A Closer Look at the Newer GP1bM Assay Sheds Light on Unusual Findings

• Published in: American journal of clinical pathology 2024-10, Vol.162 (Supplement_1), p.S173-S173
• Main Authors: Asif, Maryam, Tun, Theingi, Sabath, Daniel
• Format: Article
• Language: English
• ISSN: 0002-9173; 1943-7722
• DOI: 10.1093/ajcp/aqae129.381

Von Willibrand disease (VWD) is the most common inherited bleeding disorder. The 2021 Guidelines on VWD diagnosis recommend newer assays measuring the platelet-binding activity of von Willebrand factor (VWF), such as VWF: GPIbM and VWF: GPIbR, over the traditional ristocetin cofactor assay (VWF: RCo). Among these newer assays, the GP1bM INNOVANCE VWF Ac assay obtained de novo FDA approval in October 2022. Its ristocetin-independent activity distinguishes it from other assays. Our laboratory conducted a comprehensive validation on the Stago StaR-Max platform, utilizing Siemens Standard Plasma for calibration. The VWF activity measured by the GP1bM assay was compared with the existing VWF: Ab assay (IL, HemosIL® von Willebrand Factor Activity). The validation studies revealed a good correlation between the two assays. Since implementing the GP1bM assay in October 2023, our attention has been drawn to at least four cases exhibiting unexpectedly high activity levels compared to VWF antigen levels. On average, the activity-to-antigen ratio was 1.9. Notably, all these cases showed the presence of all multimers (low, intermediate, and high). Despite extensive investigation, encompassing considerations like heterophile antibody interference using scantibodies, retesting with different lot numbers of Innovance VWF Ac reagent, and rerunning tests at an outside laboratory, the cause of this anomaly remains elusive. A notable variable lies in the platform disparity between Siemens and Stago. Given the recent introduction of this assay, it has yet to gain widespread adoption in laboratories, resulting in a limited understanding of its practical variations. While recent studies affirm the overall superior performance of newer assays compared to the ristocetin cofactor assay, intricate details of each assay remain less explored. The novelty of the GP1bM assay and its limited usage in laboratories underscore the potential emergence of stable outliers as it becomes more widely adopted. The peculiar cases in our study pose an enigma as to why the reported activity is approximately double the antigen level. As laboratories increasingly embrace and integrate the GP1bM assay, a more complete understanding of its potential variations is expected to unfold. This may elucidate the mysteries surrounding these peculiar cases and ensure the assay’s robust interpretation in clinical settings.