Changes in the anion gap in patients undergoing hemodiafiltration

: The serum anion gap (AG) is a calculated value defined as the difference between the sum of sodium and potassium and the sum of chloride and bicarbonate concentrations. Thus, the anion gap is equal to the unmeasured cations minus the unmeasured anions (UA). To evaluate the AG changes during HDF-on line, we studied 20 patients treated with this technique. Blood pH, HCO3, NA, K, Cl, albumin, phosphorus, urea, creatinine and lactate were determined pre and post-HDF. The AG, negative charger of serum albumin (CAA) and phosphate (CAP) were computed by equations. AG decreased during HDF from 23.1 +/- 3.4 mEq/l to 17.3 +/- 3.6 mEq/l (p < 0.001). The CAA rose from 10.9 +/- 0.8 to 12.3 +/- 1.7 mEq/l (p < 0.001). The CAP and lactate fell significantly during HDF (p < 0.001 and 0.05 respectively). Other unmeasured anions (UA) decreased from 7.9 +/- 3.0 to 2.4 +/- 2.7 mEq/l (p < 0.001). The CAA contributed 47.7 +/- 6.5% and 73.01 +/- 12.7% to the pre and post-HDF serum anion gap respectively. The CAP accounted for 12.4 +/- 3.4% and 8.6 +/- 1.8%, lactate 6.4 +/- 3.9% and 6.0 +/- 3.0% and UA for 33.2 +/- 7.7% and 12.2 +/- 13.6% of the anion gap pre and post-HD respectively. AG and UA correlated significantly with blood urea pre-HDF and urea generation. The increase in serum albumin and pH can mask an decreased concentration of unmeasured anions in patients treated with HDF on-line. An adjusted anion gap without effect of CAA and CAP can be obtained. With the help of this adjustments the changes in some undetermined anions organic and inorganic (sulphate and others in renal failure) can be calculated.