Prekallikrein (PK), also known as Fletcher factor, is a serine protease that complexes with HK. Prekallikrein 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. However, prekallikrein deficiency can cause a prolonged APTT, which can be corrected by incubating the patient’s plasma. Deficiencies in prekallikrein can also be required due to some disease states, such as angioedema, infection, DIC, and sickle cell disease.