Hemophilia: How to Make a Differential Diagnosis

Written by Margaret Anne Rockwood | Last updated August 5th, 2026
Medically reviewed by Shannon Walker, MD

Hemophilia is a disorder of secondary hemostasis. Thus, making an accurate diagnosis requires integration of clinical findings with targeted laboratory testing.

Individuals with severe hemophilia typically present with spontaneous hemarthroses, deep-muscle hematomas, or prolonged bleeding after minor trauma, whereas mild disease may not become apparent until bleeding occurs after surgery, dental extraction, or a more significant injury. The severity and frequency of bleeding events generally correlates with the amount and activity level of the affected clotting factor (either factor VIII [FVIII] or factor IX [FIX]) in the individual.

Severe hemophilia often presents with spontaneous bleeding, most often into joints and muscles, but also other areas, including the brain and GI tract, can occur. It is often diagnosed at a young age, while mild hemophilia may not become apparent until adulthood. A family history of hemophilia or unexplained bleeding can provide an important diagnostic clue, although up to one-third of patients may have no known family history because of spontaneous mutations.

Diagnostic Lab Tests and Screens

Initial Screens

Initial workup typically includes:

• Complete blood count (CBC)
• Prothrombin time (PT)
• Activated partial thromboplastin time (aPTT)
• Fibrinogen level (often added when there is broader concern about coagulation, disseminated intravascular coagulation, liver disease, major bleeding or another cause of an abnormal clotting test)

In classic hemophilia:

• Platelet count is normal
• PT is normal
• Fibrinogen levels are typically normal (one distinguisher with consumptive coagulopathies such as DIC and inherited fibrinogen disorders)
• aPTT is abnormal

aPTT is usually prolonged, indicating an impaired intrinsic coagulation pathway. However, a normal aPTT does not exclude mild hemophilia, particularly when FVIII or FIX activity levels are near the lower limit of normal.

An isolated prolonged aPTT should prompt further evaluation with a mixing study. Failure to correct suggests the presence of an inhibitor or anticoagulant interference. However, some FVIII inhibitors are time- and temperature-dependent, so an incubated mixing study may reveal an inhibitor even when the immediate mixture initially corrects. Mixing studies are most useful when the cause of an isolated prolonged aPTT is unclear or when acquired disorders, such as factor inhibitors, must be excluded.

Lupus Anticoagulant Testing

Testing for a lupus anticoagulant may be appropriate when an isolated prolonged aPTT fails to correct in a mixing study or when the clinical and laboratory findings do not fit a straightforward factor deficiency. A lupus anticoagulant can prolong the aPTT and interfere with some factor assays, but it is more often associated with thrombosis than with bleeding.

Factor Assays

Definitive diagnosis of hemophilia largely depends on quantitative measurement of factor VIII and factor IX activity:

• Low factor VIII activity largely confirms hemophilia A, though reduced FVIII can also occur in von Willebrand disease and acquired hemophilia A. The result must be interpreted with the clinical history, VWF testing, mixing studies, and inhibitor testing when appropriate.

• Low factor IX activity largely confirms hemophilia B, though acquired FIX inhibitors, liver disease, vitamin K deficiency, anticoagulants, and assay interference may occasionally need consideration.

Factor levels also help classify disease severity and guide treatment decisions. Disease severity is categorized as severe (<1% activity), moderate (1% to 5%), or mild (>5% to <40%).

The one-stage clotting assay remains the most widely used method for measuring factor activity and is available broadly. Chromogenic assays are more limited to specialty labs and are increasingly used because assay discrepancies may occur in some patients if they have mild disease, are treated with certain modified factor products, or gene-therapy–associated expression patterns.

Inhibitor Testing

For individuals who have been diagnosed with hemophilia, the development of inhibitory antibodies that neutralize infused clotting factor concentrates remains one of the most significant complications of hemophilia treatment. Inhibitors should be suspected when bleeding persists despite apparently adequate factor replacement or when post-infusion factor levels are lower than expected.

The Bethesda assay and Nijmegen-modified Bethesda assay remain standard methods for inhibitor quantification. Inhibitor titers are reported in Bethesda units (BU) or Nijmegen-Bethesda units (NBU), and regular surveillance is recommended, particularly during the early exposure period to replacement products.

Bleeding Disorder Mimics

Von Willebrand disease

Von Willebrand disease (VWD) is the most common inherited bleeding disorder. VWD can resemble mild hemophilia A because the role of von Willebrand factor (VWF) is to stabilize circulating factor VIII. When VWF is reduced, it leads to decreased factor VIII activity and a prolonged aPTT, creating a laboratory picture that resembles hemophilia A.

Both disorders can present with excessive bleeding after surgery, dental procedures, or trauma. On the surface, clinical symptoms may resemble mild hemophilia; however, individuals with VWD are more likely to exhibit mucocutaneous bleeding manifestations, including epistaxis, menorrhagia, and gingival bleeding due to VWFs role in primary hemostasis.

Differential diagnostic evaluation should include:

• von Willebrand factor antigen
• von Willebrand factor activity
• factor VIII measurements

Factor XI deficiency

Factor XI deficiency is another potential confounder in hemophilia diagnosis, as it can present with excessive bleeding after surgery or trauma and prolonged bleeding after dental procedures. This disease, historically called hemophilia C or Rosenthal syndrome, is usually caused by inherited mutations in the F11 gene, which encodes coagulation factor XI. Most cases are autosomal recessive (vs. X-linked); both males and females can be affected in equal proportions. It is particularly common among individuals of Ashkenazi Jewish ancestry.

Features that distinguish Factor XI deficiency from hemophilia:

• Hemarthroses are uncommon.
• Spontaneous muscle bleeding is uncommon.
• Bleeding is often procedure-based.
• Factor level and bleeding severity are poorly correlated.
• Affects both sexes in a similar proportion.

Similar to hemophilia A and B, lab tests for factor XI deficiency typically return with a normal PT and prolonged aPTT. Therefore, despite some differences in features, measurement of factor XI is needed to distinguish it from hemophilia.

Platelet Disorders

Platelet disorders, including immune thrombocytopenia and inherited platelet function defects, may also mimic hemophilia. Unlike hemophilia, however, these conditions generally produce mucocutaneous bleeding, petechiae, and easy bruising rather than recurrent hemarthroses and deep-tissue bleeding.

Acquired hemophilia A

Acquired hemophilia A is a separate form that should be considered in adults who develop spontaneous bleeding and an isolated prolonged aPTT without a personal or family history of bleeding disorders. The condition results from autoantibodies directed against factor VIII and often presents with severe soft-tissue or mucosal hemorrhage. Mixing studies typically fail to correct, and inhibitor assays confirm the diagnosis.

Medications

Certain anticoagulants and medications used to treat liver disease can produce abnormal coagulation studies and bleeding symptoms that mimic inherited clotting disorders.

Genetic Testing and Carrier Detection

Genetic testing has become an increasingly important component of confirmatory diagnosis in hemophilia by identifying the specific disease-causing variant within the F8 or F9 gene through DNA analysis. Unlike clotting factor testing, which measures protein activity, molecular testing directly detects the underlying genetic alteration responsible for hemophilia.

Cascade Testing

Genetic testing can also be used to identify at-risk relatives and carriers and provide guidance for reproductive counseling if appropriate.

Cascade testing is the systematic genetic testing of biologically related family members after a pathogenic variant has been identified in one individual. Testing typically begins with close relatives, such as parents, siblings, and children, and may expand to more distant relatives who could also be carriers or affected individuals who were previously undiagnosed.

Genetic Counseling and Reproductive Options

Genetic counselors who specialize in rare disease can help families understand inheritance patterns, testing options, and reproductive risks.

Available testing options for hemophilia may include:

• carrier testing before pregnancy
• prenatal diagnosis through chorionic villus sampling (CVS)
• amniocentesis
• preimplantation genetic testing (PGT-M) during in vitro fertilization

Chorionic villus sampling is typically performed between 10 and 13 weeks of pregnancy, while amniocentesis is usually performed after 15 weeks. Both procedures can identify a known familial hemophilia mutation before birth.

Women and Girls May be Overlooked

Diagnosis may be delayed by the mistaken belief that hemophilia occurs only in males. Hemophilia should be considered in females with compatible bleeding histories, particularly when there is a relevant family history. Some females who are carriers have sufficiently low factor levels to experience heavy menstrual bleeding, postpartum hemorrhage, or excessive bleeding after procedures. Further, recent studies demonstrate that carriers with normal baseline factor levels can have increased clinical bleeding symptoms.

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