Calculator

Alveolar-arterial O₂ gradient

New

PAO₂ = FiO₂ × (Patm − 47) − PaCO₂/0.8; A-a = PAO₂ − PaO₂.

fraction
mmHg
mmHg
mmHg
years

A-a gradient

30 mmHg

Formula

fio2 * (patm - 47) - paco2 / 0.8 - pao2

Notes

Expected upper limit on room air ≈ age/4 + 4 mmHg. Less reliable at high FiO₂.

References

  1. Albert MS, Dell RB, Winters RW. Quantitative displacement of acid-base equilibrium in metabolic acidosis. Ann Intern Med 1967;66:312-22.

Used in

Oxygen prescription and SpO2 targetsFive-step blood gas interpretation

Calculators support, not replace, clinical judgement. Check inputs and units, and confirm doses with your unit protocol.