Evaluation of International Council for Standardization in Haematology Recommendations on Activated Partial Thromboplastin Time Mixing Tests Using an Automated Haemostasis Analyser

Introduction Plasma mixing tests are frequently performed in haemostasis laboratories to aid in the determination of the cause of prothrombin time or activated partial thromboplastin time (APTT) prolongation. The International Council for Standardization in Haematology (ICSH) has recently published recommendations for performing and interpreting mixing test; we evaluated the ICSH recommendations for APTT mixing tests on patient samples using automated mixing on a Sysmex CN‐series analyser. Methods Samples from patients with haemophilia A with and without inhibitors, or patients with positive lupus anticoagulant, or patients on rivaroxaban/edoxaban, with an APTT ≥ 4 s above normal, were tested using five different APTT reagents: Siemens Actin FS, Actin FSL, and Pathromtin SL; and Hyphen Biomed Cephen and Cephen‐LS. Results A likely/possible inhibitor was erroneously diagnosed in all haemophilia patients when assessed using the ICSH criteria, except with Actin FS (erroneous diagnosis in 95%). Using CN‐series parameters with locally‐derived reference ranges, ≤ 15% of haemophilia patients were erroneously diagnosed. Only Cephen‐LS reliably detected lupus anticoagulant by any algorithm. Conclusions When using lupus‐insensitive reagents, APTT‐mixing tests are of limited value in discriminating between factor deficiencies, lupus anticoagulants, or inhibitors. Incubated mixing tests are essential when diagnosing a FVIII‐inhibitor. Rather than perform mixing tests, it is better to be guided by clinical presentation and perform further investigations as appropriate, including analysis of anti‐Xa activity for the presence of a direct factor‐Xa inhibiting anticoagulant, factor assays in patients with bleeding (or with suspected acquired haemophilia A), and lupus anticoagulant assays in patients with no bleeding.