At a Glance

Hemophilia B, a deficiency of factor IX, is clinically indistinguishable from hemophilia A, a deficiency of factor VIII. A deficiency of factor IX results in a prolongation of the partial thromboplastin time (PTT). It is an inherited bleeding disorder that is sex-linked and affects males with rare exceptions. The female carriers do not typically have bleeding disorders. A family history of bleeding that affects primarily males should prompt a consideration of hemophilia B.

Individuals with mild hemophilia B and factor IX levels above 5% have a fairly normal life and suffer severe bleeding only after major surgery or trauma. Individuals with moderate hemophilia and a factor IX percentage between 1 and 5% experience bleeding with minor trauma. Those with severe disease have factor IX levels less than 1% and experience spontaneous bleeding, especially in the joints. These percentages and clinical presentations are the same for factor VIII deficiency and factor IX deficiency.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

The screening test for hemophilia B is the PTT. With a significant deficiency of factor IX, the PTT is prolonged. For very mild deficiencies, the PTT may be in the upper end of the reference range. The next test considered after the PTT is an assay for factor IX to determine the percentage of normal factor IX present in the circulation. If a low factor IX is obtained, a PTT mixing study should be obtained to determine if the low factor IX is a result of a simple deficiency of the factor or the presence of a neutralizing factor IX inhibitor.

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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?

It is important to determine the baseline factor IX level to determine the severity of the patient’s hemophilia B. If the patient has been treated with factor IX concentrate or any product that contains factor IX, including fresh frozen plasma, the factor IX level obtained will not reflect the baseline level, but will instead reflect the amount of factor IX produced by the patient plus the amount received by infusion.

The presence of a lupus anticoagulant can result in a low factor IX level that is not a true deficiency. In this case, the lupus anticoagulant is an in vitro inhibitor of the clotting reaction required to measure the factor IX level. Patients can usually be identified as having a lupus anticoagulant, instead of a truly low factor IX, by demonstrating a positive test for the lupus anticoagulant. In addition, in patients with a lupus anticoagulant, a low factor IX is often accompanied by low values for factor VIII, factor XI, and/or factor XII. Low factor IX levels in patients with a lupus anticoagulant are not representative of low levels in vivo, and, therefore, they are not associated with an increased risk for bleeding.

What Lab Results Are Absolutely Confirmatory?

A low factor IX level with a negative test for the lupus anticoagulant confirms hemophilia B. A PTT mixing study determines whether the low factor IX is a simple deficiency or the result of a factor IX inhibitor.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

Rare carriers of hemophilia B can have factor IX levels low enough to permit excessive bleeding with trauma. Typically, carriers have values in the 50% range and do not bleed. However, values in the low 20% range, on rare occasions, are found in carriers, and such women can have a bleeding predisposition. Hemarthrosis is a common indication for admission to the hospital for a patient with hemophilia B, as they are for hemophilia A. The hemarthrosis most commonly affects the knee. An indication that an inhibitor to factor IX has arisen is a poor hemostatic response to infusion of factor IX, which was previously effective in controlling a bleeding episode.