Factor VIII inhibitors. Laboratory diagnosis of inhibitors.

The diagnosis of inhibitors of blood coagulation is often the most challenging problem in the clinical laboratory. Immediate attention must be given to the following patient groups whose principal laboratory abnormality is the prolonged activated partial thromboplastin time (aPTT): the patient with (1) hemophilia who previously responded to an adequate dose of clotting factor product and now fails to show effective clinical response to the same replacement concentrate; (2) previously benign clinical history who now presents with soft tissue bleeding or emergent internal hemorrhaging; (3) sudden onset of generalized ecchymoses who was previously well; (4) postpartum state; (5) malignancy, lymphoma, rheumatoid arthritis, or other autoimmune disorders; and (6) drug reactions. Immediate attention must be given to the prolonged prothrombin time (PT), aPTT, and thrombin time (TT) in order to respond to urgent queries from a perplexed internist, hematologist, intensivist, or surgeon caring for a patient with unexpected bleeding. Sometimes the problem of a prolonged "clotting time" arises preoperatively, causing unanticipated delay in operative procedures. For this reason, the laboratory support, usually in the coagulation section of a clinical laboratory or reference laboratory, must be quick, unequivocal and precise. The most common finding is an isolated mild, moderate, or severe prolongation of the aPTT with a normal PT, TT, and platelet count. The aPTT mixing study (The Mix), usually modified for time and temperature, along with appropriate controls, is the seminal test. This is the basis for all further testing. It may be supported by direct factor assays, and, therefore, the laboratory must know the reagent responsiveness and sensitivity for each clotting factor. By definition, complete correction of the aPTT in a 1:1 mix of patient and reference plasma is a factor deficiency. In this article, incomplete or minimal correction of The Mix will be characterized with particular attention to the various inhibitor assays, in other words, Oxford, Bethesda, and Nijmegen assays and the enzyme-linked immunosorbent assay (ELISA). An investigative approach to final characterization of the intensity (quantification) of the inhibitor and the exclusion of a lupus anticoagulant (LA) will be discussed.