PHYScalc: Blood Gas Calculator – Critical Care Info

First try at a combined blood gas and Stewart method calculator. I’m not entirely happy with it, but refinements will come soon.

Stewart Method Acid–Base Calculator

Strong Ion Difference (SID), Effective Strong Ion Difference (SIDe), Strong Ion Gap (SIG), Partial pressure of carbon dioxide (PaCO₂), Total concentration of weak acids (A_TOT).

pH PaCO₂

Bicarbonate (HCO₃⁻, mmol/L)

using calculated

Sodium (Na⁺, mmol/L) Potassium (K⁺, mmol/L) Chloride (Cl⁻, mmol/L) Ionised Calcium (Ca²⁺, mmol/L) Magnesium (Mg²⁺, mmol/L) Lactate (Lac⁻, mmol/L) Albumin (g/L) Phosphate (Pi, mmol/L) Other measured strong anions (sum, mmol/L)

Examples (only if measured): sulfate, ketoacids, citrate, formate, glycolate, pyroglutamate, salicylate, D-lactate, bromide/iodide, urate, hippurate.

Bicarbonate (calculated): 24.3 mmol/L

Albumin charge (Alb⁻): 11.2 mmol/L

Phosphate charge (Phos⁻): 1.8 mmol/L

SIDa: 42.1 mmol/L

SIDe: 37.3 mmol/L

SIG (SIDa − SIDe): 4.8 mmol/L

A_TOT (bedside estimate): 11.0 mmol/L eq

Pattern: normal/near-zero SIG

Formulas: SIDa = (Na⁺ + K⁺ + Ca²⁺ + Mg²⁺) − (Cl⁻ + Lactate⁻ + other measured strong anions). SIDe = HCO₃⁻ (measured or calculated) + Alb⁻ + Phos⁻. SIG = SIDa − SIDe. HCO₃⁻(calc) = α·PaCO₂·10^(pH − pKₐ); α = 0.0307 mmol·L⁻¹·mmHg⁻¹ (0.2303 mmol·L⁻¹·kPa⁻¹), pKₐ ≈ 6.10 (37 °C). Alb⁻ = (0.123·pH − 0.631)·Albumin(g/L); Phos⁻ = (0.309·pH − 0.469)·Phosphate(mmol/L). A_TOT(est) ≈ 0.25·Albumin(g/L) + Phosphate(mmol/L). Use ionised Ca²⁺ where available.

Stewart PA. How to Understand Acid–Base. Elsevier; 1981. Figge J, Mydosh T, Fencl V. J Lab Clin Med. 1992;120:713–719. Figge J, Rossing TH, Fencl V. J Lab Clin Med. 1991;117:453–467.

29/08/2025

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| Category: Biochemistry, Calculators, Emergency Medicine, Intensive Care

Tags: blood gas, calculator

Critical Care Info

Stewart Method Acid–Base Calculator

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