D-dimer Reporting and Harmonization: Analysis of College of American Pathologists D-dimer Proficiency Testing Program.

D-dimer, a fibrin degradation product, is crucial for diagnosing thrombotic and fibrinolytic conditions. Despite its expanding utility, D-dimer testing faces challenges due to varying assay methodologies and non-standardized reporting. To understand D-dimer reporting practices, the College of American Pathologists (CAP) Hemostasis and Thrombosis Committee (HaTC) analyzed proficiency testing data from D-dimer surveys conducted between 2020 and 2023 across multiple laboratories. The data demonstrate that laboratories commonly report units that differ from the assay package insert when reporting their proficiency testing results. We sought to quantify the proportion of laboratories reporting units that may be considered incorrect. To accomplish this, we harmonized all D-dimer results to ng/mL fibrinogen equivalent units (FEU) for all results from one distributed sample and assessed distinct populations of entries per instrument/reagent combination. Notable trends emerged from this analysis. Data from laboratories using the same instrument/reagent combinations clustered around different means, suggesting issues with unit reporting and/or conversion. Entries reported in mg/L or ug/mL DDU commonly exhibited significant deviations from the dominant population. For one instrument/reagent combination, most reported values differed by approximately twice the COA value. This analysis highlights potential issues with D-dimer reporting in proficiency testing. Whether these issues translate into issues with clinical D-dimer reporting requires further study.