Effect of pyridostigmine bromide on serum bromide concentration and the anion gap.

B romide (Br) causes positive interference with all clinical laboratory methods for the detenmination of serum Cl (1 .2). Interference with Cl measurement is much greater with ion-selective electrode (ISE) technology (1 .3) than with coubometry (1 -4). We recently described a patient ingesting therapeutic amounts of pynidostigmine Br (MestInon1 , containing 1 8.4 mg of Br [30.6%j per 60-mg tablet). the medication of choice for the management of myasthenia gravis (4). HIs serum Br was sufficiently high to cause artifactual increases In serum Cl and HCO3, which reduced the apparent anion gap [Na (Cl + HCO3)] to -9 mmol/liter. The spuriously high bevels of Cl and HCO3 were measured on a Kodak Ektachem 700 automated analyzer (Eastman Kodak Co. , Rochester, NY) and were not apparent when a Beckman Astra8 analyzer (Beckman Instruments, Inc., Brea, CA), which uses coulometry for Cl determination, was employed (4). taking pyridostigmine Br. None had clinical evidence of Br toxicity. Fourteen had myasthenia gravis; one had the Eaton-Lambert syndrome. Thirteen were outpatients: two were hospitalized-both for decompensation of their myasthenic state. The patients’ data are compared with those of 1 8 normal controls not taking any medications whose serum Br bevels were within the normal range. All sera were analyzed for Na, Cl, HCO3. K, and creatinine (Cr) with a Kodak Ektachem 700 automated analyzer (Eastman Kodak Co.) (5). Na, Cl, HCO3, and K were also determined with a Beckman Synchron CX3 analyzer (Beckman Instruments, Inc.) (6). Serum Br was measured by the trichloroacetic acid method (7,8), which detects Br concentrations as low as 1 0 zg/mL (0. 1 25 mmob/L). With It, the serum Br bevel in normal individuals is less than 30 ig/mL (0.375 mmob/L) (8). The Ektachem 700 measures Cl and HCO3 (total CO2 content) by using simultaneous deposition of 10 zL of sample on separate halves of a multilayered “dry slide” (5). The methodology differs for each assay: Cb-ISE (potentiometry) and HCO3-ISE (membrane sensitive to carbonate ion) (5). The Synchron CX3 measures Cl with a two-phase Ag/AgC1 ISE and measures HCO3 by the differential pH rate of change for CO2 employing both measuring and reference pH electrodes (6). Statistical analysis was carried out by one-way analysis of variance followed by the Newman-Keuls multiple range test (with P set at 0.05) (9). We also used the Bonfernoni method ( 1 0), wherein dividing the P value (t test) usually required for significance (0.05) by the number of potential companisons gives the new P value necessary to achieve statistical significance.