Setting Up a Heparin Response Curve
Q: Our lab director has asked me to set up a “heparin response curve” to yield a curve to the appropriate heparin dosage for each patient, based on the particular brand of heparin in our formulary. The curve is to be specific to our brand of heparin, of APTT reagents and our geographic population. Could you help me design a curve specific for our hospital?
A:It is important for laboratories, in concert with clinicians and pharmacists, to establish a valid therapeutic range. Now that provision of recommended therapeutic ranges is mandated by our accrediting agencies (refer to CAP Inspection Checklist question 02.3712) the emphasis on the laboratory’s role in providing this service reaches a greater magnitude. We are, therefore, compelled to do the best we can given the resources we have in our individual lab settings.
The traditional in vitro method of using samples of plasma to which a range of heparin concentrations have been added is no longer favored, since the relation- ship between APTT values and heparin levels differs significantly when mea- sured in patients treated with heparin (ex vivo). This is related to the clearance rate of heparin fractions, as well as metabolism, levels of antithrombins, PF4 activity and factor VIII and fibrinogen levels.
To establish a therapeutic range for each new lot of APTT reagent: 1. Collect samples from patients on specific heparin protocols, not receiving concomitant warfarin. 2. Perform APTT within 2 hours of collection, freeze remaining aliquots, accumulating 50-60 samples. 3. Perform heparin assay by chromogenic anti-Xa method (protamine titration is much too cumbersome), using the same heparin preparation from the hospital pharmacy used in your patients’ treatment regimen to construct the standard curve. 4. Plot the clotting times vs. heparin levels using regression analysis, calculating a line of best fit. Plots will be scattered–this is expected. Use the ordinate value of the regression line corresponding to 0.3-0.7 u/ml of heparin to determine the APTT therapeutic range (equivalent to 0.2-0.4 u/ml by protamine titration).
–Lois Cantwell, MT(ASCP)
A: The in vitro heparin response curve is a time-honored technique for matching your laboratory’s PTT results to therapeutic plasma heparin concentrations measured in international units (IU). From your pharmacy, obtain a vial of the current heparin lot that has a heparin concentration of 10 IU or 100 IU/mL, if possible. Higher heparin concentrations are hard to accurately dilute. Next, calculate and prepare the following dilutions of heparin in normal saline: 0.5, 1, 2, 4, 8, and 10 IU/mL. Add 0.1 mL of each dilution to respective aliquots of 0.9 mL each of commercially prepared platelet-free normal plasma control or locally prepared normal platelet-free plasma. This yields final dilutions of 0.05, 0.1, 0.2, 0.4, 0.8, and 1.0 IU/mL. Mix well and perform a PTT on each dilution within one hour of preparation. Plot the heparin concentration on the horizontal axis and the PTT results on the vertical axis of 2’2 cycle log/log graph paper. The resulting graph may be used to convert PTT in seconds to heparin concentration in IUs/mL as long as the heparin lot, PTT reagent lot and instrumentation are unchanged. The PTT therapeutic range is the range of results, in seconds, that corresponds to 0.3-0.7 IU heparin/mL. The range may be different in your institution, and you may need to extend the number of dilutions to accommodate higher or lower therapeutic levels.
Unfortunately, the in vitro heparin response curve is no longer the best way to establish your lab’s heparin therapeutic range. In vitro heparin is absorbed to fibronectin, vitronectin, Von Willebrand factor and platelet factor 4 at rates that differ from in vivo. Further, the in vitro results “systematically underestimate the true therapeutic range.”
Since 1993, coagulation reference labs have used the Brill-Edwards approach (see Dec. 1 ADVANCE, “Top Ten Problems in Coag”) This is an “ex vivo” approach. Col-lect 50-150 plasma specimens from inpatients receiving heparin at all dosage ranges and perform PTT and heparin anti-Xa assays on each. Plot the heparin concentration on the horizontal axis and the time on the vertical axis of linear graph paper. The therapeutic range for your institution is the range of PTT seconds corresponding to 0.3-0.7 IU anti-Xa heparin/mL. Ranges vary with institutions.
If your laboratory is not equipped to perform the heparin anti-Xa assay, perform the PTT in your laboratory, then freeze aliquots from each patient at -70 degrees Celsius and ship on dry ice to a coagulation reference lab. Be sure to tell the reference laboratory customer service department what you are doing so that the assays are billed to you at a group rate.
Even if your lab director continues to insist on the heparin response curve, you should validate your curve at least once by comparing it to the Brill-Edwards adaptation.
–George Fritsma, MS, MT(ASCP)
(with input from Colorado
Coagulation Consultants)
Suggested Readings
Brill-Edwards, P., et al. Establishing a therapeutic range for heparin therapy. Ann Intern Med 1993; 199:104-109.
THIS ISSUE’S Q&A PANEL:
Lois Cantwell, MT(ASCP) is senior medical technologist, Coagulation Lab-oratory at Henry Ford Hospital, Detroit, MI.
George Fritsma, MS, MT(ASCP), is associate professor of Clinical Labora-tory Sciences at the University of Alabama at Birmingham School of Health Related Professions.
