Alteration of anion gap during almost total plasma replacement with synthetic colloids in piglets

In ten piglets (body weight 8.2-11.6 kg), acid base, electrolyte and anion gap changes were investigated during almost total plasma replacement with hydroxyethyl starch (HES) and modified fluid gelatin (GEL) in saline solution using a cell saver autotransfusion technique. During the study, there were only moderate acid base changes, but marked disturbances in anion balance. At study end, the mean chloride concentration was significantly higher (mmol/l: normal values 97-108, HES 116 +/- 1.5, GEL 108 +/- 1.1, p < 0.01) and the mean anion gap was significantly lower in the HES group in comparison to the GEL group (mmol/l: normal values 5-14, HES 3 +/- 1.7, GEL 11.9 +/- 0.9, p < 0.01). It is concluded that plasma replacement with electroneutral HES, but not with negatively charged GEL, can lower the anion gap irrespective of the underlying disease. This can be misleading when the anion gap is used for differential diagnosis of metabolic acidosis in patients after large volume infusion of synthetic colloids.