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
Version 1.0.0
Blood gas
kPa
mmol/L
Calculated bicarbonate will be used.
Strong ions and weak acids
mmol/L
mmol/L
mmol/L
mmol/L
mmol/L
mmol/L
g/L
mmol/L
mmol/L
Only include other strong anions that were measured, such as ketones, citrate, formate or salicylate.
Calculated bicarbonate
— mmol/L
Albumin charge
— mmol/L
Phosphate charge
— mmol/L
Apparent strong ion difference (SIDa)
— mmol/L
Effective strong ion difference (SIDe)
— mmol/L
Strong ion gap (SIG)
— mmol/L
ATOT bedside estimate
— mmol/L equivalents
This is a quantitative Stewart/Figge analysis. The frozen source contains no conventional primary-disorder or compensation decision tree; none is inferred here. Interpret results with the full clinical picture.
No clinical values are intentionally saved by this control.
Equations
Calculated bicarbonate (Henderson–Hasselbalch)
HCO3_calc = 0.0307 × PaCO2_mmHg × 10^(pH - 6.10)PaCO2_mmHg equals entered PaCO2 in mmHg or entered PaCO2 in kPa multiplied by 7.50061683. The fixed carbon dioxide solubility coefficient is 0.0307 mmol L^-1 mmHg^-1 and pKa is 6.10 at 37 °C. A valid measured bicarbonate replaces this calculated value only in SIDe.
Albumin weak-acid charge
Alb_charge = (0.123 × pH - 0.631) × albumin_g_per_LResult is expressed in mmol/L equivalents.
Phosphate weak-acid charge
Phos_charge = (0.309 × pH - 0.469) × phosphate_mmol_per_LResult is expressed in mmol/L equivalents.
Apparent strong ion difference
SIDa = (Na + K + Ca + Mg) - (Cl + lactate + other measured strong anions)All concentrations are entered in mmol/L, following the frozen calculator's bedside SIDa convention.
Effective strong ion difference
SIDe = HCO3_used + Alb_charge + Phos_chargeHCO3_used is measured bicarbonate when supplied and valid; otherwise it is calculated bicarbonate.
Strong ion gap
SIG = SIDa - SIDeThe preserved legacy interpretation is positive SIG when SIG > 5 mmol/L, negative SIG when SIG < -2 mmol/L, and normal/near-zero SIG from -2 through 5 mmol/L inclusive.
Total weak-acid bedside estimate
ATOT_est = 0.25 × albumin_g_per_L + phosphate_mmol_per_LThis is the frozen source's displayed bedside estimate in mmol/L equivalents and is not used to calculate SIDe or SIG.
References
- Stewart PA. How to Understand Acid-Base: A Quantitative Acid-Base Primer for Biology and Medicine. New York: Elsevier North Holland; 1981.
- Figge J, Mydosh T, Fencl V. Serum proteins and acid-base equilibria: a follow-up. J Lab Clin Med. 1992;120(5):713-719. PMID: 1431499
- Figge J, Rossing TH, Fencl V. The role of serum proteins in acid-base equilibria. J Lab Clin Med. 1991;117(6):453-467. PMID: 2045713
- Kellum JA. Determinants of blood pH in health and disease. Crit Care. 2000;4(1):6-14. PMID: 11094491 DOI: 10.1186/cc644