A post from Bryan McGill, PharmD at St. Mary’s Hospital in Madison, WI:
We have a patient with a history of HIT, so we are using Arixtra 2.5 mg q24h for prophylaxis. Unfortunately the patient also has a CrCl hovering at 30mL/min, so I am worried that the drug may start to build up. Are you aware of any guidelines for Xa monitoring for fondaparinux? Thanks for your help.
Dr. McGill, thanks for your question. This is partially address by Michael Janasik’s October 4 post describing how Blood Center of Wisconsin assays for Arixtra (fondaparinux). They use the chromogenic anti-Xa heparin assay, the same as is used to monitor low molecular weight heparin (Lovenox), however the laboratory must develop and validate a separate standard curve for Arixtra prepared by diluting a pharmaceutical preparation. For a 24h turn-around, send a specimen to Blood Center of Wisconsin or Quest Diagnostics in Chantilly, VA.
The target ranges are approximately 0.1-0.5 ug/mL for prophylaxis and 0.5 to 1.3 ug/mL for therapy. Notice the units are quantitative, and not heparin units, as Arixtra is a synthetic, and you must consult the ranges provided by the reference lab you choose.
I have a follow-up question. Is Arixtra the indicated antithrombotic for a patient with HIT? I’ve not read of any cross-reaction with the HIT (anti-PF4/heparin) antibody, but Arixtra is a form, albeit synthetic, of heparin. Would a direct thrombin inhibitor such as Novastan (argatroban) or Angiomax(bivalirudin) be preferred to avoid the risk of cross-reaction?