PiO2 = 0.21 x (760 – 47) – sea level.
  • PiO2 = 150mmHg.

Clinical use

The A–a gradient is helpful in determining the source of hypoxemia such as intrapulmonary (within the lungs) or extrapulmonary (elsewhere in the body).

Normal values

A normal A–a gradient for a young adult non-smoker breathing air, is between 5–10 mmHg.

Age and A-a gradient

Normally, the A–a gradient increases with age. For every decade a person has lived, their A–a gradient is expected to increase by 1 mmHg. A conservative estimate of normal A–a gradient is less than [age in years/4] + 4. Thus, a 40-year-old should have an A–a gradient less than 14.


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