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, SH Jr, Hathaway, WE. Disorders of Hemostasis and Thrombosis. Second edition. 2001.
- Dasanu CA, et al. "A Case of Prekallikrein Deficiency Resulting in Severe Recurrent Mucosal Hemorrhage". American Journal of Medical Sciences, 2009.
- OMIM article on prekallikrein deficiency