The activated partial thromboplastin time (aPTT) is widely used as a screening coagulation test and for monitoring unfractionated heparin therapy. Various commercial reagents are available, with different performance characteristics, particularly responsiveness to the lupus anticoagulant (LA). Because aPTT reagent selection significantly affects the interpretation of results, we reviewed College of American Pathologists proficiency testing data involving approximately 4,000 coagulation laboratories, and conducted a survey of coagulation laboratories (n = 93) using The Fritsma Factor hemostasis Web site to determine the basis for aPTT reagent selection. The data demonstrate that for routine aPTT testing, most laboratories use reagents with high/moderate responsiveness to LA. Significant misunderstanding was apparent regarding the use of appropriate aPTT reagent for routine testing and LA identification. We recommend aPTT reagents with low LA responsiveness to screen for coagulation factor deficiencies and heparin monitoring, and suggest continued education of laboratory professionals and reagent manufacturers about appropriate aPTT reagent use. © American Society for Clinical Pathology.