At a Glance
Acquired factor XII deficiency is quite rare and is often not an isolated event. It is seen in sepsis, disseminated intravascular coagulation (DIC), or in patients with lupus anticoagulants who also have specific antibodies to factor XII. Very few acquired antibodies to factor XII have been reported. Spontaneous inhibitors to factor XII have not been recognized to occur, as seen with spontaneous inhibitors to factor VIII. Inhibitors to factor XII are not associated with bleeding.
What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?
An isolated prolongation of the APTT with a normal PT should be observed. Factor XII assays show reduced activity (<10%), and a mixing study will indicate an inhibitor. A specific factor XII inhibitor assay will reveal a specific inhibitor, usually an antibody, to factor XII. (Table 1)
Table 1
| APTT | Factor XII assay |
|---|---|
| >60 sec | <10% |
Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications – OTC drugs or Herbals – that might affect the lab results?
Anticoagulants, such as unfractionated heparin, enoxaparin, fondapariux, warfarin, hirudin, argatroban, bivalitrudin and dabigatran, will appear as inhibitors on mixing or inhibitor studies.
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What Lab Results Are Absolutely Confirmatory?
Results that are confirmatory include a low factor XII activity with normal factor XII antigen being present (this latter assay is not a clinical lab assay) in a patient who was normal prior to the recognition of this problem.
What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?
Factor XII coagulant assay and a specific inhibitor assay for factor XII to confirm a diagnosis should be ordered. Studies beyond this need to be performed in a research laboratory interested in these proteins. Western blot could be performed to show normal factor XII antigen in the presence of low factor XII activity.
Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications – OTC drugs or Herbals – that might affect the lab results?
One must exclude anticoagulants being present when performing specific factor XII assays. Hyperlipidemia and hyperbilirubin can interfere with optical coagulation readers.
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