To the Editor.
—I read with great interest the article by Drs Mennemeyer and Winkelman1 evaluating patient outcomes as a tool for clinical laboratory quality assurance. The retrospective data suggest that patients are at increased risk for an adverse outcome when PT results are obtained and interpreted from a physician office laboratory. A patient tested in an office laboratory with PT test volume of less than 40 per month is at higher risk for stroke and acute myocardial infarction when compared with a similar patient tested in a commercial laboratory. An adverse outcome is also greater among patients who switch from one laboratory to another between successive PT testing.Mennemeyer and Winkelman fail to address how the variability of the sources and preparations of calcium and tissue thromboplastin were used in the various laboratories.2,3 The PT results are not comparable from laboratory to laboratory without knowing the thromboplastin