Factor X is the first enzyme in the final, common part of the coagulation pathway; its activity is vitamin-K dependent. Congenital factor X deficiency is extremely rare, but some patients are severely affected. Factor X deficiency can also be acquired; it can be associated with several conditions, among them malignancies, but acquired deficiency most often occurs in patients with amyloidosis.
Congenital factor X deficiency, an autosomal recessive disease, occurs in one per one million in the general population. As expected, it occurs more frequently in communities where consanguineous marriage is common . The manifestations of factor X deficiency are not unlike those of other coagulation factors. It is the most severe among the rare congenital coagulopathies .
The sites of bleeding depend to some extent on the severity of the disease, which is correlated with the residual activity of factor X  . Severe cases may present in the neonate with an umbilical stump bleeding 7-10 days after birth . Other spontaneous bleeding incidents in severely affected patients are hemarthroses, gastrointestinal bleeding, central nervous system hemorrhages, and post-surgical hemorrhages. Women may have severe bleeding during periods and post-partum. They may also experience bleeding into the peritoneum from ruptured luteal cysts . Patients with less severe deficiencies bruise more easily; menorrhagia and gum bleeding are also very common . Nosebleeds occur with all degrees of deficiency .
Acquired deficiency of factor X is also rare, and most cases are associated with amyloidosis  . Factor X is not the only coagulation factor affected by amyloidosis, but it is the most frequent. Liver disease and vitamin K deficiency also affect the activity of factor X as well as several other coagulation factors . Various tumors, myeloma, respiratory virus infections, leprosy and certain medications   are also associated with higher incidence of factor X deficiency.
Registries of patients of European, American, Asian Indian, and Iranian extraction with factor X deficiencies now exist     . With a cut-off value of less than 0.1 IU/ml for Factor X activity, bleeding episodes tend to be more severe, whereas above that value the symptoms are milder . The most serious types of bleeding (gastrointestinal and intracranial bleeding, hemarthroses) occur at or below values of activity that are about five times lower than the cutoff value  .
Bleeding episodes in a patient will prompt the usage of the standard hemostasis tests: the prothrombin time (PT) - together with the derived international normalized ratio (INR), - and the activated partial thromboplastin time (APTT). Factor X deficiency usually results in prolonged PT, APTT, as well as in a prolonged Russell viper venom time (RVVT) in which the viper venom activates Factor X  , and therefore directly tests factor X. The bleeding time and thrombin time are normal in Factor X deficiencies.
There are also tests for factor X activity, which is determined by one stage PT-based assay. The factor X protein (FX antigen) can be measured by immunological methods . One classification scheme for the disease involves the relationship of these last two measurements: In type I deficiency the levels of both enzyme activity and protein (antigen) are low, whereas severely depressed activity despite near normal antigen levels is type II deficiency   .
Acquired deficiency of factor X occurs in about ten percent of immunoglobulin light chain (AL)-amyloidosis patients. Mixing tests may reveal if there is an immunologic origin of factor X deficiency. Liver disease and coumarin anticoagulant administration can cause factor X deficiency, together with deficiencies in other coagulation factors.