Hemophilia A vs von Willebrand Disease: Board-Relevant Differences

Hemophilia A is an X-linked deficiency of factor VIII that causes deep bleeding into joints and muscles, almost always in males. Von Willebrand disease is a usually autosomal dominant defect in von Willebrand factor that causes mucosal bleeding, such as nosebleeds, gum bleeding, and heavy periods, in both sexes. On labs, hemophilia A prolongs only the PTT, while von Willebrand disease prolongs the bleeding time and may or may not prolong the PTT. Desmopressin helps both in mild cases, but factor VIII replacement is the main treatment for hemophilia A.

Why do boards pair hemophilia A with von Willebrand disease?

Both are inherited bleeding disorders with a normal platelet count and a normal PT. Both can prolong the PTT. A stem that only says "prolonged PTT and easy bleeding" hasn't told you enough to pick between them.

The link is biological. Von Willebrand factor (vWF) has two jobs. It glues platelets to exposed collagen by binding the platelet receptor GPIb, and it carries factor VIII in the blood and protects it from breakdown. When vWF is low, factor VIII falls with it, because unbound factor VIII is cleared fast.

That second job is why the two diseases overlap on the PTT. The first job is why they split on the clinical picture. Hemophilia A is a pure clotting factor problem, so it looks like a secondary hemostasis defect. Von Willebrand disease is mainly a platelet adhesion problem, so it looks like a primary hemostasis defect.

How do you separate deep tissue bleeding from mucosal bleeding?

This is the first split I make on any bleeding stem, before I look at a single lab. Where the patient bleeds tells you which half of hemostasis is broken.

Primary hemostasis is the platelet plug. Platelets adhere to collagen through vWF, activate, and aggregate. When this step fails, small vessels in mucous membranes and skin leak right away. Think petechiae, epistaxis, gum bleeding after brushing, heavy menstrual bleeding, and oozing right after a dental extraction.

Secondary hemostasis is the fibrin clot built by the coagulation cascade. When a clotting factor is missing, the platelet plug forms but never gets reinforced. Bleeding goes deep and often starts late. Think hemarthrosis, muscle hematomas, retroperitoneal bleeding, and bleeding that restarts hours after surgery or circumcision.

Type 3 von Willebrand disease breaks this rule. Those patients have almost no vWF, so their factor VIII is very low too, and they can bleed into joints like a hemophiliac. Boards rarely test type 3, but know that it exists.

Hemophilia A: factor VIII and deep bleeding

Inheritance and who gets it

Hemophilia A is X-linked recessive. Affected patients are almost always male, and an affected father passes the mutant X to every daughter and to none of his sons. A carrier mother gives each son a 50% chance of disease. About one third of cases have no family history and come from a new mutation.

Females are usually carriers without symptoms. A carrier can still bleed if most of her cells happen to inactivate the normal X (skewed lyonization), or if she has Turner syndrome and her single X carries the mutation. If a stem gives you a bleeding female with low factor VIII and no vWF defect, that's the explanation.

Severity and presentation

Severity tracks residual factor VIII activity. Severe disease is under 1% of normal, moderate is 1% to 5%, and mild is 5% to 40%. Severe patients bleed spontaneously. Mild patients may go undiagnosed until a surgery, a dental extraction, or major trauma.

The classic presentation is hemarthrosis. Knees are hit most often, then elbows and ankles. The joint is swollen, warm, and painful, and the child refuses to bear weight. Repeated bleeds damage the synovium and cartilage and lead to hemophilic arthropathy.

Other board clues include prolonged bleeding after circumcision, large hematomas after vaccines, and deep muscle bleeds. An iliopsoas bleed can compress the femoral nerve. Intracranial hemorrhage is the most feared complication.

Labs in hemophilia A

The mixing study is the step boards love. You mix the patient's plasma 1:1 with normal plasma. If the PTT corrects, a factor was missing and the normal plasma supplied it. If the PTT stays prolonged, something in the patient's plasma is blocking clotting, such as a factor VIII inhibitor or a lupus anticoagulant.

Treatment of hemophilia A

Recombinant or plasma-derived factor VIII concentrate is the core treatment, for bleeds and for routine prophylaxis in severe disease. Desmopressin (DDAVP) releases stored factor VIII and vWF from endothelial cells, so it can raise factor VIII enough to treat mild hemophilia A. It does nothing for severe hemophilia A or for hemophilia B.

About a quarter to a third of patients with severe disease develop neutralizing antibodies (inhibitors) against infused factor VIII. Emicizumab is a bispecific antibody that bridges factor IXa and factor X, which mimics factor VIII's job. It's given as subcutaneous prophylaxis and works whether or not inhibitors are present.

Patients should avoid aspirin, NSAIDs, and intramuscular injections. Cryoprecipitate contains factor VIII, but factor concentrates have replaced it as the answer for hemophilia.

Von Willebrand disease: vWF and mucosal bleeding

Inheritance and who gets it

Von Willebrand disease is the most common inherited bleeding disorder. Lab-defined low vWF shows up in about 1% of people, though far fewer have symptoms. Types 1 and most type 2 variants are autosomal dominant, so men and women are affected equally and the disease shows up in every generation.

Women come to attention more often because menstruation and childbirth expose the defect. Heavy menstrual bleeding since menarche is one of the most common board presentations. Chronic blood loss from menorrhagia can also produce iron deficiency anemia, which the iron deficiency vs anemia of chronic disease guide walks through.

The three types

Type 2N deserves one line. The defect sits in the factor VIII binding site of vWF, so factor VIII is low while platelet adhesion is normal. It can mimic mild hemophilia A, but it's autosomal recessive and can affect women.

Labs in von Willebrand disease

Ristocetin is an old antibiotic that was pulled from clinical use because it makes vWF bind GPIb and clumps platelets. In the lab, adding ristocetin to a sample tests that exact step. Poor aggregation means vWF is missing or defective. Boards use ristocetin as the signature test for vWD.

Several things raise vWF levels and can mask mild disease. Pregnancy, estrogen, oral contraceptives, stress, exercise, and inflammation all push vWF up. People with blood type O have lower baseline vWF than other blood types. A single normal result doesn't rule vWD out.

Treatment of von Willebrand disease

Desmopressin is first line for type 1. It's a V2 receptor agonist that triggers release of vWF from Weibel-Palade bodies in endothelial cells. It's given before dental work or minor surgery, or at the onset of a bleed. It doesn't work in type 3, and it's avoided in type 2B because the released abnormal vWF can drop the platelet count further.

Because desmopressin acts on V2 receptors, it also concentrates the urine. Hyponatremia from water retention is the side effect to know, and the central vs nephrogenic diabetes insipidus guide covers the kidney side of that drug.

Antifibrinolytics, tranexamic acid or aminocaproic acid, stabilize clots on mucosal surfaces and are useful for epistaxis, dental bleeding, and heavy periods. Hormonal contraception reduces menstrual blood loss. For major surgery, type 3, or desmopressin failure, patients get vWF-containing factor VIII concentrates or recombinant vWF.

Hemophilia A vs von Willebrand disease side by side

The bleeding time is the cleanest single separator. It reflects platelet plug formation, so it's normal in hemophilia A and prolonged in vWD. If a stem gives you a prolonged PTT plus a prolonged bleeding time with a normal platelet count, think vWD first.

Other bleeding disorders that show up as distractors

Bernard-Soulier and vWD both fail the ristocetin test, because both break the vWF to GPIb connection. The difference is which side is broken. Adding normal plasma supplies vWF and fixes vWD in the lab. It doesn't fix Bernard-Soulier, because the platelet receptor itself is missing, and the smear shows large platelets with a low count.

If the bleeding stem also has a low platelet count and schistocytes, you've left inherited bleeding disorders behind. The ITP vs TTP vs HUS vs DIC guide covers that differential.

Three bleeding stems: working the split

Here are three example stems built to show the split.

Patient 1. A 14-month-old boy is brought in because he won't walk on his right leg. His right knee is swollen and warm after a minor fall. His maternal uncle had "bleeding problems." Platelet count, bleeding time, and PT are normal. The PTT is prolonged and corrects on a 1:1 mixing study.

A male toddler with a hemarthrosis and an affected maternal uncle points to an X-linked factor deficiency. A normal bleeding time rules out a platelet problem. Factor VIII and factor IX levels decide between hemophilia A and B, with A being about four times more common.

Patient 2. A 15-year-old girl has had periods lasting 9 days with clots since menarche. She bruises easily and had bleeding for two days after a wisdom tooth extraction. Her father has frequent nosebleeds. Platelet count and PT are normal, the PTT is mildly prolonged, and ristocetin cofactor activity is low.

Mucosal bleeding, an affected father, and an abnormal ristocetin test make this type 1 vWD. The mildly prolonged PTT comes from low factor VIII that rides on vWF. Desmopressin before procedures and tranexamic acid or hormonal therapy for menorrhagia are the expected answers.

Patient 3. A 72-year-old man has new, large bruises on his arms and a thigh hematoma with no prior bleeding history. The PT and platelet count are normal. The PTT is prolonged and stays prolonged after mixing with normal plasma. Factor VIII activity is very low.

A new bleeding disorder in an older adult with a PTT that doesn't correct points to an inhibitor. This is acquired hemophilia A from an autoantibody against factor VIII. Giving more factor VIII is the trap, because the antibody neutralizes it.

Stem clues that crack these questions fast

The clues I look for in every question are age, acuity, unusual findings, matching timelines, laterality, labs and imaging, and the question part. Here's how they apply to bleeding disorders.

1. Age. A male infant or toddler with joint or circumcision bleeding points to hemophilia. A teen girl with heavy periods points to vWD. An older adult with brand-new bleeding points to an acquired inhibitor.
2. Acuity. Bleeding that restarts hours after a procedure fits a factor deficiency. Oozing that starts immediately and won't stop fits a platelet adhesion defect.
3. Unusual findings. A family tree is a gift. Affected maternal uncles and brothers mean X-linked. An affected parent of either sex means autosomal dominant.
4. Matching timelines. Lifelong symptoms point to an inherited disorder. Bleeding that begins in late adulthood or after delivery points to an acquired one.
5. Labs are king. Check the bleeding time and the PTT together. Normal bleeding time with a long PTT is hemophilia. Long bleeding time with a normal or long PTT is vWD.
6. The question part. "Most likely deficient protein," "mode of inheritance," and "best treatment before a dental procedure" each ask something different. Read the last sentence before you commit.

When two answers both look right on a coagulation question, the guide to choosing between two answers on COMLEX walks through how to break the tie.

Common board pitfalls and how to avoid them

Pitfall 1 is ruling out vWD because the PTT is normal. The PTT only prolongs when factor VIII drops enough. Many type 1 patients have a normal PTT.

Pitfall 2 is giving desmopressin for severe hemophilia A or hemophilia B. Desmopressin only releases what the body already stores, so it helps mild hemophilia A and not severe disease. It does nothing for factor IX.

Pitfall 3 is giving desmopressin in type 2B vWD. The released abnormal vWF binds platelets and can worsen the thrombocytopenia.

Pitfall 4 is assuming a female can't have a factor VIII problem. Skewed X-inactivation, Turner syndrome, and type 2N vWD all produce low factor VIII in women.

Pitfall 5 is treating an inhibitor like a deficiency. A PTT that doesn't correct on mixing means an antibody, and more factor VIII won't overcome it.

Pitfall 6 is calling every abnormal ristocetin test vWD. Bernard-Soulier also fails it. Giant platelets and a low count point to the receptor, not the factor.

Frequently asked questions about hemophilia A vs von Willebrand disease

What is the main difference between hemophilia A and von Willebrand disease?

Hemophilia A is a deficiency of clotting factor VIII, so it causes deep bleeding into joints and muscles. Von Willebrand disease is a defect in von Willebrand factor, the protein that glues platelets to injured vessel walls, so it causes mucosal bleeding such as nosebleeds, gum bleeding, and heavy periods. Hemophilia A is X-linked recessive and affects males. Most von Willebrand disease is autosomal dominant and affects both sexes.

Why can von Willebrand disease prolong the PTT?

Von Willebrand factor carries factor VIII in the blood and protects it from rapid breakdown. When vWF is low, factor VIII levels fall too. If factor VIII drops far enough, the PTT prolongs. Many patients with type 1 disease have only mildly low factor VIII and a normal PTT, so a normal PTT does not rule out von Willebrand disease.

What labs distinguish hemophilia A from von Willebrand disease?

The bleeding time, or its modern replacement the PFA-100, is the cleanest separator. It is normal in hemophilia A and prolonged in von Willebrand disease. Ristocetin-based testing is abnormal in von Willebrand disease and normal in hemophilia A. Both have a normal platelet count and PT. Factor VIII activity is low in hemophilia A, and vWF antigen and activity are low in von Willebrand disease.

How does desmopressin work in von Willebrand disease and hemophilia A?

Desmopressin is a V2 receptor agonist that releases stored von Willebrand factor and factor VIII from endothelial cells. It is first line for type 1 von Willebrand disease and can treat mild hemophilia A. It does not work in severe hemophilia A, hemophilia B, or type 3 von Willebrand disease, and it is avoided in type 2B. Hyponatremia from water retention is the main side effect.

Can women have hemophilia A?

Yes, though it is uncommon. Female carriers can bleed if most of their cells inactivate the normal X chromosome, which lowers factor VIII activity. Women with Turner syndrome have only one X chromosome and are fully affected if that X carries the mutation. A woman can also inherit a mutant X from an affected father and a carrier mother. Type 2N von Willebrand disease is a separate cause of low factor VIII in women.

What does a mixing study show in hemophilia A?

In inherited hemophilia A, the prolonged PTT corrects when the patient's plasma is mixed 1:1 with normal plasma, because the normal plasma supplies the missing factor VIII. If the PTT stays prolonged after mixing, the patient has an inhibitor. In a new bleeding disorder of an older adult or a postpartum woman, that points to acquired hemophilia A from an autoantibody against factor VIII.

Related guides

• ITP vs TTP vs HUS vs DIC
• Iron deficiency anemia vs anemia of chronic disease on board exams
• Central vs nephrogenic diabetes insipidus on COMLEX and USMLE
• How do I choose between two answers on COMLEX?
• Doctor Lucas DO on YouTube: hematology question breakdowns and high-yield clinical comparison videos