I thought that I should respond to the August 4 comment from “juliahil” that asks about monitoring low molecular weight heparin (LMWH, enoxaparin, Lovenox). A CDC surveyreported in 2001 that 72% of US respondents were attempting to use the partial thromboplastin time (PTT) to monitor Lovenox. The survey referenced Arch Pathol Lab Med 1998;122:799–807, in which the College of American Pathologists recommends the heparin anti-Xa assay for LMWH. I attended the Mayo Medical Laboratories Thrombophilia conference in Rochester, MN yesterday, and discussed the effects of LMWH on the PTT with Dr. Rajiv Pruthi of Mayo. He reiterated that LMWH has little effect on the PTT, and that the insensitivity of the PTT for LMWH, as well as its non-linearity, renders it useless for that purpose, though it is used widely to monitor unfractionated heparin. If your physicians are requesting the PTT as a means for monitoring LMWH, you may wish to point them to the CDC survey and the Archives reference.