Factor XI Deficiency Workup

Updated: Jun 15, 2022
  • Author: Jamie E Siegel, MD; Chief Editor: Srikanth Nagalla, MD, MS, FACP  more...
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Laboratory Studies

Activated partial thromboplastin time (aPTT) should be measured. In patients with severe factor XI (FXI) deficiency, the aPTT value will be more than two standard deviations above the normal mean; in heterozygotes, the aPTT may be slightly prolonged APTT or within the normal range. [11]

An FXI assay may help confirm the diagnosis, although levels can be in the normal range. [11] Homozygotes and compound heterozygotes will have an FXI level of less than 15%. The expected FXI level in heterozygotes is 25-70%.

A mixing study using normal pooled plasma may help identify a factor deficiency. If the sample is incubated and, subsequently, the aPTT is prolonged, then the presence of an inhibitor needs to be considered. Based on the data regarding high risk of inhibitor development in patients who have severe (< 1%) FXI deficiency, checking an inhibitor titer before proceeding with surgery is recommended.

Factor assays for the intrinsic coagulation system should be performed with at least three dilutions.

In a patient who is newly diagnosed and without previous bleeding history or family history (neither is uncommon in a patient with FXI deficiency), care must be taken by the coagulation laboratory to separate out a nonspecific inhibitor or lupus anticoagulant versus a true FXI deficiency.

Researchers are exploring the use of novel laboratory tests, such as thrombin generation assays and clot stability assays, to predict bleeding risk in patients with FXI deficiency. [1]  For example, Gidley et al reported that combining the apTT with the rate of clot formation and area under the curve in fibrinolysis assays identifies most FXI-deficient patients with a bleeding tendency. [17]