The commonly recommended therapeutic range for patients receiving unfractionated heparin of 1.5 to 2.5 times the control activated partial thromboplastin time (aPTT) is not universally applicable. It has been suggested that the therapeutic range for each aPTT reagent should be based on plasma heparin levels. We sought to identify an aPTT ratio that corresponds to therapeutic anti–factor Xa heparin levels for combinations of several reagents and coagulometers that are commonly used.
Citrated plasma was collected from 126 unselected patients receiving unfractionated heparin. Four automated coagulometers and 6 commercial aPTT reagents were used to measure the aPTT. Plasma anti–factor Xa levels were measured by means of a commercially available assay. The relationship between the aPTT results and anti–factor Xa heparin levels for each reagent-coagulometer combination was determined by linear regression analysis, and the aPTT results corresponding to therapeutic anti–factor Xa heparin levels were calculated.
For all reagent-coagulometer combinations studied, an aPTT ratio of 1.5 resulted in anti–factor Xa heparin levels considerably below the lower limit of the therapeutic range. When the aPTT was performed on any of the coagulometers assessed with the use of Actin (Dade Diagnostics, Aguada, Puerto Rico) and IL Test (Instrumentation Laboratories, Fisher Scientific, Unionville, Ontario) reagents, aPTT ratios necessary to achieve therapeutic anti–factor Xa heparin levels approximated 2.0 to 3.5.
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 alternative.