Stability of the Strong Ion Gap versus the Anion Gap over Extremes of PCO 2 and pH

The strong ion gap (SIG) is under evaluation as a scanning tool for unmeasured ions. SIG is calculated by subtracting [buffer base], which is ([A^-]+[HCO₃^-]), from the apparent strong ion difference, which is ([Na⁺] + [K⁺]+[Ca⁺⁺]+[Mg⁺⁺]-[Cl^-]-[L-lactate]). A^- is the negative charge on albumin and phosphate. We compared the pH stability of the SIG with that of the anion gap (AG).

In normal and hypoalbuminaemic hyperlactaemic blood, PCO₂ was reduced stepwise in vitro from >200 mmHg to < 20 mmHg, with serial blood gas and electrolyte analyses, and [albumin] and [phosphate] measurement on completion.

Respective [haemoglobin], [albumin], [phosphate] and [lactate] in normal blood were 156 (0.9) g/l, 44 (2) g/l, 1.14 (0.06) mmol/l and 1.7 (0.8) mEq/l, and in hypoalbuminaemic blood 116 (0.9) g/l, 24 (2) g/l, 0.78 (0.06) mmol/l and 8.5 (0.5) mEq/l. pH increased from <6.85 to >7.55, causing significant falls in [Na⁺] and elevations in [Cl^-]. Initial and final SIG values did not differ, showing no correlation with pH. Mean SIG was 0.5±1.5 mEq/l. AG values were directly correlated with pH (normal: R²=0.51, hypoalbuminaemic: R²=0.65). Final AG values significantly exceeded initial values (normal blood: 15.9 (1.7) mEq/l versus 8.9 (1.8) mEq/l, P<0.01; hypoalbuminaemic blood: 16.5 (0.8) mEq/l versus 11.8 (2.0) mEq/l, P<0.05).

We conclude that, unlike the AG, the SIG is not affected by severe respiratory acidosis and alkalosis, enhancing its utility in acid-base disturbances.