Serum sulfate concentration and the anion gap in hemodialysis patients

First-of-the-month predialysis serum sulfate (SO4) and other blood chemistry values were measured prospectively for 5 to 7 months in 14 patients undergoing single pass chronic tri-weekly maintenance hemodialysis with bicarbonate dialysate. Blood was also obtained predialysis and again immediately postdialysis from seven patients (five of whom also participated in the chronic study). As expected, the patients manifested a high anion gap (AG) metabolic acidosis. Serum SO4 was only moderately stable from month to month (the average coefficient of variation was 0.30; correlation between the serum SO4 value of month one and months two and five were r = 0.59, p = 0.026; and r = 0.38, p = 0.182, respectively). The ratio of mean serum SO4 to mean AG (5.0 +/- 0.4 [SE] mEq/L divided by 19.1 +/- 0.5 mEq/L) was 0.26. Although there was a statistically significant correlation between the serum SO4 and the blood urea nitrogen (BUN), there was no such correlation between SO4 and AG. A single hemodialysis reduced serum SO4 by 54% (from 3.5 +/- 0.5 mEq/L to 1.6 +/- 0.1 mEq/L), but there was no correlation between the change in SO4 and the change in AG. The authors concluded that SO4 contributes importantly to the elevated AG in patients receiving chronic hemodialysis. Single pass bicarbonate hemodialysis temporarily reduces, but does not normalize, both the serum SO4 and the AG of such patients.