We agree with the comments of Chung and colleagues. We have encountered similar problems and have devised practices for minimizing them. The most important are the development of a close working relationship between interested clinicians and laboratory personnel and proper implementation of standardized heparin order sheets. After years of practice, our coagulation laboratory staff have become adept at calibrating the therapeutic aPTT range for new thromboplastin reagents. We have experienced several major shifts in aPTT therapeutic ranges over the years: Our lowest calibrated range was 46 to 70 seconds, and our highest was 75 to 105 seconds. The calculated aPTT therapeutic range in our hospital is published as part of a preprinted heparin order sheet that is available in all patient care areas. This order sheet incorporates weight-based dosing (1) and has been widely accepted at our institution (2). Whenever the therapeutic range is recalibrated because of a change in reagents, all of the previous order sheets are collected and destroyed, and new ones are distributed. Staff from our coagulation laboratory carry out this procedure on the morning that the new reagent is introduced. It hasn't been necessary to do this more often than once every few years because our laboratory does not change thromboplastin reagents unless there is a compelling reason, and lot-to-lot variation in our reagents has not been significant enough to warrant a change in the heparin order sheets. Over the years, inappropriate aPTT ranges have been introduced into our institution as part of multi-institutional clinical trials. We have declined participation in trials that do not allow participating institutions to use properly calibrated aPTT ranges.

The solution to the problem of maintaining an appropriate therapeutic range for monitoring unfractionated heparin requires a team effort and institutional support. Individual clinicians cannot do all this on their own.