Stability of the strong ion gap versus the anion gap over extremes of PCO2 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.