Antithrombin Affects the Anti-Xa Heparin Assay

Here is a question from my friend and colleague, Vicki Cardone at Children’s Hospital of Alabama, just down the street from UAB:

I am having a hard time getting the concept behind finding a decreased anti-Xa level when you have decreased antithrombin (AT, antithrombin III, AT III). What part of the patient’s heparin is the anti-Xa heparin assay measuring?

Thanks for your time. Vicki

Hi, Vicki, thank you for your question. The chromogenic anti-Xa heparin assay procedure employs a three-stage sequence. Patient therapeutic heparin first binds withantithrombin; the complex of heparin + antithrombin then binds the reagent bovine coagulation factor Xa (activated X). The unbound “leftover” bovine Xa then reacts with the substrate to generate a colored product, usually yellow para-nitroanaline (pNA). The color intensity is inversely proportional to the amount of heparin present.

Your question relates to the source of the antithrombin. Some kits, for instance, Diagnostica Stago’s STACHROM HEPARIN, provide antithrombin as a reagent. Others, for instance, Stago’s STA ROTACHROM HEPARIN, provide no antithrombin, thus the reaction relies on antithrombin from the patient’s plasma. In the United States, the majority of coagulation laboratory supervisors prefer STA ROTACHROM HEPARIN on the theory that it detects antithrombin deficiency (consumption) and reports back a lower result, alerting the operator to possible “heparin resistance.”

From a lab point of view you cannot distinguish between inadequate heparin therapy and antithrombin deficiency when using STA ROTACHROM HEPARIN. This may be a frustration, however, it is important to be able to detect antithrombin deficiency,  as the patient fails to gain antithrombotic benefit from the heparin when they are antithrombin deficient.