Reference intervals for anion gap and strong ion difference in pregnancy: a pilot study

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).

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).

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 PCO(2).

Anion gap and SID both decrease in pregnancy, events which must be taken into account when managing acid-base disorders in gestation.