Acquired hemophilia is a rare bleeding disorder that develops when your immune system mistakenly produces antibodies that attack a specific clotting protein in your blood called factor VIII. Unlike the inherited form of hemophilia that people are born with, acquired hemophilia strikes suddenly in people who have never had bleeding problems before. It affects roughly 1.4 people per million each year, and about half the time, no underlying cause is ever identified.
How It Differs From Inherited Hemophilia
The word “acquired” is the key distinction. Inherited (congenital) hemophilia is caused by a genetic mutation present from birth, almost always affecting males. Acquired hemophilia appears later in life when the immune system begins producing autoantibodies that neutralize factor VIII, one of the proteins your blood needs to form clots. These antibodies don’t completely eliminate factor VIII the way a genetic deficiency does. Instead, they progressively inactivate it, which creates an unpredictable and often severe bleeding pattern.
The bleeding pattern itself looks different too. People with inherited hemophilia typically bleed into their joints and muscles, especially in the knees, elbows, and ankles. Acquired hemophilia tends to cause widespread bleeding under the skin (large, spreading bruises), bleeding into soft tissues and muscles, and bleeding from mucous membranes like the gums, nose, or gastrointestinal tract. Joint bleeding is actually uncommon in the acquired form. This difference in bleeding pattern is one reason the condition can be difficult to recognize at first.
Who Gets It and Why
Acquired hemophilia most commonly affects older adults, though it can occur at any age. Mortality is notably higher in people over 65 (8%) compared to younger patients (3%). In roughly half of all cases, no trigger or underlying condition is found. For the other half, several patterns emerge:
- Autoimmune diseases (17 to 18%): Conditions like lupus, rheumatoid arthritis, and other disorders where the immune system is already overactive.
- Cancer (about 25%): Both solid tumors (21%) and blood cancers (roughly 4%) are associated with acquired hemophilia.
- Postpartum (8.4%): Some women develop the condition one to four months after giving birth, though cases have been reported anywhere from pregnancy through one year postpartum.
- Medications: Certain antibiotics, blood thinners, anti-inflammatory drugs, and other medications have been linked to the condition.
- Infections: Hepatitis B and C, and more recently COVID-19, have been associated with cases.
Symptoms and Warning Signs
The hallmark of acquired hemophilia is sudden, unexplained bleeding in someone with no personal or family history of bleeding disorders. Large bruises that spread across the skin are the most common presentation, often appearing without any clear injury. These bruises tend to be extensive and may feel firm or raised rather than flat.
Bleeding can also occur internally, into muscles or the gastrointestinal tract, or from the urinary tract. Surgical wounds or minor procedures that won’t stop bleeding can be the first clue. In postpartum women, abnormally heavy or prolonged bleeding after delivery that doesn’t respond to standard treatment is a red flag. Because the condition is so rare, there is often a delay between the first symptoms and a correct diagnosis.
How It Is Diagnosed
Diagnosis starts with routine blood clotting tests. A test called the activated partial thromboplastin time (aPTT) will be prolonged, meaning the blood is taking longer than normal to clot. The next step is a mixing study, where the patient’s blood is mixed with normal blood to see if the clotting time corrects. In acquired hemophilia, the clotting time does not fully correct because the antibodies in the patient’s blood begin inactivating the factor VIII in the normal blood.
One important technical detail: most of the antibodies in acquired hemophilia are “time and heat dependent,” meaning they don’t act instantly. A mixing study that only checks results immediately after mixing can miss the diagnosis entirely. The sample needs to be incubated for about two hours before being re-tested. This is a well-known diagnostic pitfall.
Once the mixing study raises suspicion, a specific factor VIII activity level is measured, and a Bethesda assay quantifies how much inhibitor (antibody) is present. The inhibitor level, measured in Bethesda units, helps guide treatment decisions. Some low-level inhibitors can be tricky to detect and may require additional sample dilutions to confirm.
Treatment: Stopping the Bleeding
Treatment has two parallel goals: stop any active bleeding and eliminate the antibodies so the body can produce functional factor VIII again.
For active bleeding, the approach depends on the inhibitor level. When antibody levels are high, standard factor VIII replacement won’t work because the antibodies will simply neutralize it. Instead, doctors use “bypassing agents,” medications that activate the clotting process through an alternate route that doesn’t require factor VIII. The only bypassing agent specifically approved for acquired hemophilia in the U.S. works by providing an activated form of another clotting factor. In a large registry study, this type of agent was used as the first-line treatment in the majority of bleeding episodes. Red blood cell transfusions are sometimes needed as well, particularly when bleeding has been significant.
Eliminating the Antibodies
Controlling bleeding is only half the battle. The longer-term goal is to suppress the immune system enough to stop it from producing the destructive antibodies. First-line treatment typically involves corticosteroids, either alone or combined with an immune-suppressing drug. If this combination fails, a targeted therapy that depletes the specific immune cells producing the antibodies can be added, with response rates between 59% and 90%.
The timeline for antibody elimination varies widely. Some patients respond within weeks, while others require months of treatment. Relapses can occur even after successful treatment, and there is unfortunately little correlation between initial antibody levels and how quickly or durably someone responds.
Prognosis and Recovery
Acquired hemophilia is serious. In a large analysis, the inpatient mortality rate was 7%, with a median hospital stay of seven days. The 30-day readmission rate was 27%, and infections followed by bleeding were the most common reasons for readmission. Mortality during readmission reached nearly 11%.
Complications like blood clots or stroke during treatment significantly worsen outcomes. Patients who developed a stroke or blood clot during their hospital stay had mortality rates of 34% and 20%, respectively, compared to roughly 6% for those without these complications. This reflects the difficult balancing act in treatment: the condition causes dangerous bleeding, but the clotting therapies used to control it can tip the balance toward excessive clotting.
For postpartum acquired hemophilia, the outlook is generally more favorable. Many of these cases resolve as the immune stimulus from pregnancy fades, though the timeline is unpredictable and relapses remain possible. Younger patients overall have significantly better survival than those over 65, largely because older patients are more likely to have underlying cancer or other serious conditions contributing to the disorder.