This post comes from Scott Miller, St. Mary’s of Michigan: We will soon be validating our activated partial thromboplastin time (APTT, PTT) heparin therapeutic range by using a comparison between an anti-Xa assay and APTTs run with our new lot of APTT reagent. I have always understood that when calibrating an anti-Xa assay for this procedure, one should always make the calibrator for the anti-Xa assay by careful dilution of the same type of heparin used at the institution. Yet the kit we received from IL includes a box of WHO-certified calibrators. Should these be used instead of the homemade stuff for our purpose?
Hello, Scott, and thank you for your question, I always enjoy getting notes from Grand Rapids, Michigan, my home town, even though, when I was training at Butterworth Hospital, now Spectrum, St. Mary’s was our cross-town rival. At least St. Mary’s has retained their identifiable name!
I would definitely recommend you use the calibrators provided in your Instrumentation Laboratory (IL) liquid heparin kit. Not only are they traceable to World Health Organization (WHO) international unfractionated (UFH) and low molecular weight heparin (LMWH) standards, they also enable you to construct a hybrid curve, one that may be used to report both UFH and LMWH results from the anti-Xa assay without have to change curves. Plus, home-brew curves tend to flatten out at the top end.
By the way, while you are going to the trouble of validating the chromogenic anti-Xa liquid heparin assay, have you considered just using it to report your heparin results, thus eliminating the PTT altogether and avoiding the interferences that affect the PTT and the need to aliquot a number of heparin specimens? Just a suggestion!