Our purpose was to determine whether the in vitro (or "spiked curve") method for the therapeutic heparin range is an accurate and valid method for heparin monitoring. Many laboratories use the in vitro method for determining the activated partial thromboplastin time (APTT)-based unfractionated heparin (UFH) therapeutic range as a more practical method to determine the therapeutic heparin range. Is this a valid method compared with the recommended ex vivo method? Plasma samples from patients receiving UFH and a normal plasma pool spiked with UFH were compared for 8 APTT reagents (18 lots). The in vitro curve has significantly increased lower limit and upper limit values and has a significantly widened range compared with the ex vivo method. When APTT values are compared with both methods, more samples are underheparinized with the in vitro curve method. The in vitro method is not a valid method to determine an accurate therapeutic heparin range.