From Allen Quigley, Hematology/Coagulation Technical Specialist, Children's Hospitals and Clinics of Minnesota. George, what is the best way to measure factor VIII and IX levels on patients that are being infused with different products? We perform one stage clotting assays using Actin FSL, which is an ellagic acid activator, and other labs may use reagent systems that have silica activators. Depending on the product being infused, especially with factor IX (Idelvion, Alprolix) the results could be biased lower by 25-50%. Would chromogenic assays help for factor VIII? I don't think there are any approved for factor IX at this time. Thanks, Al.
Hello, Al, and thank you for your question. Monitoring the new extended half-life factors VIII and IX raises new issues of accuracy and reproducibility. I attended the Mayo Medical Laboratories Bleeding and Thrombosing Diseases conference Sept 13–15, 2017 in Rochester, where Dr. Rajiv Pruthi reviewed CAP survey results and manufacturer recommendations, and for now at least, we are in a swamp. It appears that for factor VIII assays, our best approach is the chromogenic substrate assay, as you suggest, with the caveat that we be sure to check the manufacturers' claims for low-range accuracy. Some chromogenic assays go non-linear below 10 units/dL (10%) factor VIII, a serious limitation. For the pegylated factor VIII concentrates like Adynovate and N8-GP, Dr. Pruthi indicates the ellagic acid-based one-stage assays like Actin FSL work well, however this is not true for the Fc-fusion materials such as Eloctate, which tends to be underestimated in all the one-stage assays. For Afstyla, CSL's package insert instructs us to simply double the one-stage assay result, advice that tends to make lab scientists fibrillate.
For the factor IX concentrates, there are no chromogenic substrate assays in the US, though many manufacturers have them under development. Until these come along, Dr. Pruthi's best advice is to try to maximize your one-stage assays (ellagic acid, silica, kaolin) with factor IX product calibrators and use the same assay consistently, perhaps with a "correction factor."
Sorry I can't provide a more definitive answer. Eventually, chromogenic assays will provide the answers we are looking for, but this may be a year or two away.
This entry is likely to attract some additional comments from experts around the world, so keep watching.