Use of a fixed activated partial thromboplastin time ratio to establish a therapeutic range for unfractionated heparin

Background: The commonly recommended therapeutic range for patients receivi ng unfractionated heparin of 1.5 to 2.5 times the control activated partial thromboplastin time (aPTT) is not universally applicable. It has been sugg ested that the therapeutic range for each aPTT reagent should be based on p lasma heparin levels. We sought to identify an aPTT ratio that corresponds to therapeutic anti-factor Xa heparin levels for combinations of several re agents and coagulometers that are commonly used. Methods: Citrated plasma was collected from 126 unselected patients receivi ng unfractionated heparin. Four automated coagulometers and 6 commercial aP TT reagents were used to measure the aPTT. Plasma antifactor Xa levels were measured by means of a commercially available assay. The relationship betw een the aPTT results and anti-factor Xa heparin levels for each reagent-coa gulometer combination was determined by linear regression analysis, and the aPTT results corresponding to therapeutic anti-factor Xa heparin levels we re calculated. Results: For all reagent-coagulometer combinations studied, an aPTT ratio o f 1.5 resulted in anti-factor Xa heparin levels considerably below the lowe r limit of the therapeutic range. When the aPTT was performed on any of the coagulometers assessed with the use of Actin (Dade Diagnostics, Aguada, Pu erto Rico) and IL Test (Instrumentation Laboratories, Fisher Scientific, Un ionville, Ontario) reagents, aPTT ratios necessary to achieve therapeutic a ntifactor Xa heparin levels approximated 2.0 to 3.5. Conclusion: For laboratories that cannot perform heparin levels, the use of less responsive reagents and any of the coagulometers studied, along with target aPTT ratio between 2.0 and 3.5, appears to be a reasonable alternati ve.