STRONG ION GAP - A METHODOLOGY FOR EXPLORING UNEXPLAINED ANIONS

Purpose: This paper describes the calculation of the strong ion gap (S IG), a physical chemical methodology similar to the anion gap (AG), as a measure of the anion/cation balance exclusive of sodium, potassium, chloride, and bicarbonate. We compared the SIG and AG methodologies i n three groups of subjects with and without unexplained anions. These groups were (1) healthy volunteers with hyperlacticemia during exercis e; (2) intensive care unit (ICU) patients with sepsis; and (3) ICU pat ients with severe liver disease. Methods: The SIG, AG, and corrected A G (AGc) were calculated for each group from data available in the orig inal reports (groups 1 and 2) and by retrospective chart review (group 3). Results: The SIG correlated poorly with the AG in group 2, wherea s no correlation was seen in groups 1 and 3. The AGc correlated with S IG in all three groups (r = .99, .93, and .91 respectively; P < .01 fo r each group). Although the AG was similar, the SIG differed for each group. Group 1 had levels of SIG near zero, and groups 2 and 3 had mea n SIG's of 4.80 +/- 4.67 mEq/L and 9.60 +/- 6.43 mEq/L respectively. T he composition of the anion gap differed markedly among subject types. Conclusions: The SIG correlates with the AG once corrected for all kn own anions. The SIG technique can detect unknown anions in a patient p opulation known to have them and does not detect unknown anions in hea lthy volunteers during exercise. This test detects large amounts of un known anions in some patients with sepsis or liver disease. Therefore, the test is both sensitive and specific in characterizing metabolic a cidosis.