Congenital PK deficiency is a very rare condition that is presumed to be asymptomatic, but has been linked to increased risk of thrombotic events.7, 8, 9, 10Congenital PK deficiency is usually detected when coagulation assays are performed, as the activated partial thromboplastin time is prolonged in the absence of PK.6, 11The critical roles PK play in the kallikrein/kinin system and in in vitro coagulation are well known, but paradoxically PK does not contribute to in vivo hemostasis.12The link between reduced levels of PK and thrombotic risk is less well established. Here, KLK4 is linked to pyruvate kinase deficiency of red cells.