Background: In an effort to improve interlaboratory agreement in the monitoring of unfractionated heparin (UFH), the College of American Pathologists (CAP) recommends that the therapeutic range of the activated partial thromboplastin time (APTT) be defined in each laboratory through correlation with a direct measure of heparin activity such as the factor Xa inhibition assay. Whether and to what extent this approach enhances the interlaboratory agreement of UFH monitoring has not been reported. Objectives: We conducted a cross-validation study among four CAP-accredited coagulation laboratories to compare the interlaboratory agreement of the anti-FXa-correlated APTT with that of the traditional 1.5-2.5 times the midpoint of normal (1.5-2.5:control) method for defining the therapeutic APTT range. Patients and methods: APTT and FXa inhibition assays were performed in each laboratory on plasma samples from 44 inpatients receiving UFH. Results: Using the anti-FXa-correlation method, there was agreement among all four laboratories as to whether a sample was subtherapeutic, therapeutic or supratherapeutic in seven (16%) patient samples. In contrast, consensus was achieved in 23 (52%) samples when the 1.5-2.5:control method was employed. Conclusions: The anti-FXa-correlation method does not appear to enhance interlaboratory agreement in UFH monitoring as compared with the traditional 1.5-2.5:control method. Adoption of the anti-FXa-correlation method produces considerable disparity in UFH dosing decisions among different centers, although the clinical impact of this disparity is not known. © 2009 International Society on Thrombosis and Haemostasis.