INTRODUCTION: Activated protein C resistance (APCR) represents a risk factor for thrombosis and is usually due to factor V Leiden (FVL). Clinicians may order either test (i.e., APCR or FVL) to help assess 'thrombophilia' in patients who present with thrombosis. APCR testing is usually achieved using clot-based assays, whereas FVL is assessed by genetic testing. There are advantages and disadvantages to either approach.

METHODS: We report updated findings for APCR testing in our geographic region, in part using recent data from the RCPAQAP, an international external quality assessment (EQA) program, with some 50-60 participants for APCR testing over the past decade. Data have been updated to cover the past 13 years (2010-2023 inclusive), with four samples assessed each year, but with a primary focus on new data from 2020 to 2023 inclusive. In addition, data for APCR testing over several years from four large tertiary-level hospital laboratories have been assessed following a recent change in instrumentation and haemostasis methods.

RESULTS: EQA data continue to show variable performance in both numerical values and their interpretation for APCR testing, with certain methods providing more consistently correct findings than others. In addition, participant interpretation of their own numerical values and transcription errors seem problematic. Finally, the change in recent laboratory testing has also evidenced local improvements.

CONCLUSION: APCR assays and testing laboratories continue to show variability in performance, with two methods (Pefakit and Staclot) showing the best performance overall. Targeted education may be of benefit, as most of the errors appear to originate from a small proportion of laboratories.