Because protein Z helps inactivate factor Xa, deficiency or dysfunction of protein Z predisposes to venous thrombosis (mainly in patients with other clotting abnormalities—see Overview of Thrombotic Disorders).
Protein Z, a vitamin K–dependent protein, functions as a cofactor to down-regulate coagulation by forming a complex with the plasma protein, Z-dependent protease inhibitor (ZPI). The complex predominantly inactivates factor Xa on phospholipid surfaces.
The consequence of protein Z or ZPI deficiency, or of autoantibodies to protein Z, in the pathophysiology of thrombosis and fetal loss is unresolved; however, either defect may make thrombosis more likely if an affected patient also has another congenital coagulation abnormality (eg, factor V Leiden—see Factor V Resistance to Activated Protein C (APC)).
Quantification of protein Z, ZPI, and protein Z autoantibodies is done in specialized regional laboratories by plasma electrophoresis, immunoblotting, and enzyme-linked immunosorbent assay.
It is not yet known whether anticoagulant therapy or prophylaxis is indicated in protein Z or ZPI deficiency.
Last full review/revision October 2014 by Joel L. Moake, MD
Content last modified October 2014