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Anti-Xa Dilution

Marietta Tomlin asks, Can Owren-Koller buffer be used instead of normal plasma to dilute anti-Xa when the are results above linearity? And if not, can you provide information on the reasoning/mechanisms as to why not?

From LaShanta Brice, DCLS, MLS (ASCP)cmSH (ASCP)cm Scientific Engagement & Clinical Education Associate, Diagnostica Stago, Inc. Because our assay uses antithrombin in the patient sample, it is not recommended to use OK Buffer as it could dilute out the antithrombin in the patient.

From Geo: Ms Tomlin, Dr. Brice’s answer applies if you are measuring unfractionated or low molecular weight heparin with your chromogenic anti-Xa assay. If you are measuring rivaroxaban or apixaban I speculate the OK buffer would factitiously reduce the factor X concentration and affect your results, as you are measuring DOAC suppression of factor X activity. I’m inviting additional expert opinions from our participants.

Comments (1)
Anticoagulant Therapy
Dr. Emmanuel Favaloro
Sep 5, 2023 9:12pm

There are several things to consider here: (1) if assessing anti-Xa for heparin, you have to ensure any normal plasma used for dilution does not contain excess free PF4 to quench the heparin in the patient sample; (2) if the method used does not indicate dilution of samples when above the linearity limit, then dilution of samples may be seen as using the method outside manufacturer recommendations, and thus be potentially perceived as a LDT; (3) clarify what you are using the anti-Xa for – you will always be over the linearity limit if you have a DOAC sample and you use the heparin calibration curve.

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