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Anti-Xa to Replace PTT

From frequent contributor, Kim Kinney: Hi George, our Indiana University Health Pathology Laboratory is thinking of making the push again to use the Anti-Xa to monitor unfractionated (UFH) heparin therapy in place of the activated partial thromboplastin time (aPTT). I would like to hear from institutions that have made the transition successfully even in light of the fact that the chromogenic anti-Xa is a more expensive test than the aPTT. That seems to be where our admin gets stuck.

Hello, Kim and thank you for bringing up this important topic, which is addressed most recently in Wool GD, Lu CM; Education Committee of the Academy of Clinical Laboratory Physicians and Scientists. Pathology consultation on anticoagulation monitoring: factor X-related assays. Am J Clin Pathol. 2013; 140: 623–34. I’m eager to hear from our participants, but just to throw in my own opinion, when you compare and graph the anti-Xa to the PTT using routine daily specimens, and not just your carefully selected set of therapeutic target range aliquots, you strikingly illustrate the PTT’s unreliability. In addition, the anti-Xa can be used to measure not only unfractionated standard heparin, but also low molecular weight heparin and the pentasaccharide fondaparinux. Further, it is likely to be cleared by the FDA soon to measure the direct oral anti-Xa anticoagulants (DOACs) rivaroxaban, apixaban, and edoxaban. Last week at the Thrombosis and Hemostasis Summit of North America, Dr. Dorothy Adcock indicated it may be possible to assay all the new DOACs using a single, LMWH calibrator set, which would make it even more practical. There already exist “hybrid” calibrators that enable you to use a single curve for both UFH and LMWH, perhaps that curve could even be extended to the DOACs. You may also choose to talk to your ICU nurses, who are faced with the task of adjusting heparin drip rates based on the PTT results. Their efforts are likely to be reduced when using a more precise assay. I hope this helps, and I look forward to our participants’ responses.

Comments (4)
Anticoagulant Therapy
May 7, 2014 6:30am

Heparin-binding site antithrombin deficiency type II might b
Heparin-binding site antithrombin deficiency type II might be responsible for abnormally low anti-Xa level, especially if Stago Rotachrome assay (w/o exogenous AT) was utilized.

Apr 23, 2014 11:31am

Kim you always have good questions. I submit my own manuscri
Kim you always have good questions. I submit my own manuscripts as obvious reasons why you should get rid of the aPTT for monitoring heparins.
McGlasson DL. Monitoring Unfractionated Heparin and Low Molecular Weight Heparin Anticoagulation with an anti-Xa Chromogenic Assay using a Single Calibration Curve. Lab Medicine. 2005;36(5):297-299.

McGlasson DL et al: Effects of Pre-analytical Variables on the anti-Fxa Chromogenic Assay when Monitoring Unfractionated Heparin and Molecular Weight Heparin Anticoagulation. Blood Coagulation and Fibrinolysis. 2005;16(3):173-176.

Since we published these papers that dealt with the hybrid curve four companies have come out with reagents to support that assay. It works and no one has ever shown that it wasn’t better than having single calibrators for every heparinoid with the exception of fondaparinux.

Apr 22, 2014 4:17pm

I take it that this patient has got normal level of antithro
I take it that this patient has got normal level of antithrombin.

Apr 22, 2014 6:58am

We finally got pharmacy to switch over about a year ago. The
We finally got pharmacy to switch over about a year ago. They now see what we had been talking about for several years. I had no repurcussions from the physicians. So far, we have only had one patient that did not respond to heparin. He has been in several times and they can load him up with thousands of units of heparin and his Heparin levels by the Stago method is still <0.1!!!
Sue Osier
Northside hospital – Atlanta

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