Statistical & Clinical Analysis
Stewart Acid-Base Formula: Apparent Strong Ion Difference (SID_a) & Strong Ion Gap (SIG)
Unmask hyperchloremic acidosis and unmeasured anions in hypoalbuminemic ICU patients using Peter Stewart’s physicochemical SID_a, Figge-Fencl effective SID_e, and Strong Ion Gap SIG = SID_a - SID_e.
Why 0.9% normal saline resuscitation drops SID_a toward zero—and why Figge-Fencl SID_e reveals hidden unmeasured anions!
Peter A. Stewart (1983) revolutionized ICU acid-base physiology by recognizing that plasma electroneutrality forces [H+] and [HCO3-] to respond to three independent variables: PaCO2, total weak acids [A_tot] (albumin and inorganic phosphate), and Apparent Strong Ion Difference SID_a = ([Na+] + [K+] + [Ca2+] + [Mg2+]) - ([Cl-] + [Lactate-]) (normal ~40–44 mEq/L; 0.9% NaCl has equal 154 Na+ and 154 Cl- -> SID = 0 mEq/L, directly causing hyperchloremic metabolic acidosis)!
James Figge, Vladimir Fencl, and John Kellum quantified Effective Strong Ion Difference SID_e = [HCO3-] + Albumin_g/L * (0.123*pH - 0.631) + Phosphate_mmol/L * (0.309*pH - 0.469), defining Strong Ion Gap SIG = SID_a - SID_e (normal ≈ 0–2 mEq/L)! In a septic patient ([Na+]=140, [K+]=4.2, [Ca2+]=2.4, [Mg2+]=1.6, [Cl-]=112, [Lac-]=5.5, HCO3=16.0, Alb=28 g/L, Pi=1.8 mmol/L, pH=7.24), narrowed SID_a = 30.70 mEq/L and elevated SIG = +4.25 mEq/L diagnose combined hyperchloremic + unmeasured-anion acidosis!
| ICU Plasma Chemistry Profile | Apparent SID_a (mEq/L) | Strong Ion Gap SIG = SID_a - SID_e | Stewart Physicochemical Diagnosis |
|---|---|---|---|
| Cl=112, Lac=5.5, HCO3=16, Alb=28, pH=7.24 | 30.70 mEq/L (Low!) | +4.25 mEq/L (> 2 Elevated!) | Hyperchloremia + Lactate + Unmeasured Anions |
| Cl=102, Lac=1.2, (SID_a Normalized) | 45.00 mEq/L (Normal) | Normal <= 2.0 mEq/L Reference | Normal Strong-Ion Balance |
| Invalid pH <= 6.5 or Negative Ion Level | Error Guard | "Invalid Plasma Electrolyte Panel" | Physiological Bounds Guard |
Apparent Strong Ion Difference SID_a = (Na + K + Ca + Mg) - (Cl + Lactate) (mEq/L)
Computes net strong cation-minus-anion charge in mEq/L across B2:B7.
=IF(OR(MIN(B2:B10) < 0, B11 < 6.6, B11 > 7.9), "Invalid Plasma Electrolyte Panel", ROUND((B2 + B3 + B4 + B5) - (B6 + B7), 2))Figge-Fencl-Kellum Strong Ion Gap SIG = SID_a - SID_e (mEq/L)
Subtracts bicarbonate + pH-dependent albumin (g/L, B9) + phosphate (mmol/L, B10) buffer charge SID_e from SID_a.
=IF(OR(MIN(B2:B10) < 0, B11 < 6.6, B11 > 7.9), "Invalid Plasma Electrolyte Panel", ROUND(((B2 + B3 + B4 + B5) - (B6 + B7)) - (B8 + B9 * (0.123 * B11 - 0.631) + B10 * (0.309 * B11 - 0.469)), 2))Try the example
Use the original values or change them to test your understanding.