REPLY
Monitoring Heparin Therapy
Patrick Brill-Edwards, MD;
Jeffrey S. Ginsberg, MD; and
Jack Hirsh, MD
15 January 1994 | Volume 120 Issue 2 | Pages 169-170
IN RESPONSE:
The results of our study show that the variation in the responsiveness of the aPT reagents to heparin is problematic. We presented one method to compensate for the lack of standardization of aPT reagents.
Drs. Raschke and Reilly agree that a need exists to improve the common target of 1.5 to 2.5 times the control value. The method they propose would be reasonable if the responsiveness of different lots of the reagent did not change significantly over time; however, we have shown significant lot-to-lot variation using the same reagent. Further, the use of different laboratory instruments may alter the therapeutic range in different centers.
Dr. Shojania and colleagues have previously published that the responsiveness of aPT reagents to heparin differs when using ex vivo compared with in vitro plasma samples [1]; the use of in vitro plasma samples produces response curves that have a steeper slope. A choice must therefore be made concerning the source of plasma samples that are used to establish the therapeutic range. We chose to base our range on ex vivo samples because prospective, randomized controlled trials have shown a strong correlation between clinical outcomes and a therapeutic range of aPT results corresponding to protamine titration heparin levels of 0.2 to 0.4 U/mL when using ex vivo plasma from patients receiving heparin [2, 3]. The method proposed by Dr. Shojania [4] has been evaluated in a retrospective chart review of nonconsecutive patients, in which the decision to use the therapeutic range as suggested was optional, and no systematic diagnosis of thromboembolism or recurrent events was made. The laboratory results presented in their study represented the first 72 hours of heparin therapy only, and no clinical follow-up was done. It may not be appropriate, therefore, to recommend that in vitro plasma be used to establish a therapeutic range of aPT results based on their results.
The ideal method to develop a therapeutic range for aPT results would account for aPT reagent variation and would allow individual institutions to control for sources of variation other than the reagents. We agree that, as alternative methods are developed [5], feasibility must also be emphasized.
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Author and Article Information
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McMaster University Medical Centre; Hamilton, Ontario LSN 375; Canada
Hamilton Civic Hospitals Research Centre; Hamilton, Ontario LSN 1C3; Canada
1. Shojania AM, Tetreault J, Turnbull G. The variations between heparin sensitivity of different lots of activated partial thromboplastin time reagent produced by the same manufacturer. Am J Clin Pathol. 1988; 89:19-23.
2. Hull RD, Raskob GE, Rosenbloom D, Panju A, Brill-Edwards P, Ginsberg JS, et al. Heparin for 5 days as compared with 10 days in the initial treatment of proximal venous thrombosis. N Engl J Med. 1990; 322:1260-4.
3. Turpie AG, Robinson JG, Doyle DJ, Mulju AS, Mishkel GJ, Sealey BJ, et al. Comparison of high dose with low dose subcutaneous heparin to prevent left ventricular mural thrombosis in patients with acute transmural anterior myocardial infarction. N Engl J Med. 1989; 320:352-7.
4. Zanke B, Shojania AM. Comparison of two aPT methods of monitoring heparin therapy. Am J Clin Pathol. 1990; 93:684-9.
5. Johnston M, Moffat K, Ginsberg J, Hirsh J, Brill-Edwards P. A simplified procedure for the establishment of APT therapeutic ranges for heparin (HEP) therapy. Thromb Haemost. 1993; 69:652.
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