Patients with thrombotic thrombocytopenic purpura commonly develop metabolic alkalosis during therapeutic plasma exchange

Thrombotic thrombocytopenic purpura (TTP) and myasthenia gravis (MG) are ca tegory I indications for therapeutic plasma exchange (TPE). This study was based on the hypothesis that the development of metabolic alkalosis during TPE is more common in TTP than in MG, based on our previous observations. I n order to test it, we compared the levels of bicarbonate and potassium in both groups of patients undergoing plasmapheresis. Fifteen patients with TT P (190 procedures) and ten MG patients seen concurrently were studied. Whil e baseline bicarbonate levels were similar among all patients, the post-pro cedure bicarbonate levels in TTP patients were mostly elevated with a mean SD of 29.4 +/- 3.5 mEq/L, as opposed to decreased or unchanged in MG patien ts 26.3 +/- 3.1 mEq/L (mean +/- SD) (P = 1.4 x 10(-8)). Furthermore, alkalo sis in the TTP group persisted throughout subsequent daily treatments. Ther e was also a significant decrease between pre- and post-TPE potassium level s in TTP patients (P = 3 x 10(-21)) by paired Student's t test. Additionall y, samples with levels <3.3 mEq/L were alkalotic 75% of the time. In the MG group, however, potassium was normal in 85% and 83% of the pre- and post-T PE samples, respectively. Consequently, the hypokalemia was significantly m ore marked in the TTP group (P = 0.0008). These data confirm that plasmaphe resis commonly induces metabolic alkalosis in TTP patients, probably due to high citrate in fresh frozen plasma, the frequency of treatments, and perh aps decreased renal clearance due to disease involvement of the kidneys. (C ) 2001 Wiley-Liss, Inc.