Prekallikrein (PK), also know as Fletcher Factor, is the precursor of plasma kallikrein, which is a serine protease that activates kinins. Prekallikrein is cleaved to produce kallikrein by Factor XII (Hageman factor).

Hereditary deficiencies in prekallikrein are very rare resulting in little to no bleeding problems, but can cause a prolonged APTT. Deficiencies in prekallikrein can be caused by disease states such as angioedema, infection, DIC, and sickle cell disease.