ABG Interpretation Calculator with A-a Gradient

Step 1. pH

pH below 7.35 = acidemia; above 7.45 = alkalemia. If the pH is within 7.35 to 7.45, use 7.40 as the cutoff (7.37 counts as the acid side, 7.42 as the alkaline side).

Source: Arterial Blood Gas Analysis: Fundamentals, Interpretation, and Clinical Utility (NCBI Bookshelf, NIH) ^[1]

Step 2. Primary disorder

Acid side: PaCO₂ above 40 mm Hg = respiratory acidosis; HCO₃ below 24 mEq/L = metabolic acidosis. Alkaline side: PaCO₂ below 40 = respiratory alkalosis; HCO₃ above 24 = metabolic alkalosis. The value that matches the pH is the primary disorder; the other one shows compensation. If both match the pH, two disorders are present (mixed).

If pH, PaCO₂ and HCO₃ are all inside the official USMLE reference ranges (pH 7.35-7.45, Pco₂ 33-45 mm Hg, HCO₃ 22-28 mEq/L), the calculator reports no disorder.

Source: Arterial Blood Gas Analysis: Fundamentals, Interpretation, and Clinical Utility (NCBI Bookshelf, NIH) ^[1]; Anion Gap and Non-Anion Gap Metabolic Acidosis (NCBI Bookshelf, NIH) ^[2]; USMLE Step 1 Sample Test Questions: USMLE Laboratory Values, pages 7 to 9 (USMLE program, usmle.org) ^[3]

Step 3. Compensation: metabolic disorders

Metabolic acidosis (Winter’s formula): expected PaCO₂ = 1.5 × HCO₃ + 8 ± 2

Metabolic alkalosis: expected PaCO₂ = 0.7 × HCO₃ + 20 ± 5

PaCO₂ higher than expected = an additional respiratory acidosis; lower = an additional respiratory alkalosis.

Source: Partial Pressure of Carbon Dioxide (NCBI Bookshelf, NIH) ^[4]; Physiology, Metabolic Alkalosis (NCBI Bookshelf, NIH) ^[5]

Step 3. Compensation: respiratory disorders

Respiratory acidosis: HCO₃ rises about 1 mEq/L per 10 mm Hg rise in PaCO₂ when acute, and 4 to 5 mEq/L per 10 mm Hg when chronic (after 3 to 5 days).

Respiratory alkalosis: HCO₃ falls about 2 mEq/L per 10 mm Hg fall in PaCO₂ when acute, and 4 to 5 mEq/L per 10 mm Hg when chronic.

The change is counted from a PaCO₂ of 40. Because the patient’s own starting bicarbonate is unknown, the calculator applies the change to the whole official HCO₃ range (22 to 28 mEq/L) and reports whether the measured value fits the acute range, the chronic range, both, or neither (a possible extra metabolic disorder).

Source: Partial Pressure of Carbon Dioxide (NCBI Bookshelf, NIH) ^[4]; Hypocarbia (NCBI Bookshelf, NIH) ^[6]; USMLE Step 1 Sample Test Questions: USMLE Laboratory Values, pages 7 to 9 (USMLE program, usmle.org) ^[3]

Step 4. Oxygenation: alveolar gas equation and A-a gradient

PAO₂ = FiO₂ × (P_atm − 47) − PaCO₂ / 0.8 (47 mm Hg = water vapour; 0.8 = respiratory quotient). At sea level on room air with a PaCO₂ of 40: (760 − 47) × 0.21 − 40 / 0.8 = 99.7 mm Hg.

A-a gradient = PAO₂ − PaO₂. Expected for age = (age + 10) / 4.

Higher than expected: a problem at the alveolar-capillary unit, such as V/Q mismatch, shunt or diffusion impairment (for example pneumonia). Hypoxemia from hypoventilation keeps a normal A-a gradient. The age equation assumes room air; the A-a gradient rises about 5 to 7 mm Hg for every 10% rise in FiO₂.

Source: Partial Pressure of Oxygen (NCBI Bookshelf, NIH) ^[7]; Alveolar Gas Equation (NCBI Bookshelf, NIH) ^[8]; Physiology, Alveolar to Arterial Oxygen Gradient (NCBI Bookshelf, NIH) ^[9]; Arterial Blood Gas Analysis: Fundamentals, Interpretation, and Clinical Utility (NCBI Bookshelf, NIH) ^[1]