1A-a Gradient — Formula & Interpretation
PAO₂ = (FiO₂ × (Patm − PH₂O)) − (PaCO₂ ÷ RQ)
At sea level: PAO₂ = (FiO₂ × 713) − (PaCO₂ ÷ 0.8)
A-a Gradient = PAO₂ − PaO₂
Age-adjusted normal A-a gradient = (Age ÷ 4) + 4 mmHg
Normal range at rest (room air): 5–15 mmHg (young) up to 25 mmHg (elderly)
Clinical Significance of A-a Gradient
The A-a gradient measures the efficiency of oxygen transfer from alveoli to blood. A normal A-a gradient in a hypoxaemic patient suggests a non-pulmonary cause — such as hypoventilation or low inspired oxygen. An elevated A-a gradient confirms a pulmonary problem — either V/Q mismatch, diffusion impairment, or shunt.
✅ Normal A-a Gradient — Non-Pulmonary Causes
- Hypoventilation (↑PaCO₂)
- Opioid/sedative overdose
- Neuromuscular disease
- High altitude (low FiO₂)
- Central hypoventilation
⚠️ Elevated A-a Gradient — Pulmonary Causes
- V/Q mismatch (PE, COPD, asthma)
- Pulmonary oedema
- Pneumonia / consolidation
- ARDS
- Pulmonary fibrosis
- Intracardiac shunt (ASD, VSD)
- Atelectasis
A-a Gradient in PE Diagnosis
An elevated A-a gradient is present in ~85% of patients with pulmonary embolism. However, a normal A-a gradient does NOT exclude PE — ~15% of PE patients have a normal A-a gradient, particularly young patients with smaller emboli. The A-a gradient is best used in conjunction with clinical pre-test probability (Wells Score) rather than as a standalone rule-out test. A normal A-a gradient combined with low Wells score significantly reduces PE probability.
A-a Gradient on Supplemental Oxygen
On higher FiO₂, the normal A-a gradient increases — the formula assumes this is accounted for. On 100% FiO₂, the normal A-a gradient can be up to 100 mmHg in healthy adults. A useful rule: on 100% O₂, PaO₂ should be >500 mmHg. If PaO₂ <300 mmHg on 100% O₂, there is significant intrapulmonary or intracardiac shunt.
2Frequently Asked Questions
What is the A-a gradient? +
The alveolar-arterial (A-a) gradient is the difference between the partial pressure of oxygen in the alveoli (PAO2, calculated) and in arterial blood (PaO2, measured on ABG). A normal gradient means efficient oxygen transfer. An elevated gradient indicates a gas exchange problem — V/Q mismatch, diffusion defect, or right-to-left shunt.
What is the normal A-a gradient? +
The normal A-a gradient is age-dependent: approximately Age/4 + 4 mmHg on room air. For a 40-year-old, normal is ~14 mmHg. On 100% O2, the normal upper limit rises to ~100 mmHg. Values significantly above these thresholds suggest impaired gas exchange.
What causes an elevated A-a gradient? +
Common causes: V/Q mismatch (PE, pneumonia, COPD, asthma, pulmonary oedema), diffusion defect (interstitial lung disease, ARDS), right-to-left shunt (intracardiac defect, hepatopulmonary syndrome). A normal A-a gradient with hypoxia suggests hypoventilation — check PaCO2.
When should A-a gradient be used over P/F ratio? +
A-a gradient is most reliable on room air (FiO2 0.21). At higher FiO2 levels the gradient naturally widens, reducing discriminatory value. In patients on supplemental oxygen, the P/F ratio (PaO2/FiO2) is preferred — it is independent of FiO2 and is the standard oxygenation index in ICU (normal >400, ARDS <300).
What is the PAO2 formula? +
PAO2 = (FiO2 × [Patm − PH2O]) − (PaCO2 / RQ). At sea level on room air: PAO2 = (0.21 × 713) − (PaCO2 / 0.8). PH2O at 37°C = 47 mmHg, Patm = 760 mmHg, RQ = 0.8 assumed. The A-a gradient = PAO2 − PaO2 (from ABG).
Is A-a gradient useful for diagnosing PE? +
A raised A-a gradient raises suspicion for PE but is non-specific — nearly 20% of proven PEs have a normal gradient. It should not be used in isolation. Use Wells score + D-dimer → CTPA pathway for PE diagnosis. A-a gradient is more useful to confirm that hypoxia has a pulmonary cause rather than hypoventilation alone.
3Related Calculators