Non-parallelism in the one-stage coagulation factor assay is a phenomenon of lupus anticoagulants and not of individual factor inhibitors.

Dear Sirs, Individual coagulation factor inhibitors and lupus anticoagulants generally result in prolonged clotting times in coagulation assays. By far the most frequently occurring individual coagulation factor inhibitors are allo-antibodies against factor (F) VIII and FIX. These inhibitors arise in 10–30% of haemophilia A and 4–5% of haemophilia B patients treated with FVIII or FIX concentrates, respectively (1). Acquired FVIII deficiencies due to auto-antibodies rarely occur with an incidence of 1 to 4 per million/year, and mainly in the elderly (2). Most individual coagulation factor inhibitors have fast-acting kinetic properties and completely inhibit the factor concerned except for FVIII inhibitors that are known to be time and temperature dependent (3, 4). Furthermore, two different types of FVIII inhibitors can be recognised, type 1 and type 2. Allo-antibodies are often of type 1 while auto-antibodies generally are type 2. Type 1 antibodies completely inhibit FVIII according to second order kinetics, whereas type 2 antibodies only partially inhibit FVIII and show a non-linear dose-related response. Lupus anticoagulants are directed to proteins in complex with phospholipids, like prothrombin and β2-glycoprotein-1. These antibodies are associated with an increased risk of thrombosis (5) which is in contrast with the prolonged clotting assays in these patients. Furthermore, as both thrombosis and bleeding may occur in patients with lupus anticoagulants and in patients with individual factor inhibitors, both presenting with a prolonged activated partial thromboplastin time (APTT), further laboratory investigation has to confirm the underlying cause of the prolongation as this may determine the choice of the therapeutic strategy (6,7). In accordance with the prolongation of clotting tests, a further common laboratory finding in patients with lupus anticoagulants and patients with individual factor inhibitors is one or more low-factor activities when assayed with the one-stage coagulation factor assay according to a Parallel Line Bioassay (8–10). The results of these assays are reliable when the curves of reference and patient plasma are parallel ( Fig. 1A). In case of non-parallelism factor activity results should be regarded as incorrect. It is generally accepted that one of the main reasons for non-parallelism is the presence of antibodies like specific coagulation inhibitors, lupus anticoagulants or other non-specific inhibitors. In case of lupus anticoagulants, mixing increasingly diluted plasma samples with standard volumes of APTT reagent, the lupus activity will be disproportionately neutralised at higher dilutions resulting in increased coagulation factor activity due to non parallelism. For inhibitors against individual coagulation factors like FVIII and FIX inhibitors, non-parallelism in the onestage factor assay can only be expected when the inhibitor can dissociate from the coagulation factor. However, anti FVIII inCorrespondence to: Janneke Ruinemans-Koerts Laboratory of Clinical Chemistry and Haematology Rijnstate Hospital Wagnerlaan 55 6815 AD Arnhem, The Netherlands Tel.: +31 88 0058888, Fax: +31 88 0056086 E-mail: [email protected]