Factor X Padua: A ‘New’ Congenital Factor X Abnormality with a Defect Only in the Extrinsic System

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Abstract
A family with a new factor X defect is reported. The proposita is a 56-yr-old female. She is asymptomatic and no consanguinity is present between the parents. The main features of the defect are: prolongation of prothrombin time and derivative tests, but normal partial thromboplastin time. Factor X was found to be low (.apprx. 25-30% of normal) only if tissue thromboplastins were used in the assay system. Chromogenic substrate S-2222 also yielded decreased factor X levels. Factor X activity was normal with cephalin and cephalin-RVV mixture. Factor X antigen was normal in 3 immunological systems (electroimmunoassay, an ELISA [enzyme-linked immunosorbent assay] method and laser nephelometry). Crossed immunoelectrophoresis and antigen-antibody kinetics recorded in a laser nephelometer failed to show major differences from normal factor X. Both sons of the proposita, the father and other family members showed slightly decreased factor X levels and normal factor X antigen, and were considered heterozygous for the abnormality. The toponym factor X Padua is proposed to indicate this peculiar abnormality.
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