Causes of Abnormal Screening Results
Causes of Abnormal Screening Results | Specimen Collection and Transport Issues | Medication | Pathologic Conditions | Factor Deficiencies | Factor Inhibitors
Factor Inhibitors
Specific Factor Inhibitors. Antibodies to specific factors can prolong screening tests and are frequently associated with bleeding. The most commonly observed specific factor inhibitors affect factors of the intrinsic pathway and produce an extended aPTT with a normal PT. Approximately 15% of patients who are treated for severe factor VIII or IX deficiencies with exogenous factor will develop alloantibodies that recognize the factor when it is administered therapeutically. Alternatively, some patients spontaneously develop autoantibodies that rapidly clear their own coagulation factors. By far the most common of these specific autoantibodies are to the factor VIII molecule. Acquired antifactor VIII antibodies often occur in older patients and in pregnancy and can produce significant hemorrhage. Inhibitors to other factors are quite rare.5
Lupus Anticoagulants (LA). These nonspecific antibodies extend the clotting time of phospholipid-dependent clotting assays such as the aPTT.4,9 Unlike specific factor antibodies, LA are usually associated with venous thrombosis, pulmonary embolism, arterial thrombosis, and recurrent fetal loss.10 Lupus anticoagulants do not specifically inhibit individual coagulation factors; rather, they neutralize anionic phospholipid-protein complexes that are involved in the coagulation process. Testing for lupus anticoagulants and the antiphospholipid syndrome that is associated with these antibodies is described in more detail in the APS section of this appendix.
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