The Contact System

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Abstract
Objectives.—To review the literature for conditions, diseases, and disorders that affect activity of the contact factors, and further to review the literature for evidence that less than normal activity of any of the contact factors may be associated with thrombophilia. Data Sources.—MEDLINE search for English-language articles published from 1988 to 2001 and pertinent references contained therein, as well as search of references in recent relevant articles and reviews. Study Selection.—Relevant clinical and laboratory information was extracted from selected articles. Meta-analysis was not feasible because of heterogeneity of reports. Data Extraction and Synthesis.—Evidence for association of altered levels of the contact factors and thrombophilia was sought. A wide variety of disorders is associated with decreased activity of the contact factors; chief among these disorders are liver disease, hepatic immaturity of newborns, the antiphospholipid syndrome, and, for factor XII, being of Asian descent. These disorders are more common than homozygous deficiency. The few series and case reports of thrombophilic events in patients homozygous for deficiency of contact factors are not persuasive enough to support causality. The apparent association between levels consistent with heterozygosity (40%–60% of normal) of any of the contact factors (but especially factor XII) in persons with antiphospholipid antibodies appears to be due to falsely decreased in vitro activity levels of these factors, which are normal on antigenic testing. The apparent association with thrombosis is better explained by the antiphospholipid syndrome than by the modest reduction of the levels of contact factors. Conclusions.—Presently, it is not recommended to measure activity of contact factors during routine evaluation of patients who have suffered venous or arterial thromboembolism or acute coronary syndromes.