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Alveolar to Arterial Oxygen Difference

Estimate the oxygen difference with stated37°C and respiratory quotient assumptions.

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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Use the result with context

Health results are educational estimates, not a diagnosis or treatment recommendation. A qualified clinician can interpret measurements in the context of your health.

Formula and method

At101.325kPa,21% inspired oxygen,arterialCO₂40mmHg andarterialO₂80mmHg,the calculated difference is about19.73mmHg. Age40 gives the separate room-air heuristic14mmHg.

A–a difference = (FiO₂ percent /100) × (atmospheric pressure −47mmHg) − arterial CO₂/0.8 − arterial O₂. Room-air age reference = age/4 +4mmHg.

Worked example

Enter these known values and leave the other values blank.

Adult age (years)
40 years
Inspired oxygen fraction
21 %
Arterial CO₂ pressure
40 mmHg
Arterial O₂ pressure
80 mmHg
Atmospheric pressure
101.325 kPa
Calculated A–a oxygen difference
19.7 mmHg
Room-air age reference heuristic
14 mmHg

Assumptions and limitations

  • Adult measurement model at37°C with fixed respiratory quotient0.8. Use an arterial blood gas and the actual inspired oxygen fraction and atmospheric pressure from the same steady state. The separate age/4+4 output is a historical room-air reference heuristic; do not apply it as the normal limit on supplemental oxygen or use the gradient to rule out pulmonary embolism. Negative calculated differences can reflect measurement error or limitations of the fixed quotient, and are not a treatment indication.
  • Conventional millimeters of mercury use133.322387415Pa per mmHg.101.325kPa equals about759.99989mmHg; the760mmHg shortcut is a rounded approximation.
  • This calculation and its inverses do not establish a diagnosis or choose oxygen or other therapy.

Common questions

How is the oxygen difference calculated?

Estimate alveolar oxygen from the dry inspired pressure and arterial carbon dioxide under the fixed quotient, then subtract measured arterial oxygen. All pressures must use matching units.

Can the age reference rule out pulmonary embolism?

No. The original PIOPED analysis studied several reference criteria and found that normal A–a results can occur in pulmonary embolism. The displayed age reference is an approximate room-air comparison, not a diagnostic exclusion rule.

Does this apply to every oxygen and temperature setting?

Adult measurement model at37°C with fixed respiratory quotient0.8. Use an arterial blood gas and the actual inspired oxygen fraction and atmospheric pressure from the same steady state. The separate age/4+4 output is a historical room-air reference heuristic; do not apply it as the normal limit on supplemental oxygen or use the gradient to rule out pulmonary embolism. Negative calculated differences can reflect measurement error or limitations of the fixed quotient, and are not a treatment indication.

References

Bookify uses saturated water-vapor pressure47mmHg at37°C rather than the captured45, correcting the complete equation and all five inverse branches. Adult-age, positive inspired fraction and dry atmospheric pressure bounds define the stated model scope; the age/4+4 room-air reference remains a heuristic, not a diagnostic rule.

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