Reference Intervals for Anion Gap and Strong Ion Difference in Pregnancy: A Pilot Study

Abstract

Objective: Acid-base status and electrolyte levels change in normal pregnancy. We hypothesized that these physiological changes affect both anion gap and strong ion difference (SID). Methods: A cross-sectional study of normal pregnant women (6 in the first trimester, 47 in the second trimester, 59 in the third trimester, and 13 postpartum). Results: The anion gap in pregnancy (8.5 ± 2.9 mEq/L) was less, compared to postpartum values (10.7 ± 2.5 mEq/L), (p < 0.005). The SID, too, was lower (pregnancy, 38.3 ± 2.9 mEq/L, postpartum 43.5 ± 2.3 mEq/L, p < 0.001), reflecting the combined influence of changes in albumin, SID, and PCO₂. Conclusion: Anion gap and SID both decrease in pregnancy, events which must be taken into account when managing acid-base disorders in gestation.

Keywords:

• Anion gap
• Strong ion difference
• Reference intervals
• Pregnancy