Prekallikrein (PK), also known as Fletcher factor, is an 85,000 Mr serine protease that complexes with high-molecular-weight kininogen. PK is the precursor of plasma kallikrein, which is a serine protease that activates kinins. PK is cleaved to produce kallikrein by activated Factor XII (Hageman factor). 
Hereditary deficiencies in PK are very rare. They can cause a prolonged APTT, which can be corrected by incubation of the patient’s plasma.
Although most cases of prekallikrein deficiency are asymptomatic, a few reports link severe prekallikrein deficiency with thrombotic phenomena and recurrent pregnancy loss. More recently, a case of prekallikrein deficiency was shown to be associated with severe mucosal bleeding.
Discovery of prekallikrein
PK was initially described by Hathaway et al. in 1965 after encountering a Kentucky family who exhibited strikingly abnormal APTT results, but showed no bleeding symptoms. The family appeared to have a hereditary deficiency in an unknown coagulation factor, dubbed “Fletcher factor” after the family. In 1973 Kirk Wuepper determined that Fletcher factor and prekallikrein were the same.
- Goodnight, S.H.; Hathaway, W.E. (2001). Disorders of Hemostasis & Thrombosis: A Clinical Guide (2nd ed.). McGraw Hill Professional. ISBN 0071348344.
- Dasanu CA, Alexandrescu DT (November 2009). "A case of prekallikrein deficiency resulting in severe recurrent mucosal hemorrhage". Am. J. Med. Sci. 338 (5): 429–30. doi:10.1097/MAJ.0b013e3181b270bb. PMID 19773642.
- Online 'Mendelian Inheritance in Man' (OMIM) KALLIKREIN B, PLASMA, 1; KLKB1 -229000