Hemophilia is a rare, usually inherited bleeding disorder in which the blood does not clot the way it should, because one of the proteins needed to form a stable clot is missing or in short supply. People with hemophilia do not bleed faster than anyone else, but they bleed for longer, and the most important bleeds are often the ones inside the body, especially into joints and muscles. Thanks to modern treatment, the outlook has been transformed: with regular preventive therapy, many people with hemophilia now have a life expectancy approaching that of the general population. This guide explains what hemophilia is, its main types, the symptoms and how severity is measured, the clotting tests used to diagnose it, today’s treatments including one-time gene therapies, and the latest research.
What is hemophilia?
Clotting is a team effort involving a series of proteins called clotting factors that work in sequence to seal an injured blood vessel. In hemophilia, one of these factors is deficient or absent, so the clotting cascade stalls and bleeding continues longer than normal. The result is not usually dramatic external bleeding from small cuts, but rather prolonged bleeding after injury, surgery, or dental work, and internal bleeding into joints and muscles that can cause lasting damage if untreated.
Hemophilia is uncommon, affecting about 1 in 5,000 male births, with roughly 33,000 males living with hemophilia A or B in the United States. It is a lifelong condition, but it is highly manageable, and specialized hemophilia treatment centers help people prevent bleeds, protect their joints, and live active lives. There is no routine cure yet, though one-time gene therapies have recently changed what is possible for some people.
Types of hemophilia
Hemophilia is classified by which clotting factor is affected. The three types differ in frequency and inheritance, as summarized below.
| Type | Missing factor | Notes |
|---|---|---|
| Hemophilia A | Factor VIII | The most common type, three to four times more frequent than hemophilia B |
| Hemophilia B | Factor IX | Also called Christmas disease; less common than type A |
| Hemophilia C | Factor XI | The rarest form; affects both sexes and rarely causes joint bleeding |
Symptoms of hemophilia
The symptoms of hemophilia reflect prolonged or excessive bleeding rather than easy cuts. Common signs include large or unexplained bruises, prolonged bleeding after injury, surgery, or dental work, frequent or hard-to-stop nosebleeds, and blood in the urine or stool. The most characteristic problem is bleeding into joints such as the knees, ankles, and elbows, which causes pain, swelling, and stiffness and, over years, can damage the joint.
How often and how seriously a person bleeds depends on how much working clotting factor they have. In severe hemophilia, factor activity is below 1 percent of normal and bleeding can happen spontaneously, without any obvious injury. Moderate hemophilia, at 1 to 5 percent, tends to bleed after minor injuries, while mild hemophilia, above 5 percent, may only become apparent after surgery, dental procedures, or significant trauma. A small share of people develop an inhibitor, an antibody that blocks replacement factor and makes bleeds harder to treat.
What causes hemophilia?
Hemophilia is caused by changes in the genes that carry instructions for clotting factors. Hemophilia A and B are inherited in an X-linked recessive pattern, which is why they mainly affect males, who have a single X chromosome. Females have two X chromosomes and are usually carriers who may have no symptoms, though some carriers do bleed more than average, and rarely a female can be affected as severely as a male. Hemophilia C follows a different, non-X-linked pattern and affects both sexes.
Because the condition is genetic, it is present from birth and cannot be caught from another person. In about a third of cases there is no family history, because the gene change arises new in that individual. For families affected by hemophilia A or B, genetic testing can identify carriers and support family planning and prenatal counseling.
How hemophilia is diagnosed
Diagnosis relies on blood tests that measure how well and how quickly the blood clots. A complete blood count is often normal, so the key tests are the ones that time the clotting cascade and then measure specific factors. In hemophilia A and B, the prothrombin time is normal while the activated partial thromboplastin time is prolonged, a pattern that points toward a problem in the pathway that includes factors VIII and IX.
Doctors then confirm the diagnosis and type with factor activity assays, and the result also defines severity, since factor levels below 1 percent indicate severe disease, 1 to 5 percent moderate, and above 5 percent mild. Understanding these clotting tests is easier with a few plain-language guides: this overview explains the clotting time measured by a partial thromboplastin time test, while another covers the separate pathway checked by a prothrombin time test. Because a normal platelet count with a prolonged clotting time helps separate hemophilia from platelet problems, it also helps to know what a platelet count reveals on a blood test. Genetic testing can then confirm the specific gene change and identify carriers in the family.
Treatment options for hemophilia
Treatment aims to replace or mimic the missing clotting factor, prevent bleeds before they start, and protect the joints. Care is highly individualized and coordinated through specialized hemophilia treatment centers.
| Approach | Examples | Role |
|---|---|---|
| Factor replacement | Recombinant or plasma-derived factor VIII or IX | Given on demand for bleeds or on a schedule to prevent them |
| Non-factor prophylaxis | Emicizumab and newer injectable agents | Subcutaneous drugs that reduce bleeds, including with inhibitors |
| Gene therapy | One-time infusions for hemophilia A or B | Enable the body to make its own clotting factor |
| Supportive medicines | Desmopressin, antifibrinolytics | Help mild hemophilia A or control mucosal bleeding |
The mainstay of care has long been clotting factor replacement, delivered intravenously either to stop an active bleed or, more often now, as regular prophylaxis to keep bleeds from happening. Emicizumab, a subcutaneous antibody that mimics the action of factor VIII, has made prevention easier for many people with hemophilia A, including those with inhibitors, and newer injectable prophylaxis options have followed. One-time gene therapies for both hemophilia A and B can prompt the body to produce its own factor, reducing or eliminating the need for infusions in eligible adults. Desmopressin can raise factor VIII in some people with mild hemophilia A, and antifibrinolytic drugs help with bleeding from the mouth and other mucous membranes. People with hemophilia are also advised to avoid aspirin, other nonsteroidal anti-inflammatory drugs, and blood thinners, which further impair clotting.
Living with hemophilia and long-term outlook
With consistent preventive treatment, most people with hemophilia lead full, active lives, and life expectancy now approaches that of the general population. The main long-term concern is joint damage from repeated bleeds, so protecting the joints through prophylaxis, prompt treatment of bleeds, physical therapy, and appropriate exercise is central to staying well. Care at a hemophilia treatment center helps coordinate all of this.
Repeated or heavy bleeding can sometimes lead to low iron and anemia, so monitoring blood counts is part of routine follow-up, and understanding the low red blood cell counts behind anemia can help people recognize related fatigue. Staying up to date with vaccinations, choosing safe physical activities, and reviewing any new medication with the care team all help prevent complications over a lifetime.
Latest scientific advances in hemophilia research
Hemophilia care is in the middle of a genuine revolution, with long-term data now arriving for gene therapies and next-generation prophylaxis. According to PubMed-indexed research, the five-year final analysis of the HOPE-B trial, published in the New England Journal of Medicine in 2025, found that a single infusion of the hemophilia B gene therapy etranacogene dezaparvovec cut the annual bleeding rate by 63 percent and kept factor IX activity at useful levels years later, with most participants no longer needing routine factor infusions (Pipe et al., 2025). What this means for you: for some people with hemophilia B, a one-time treatment can meaningfully reduce bleeds and infusions for years, though it is not guaranteed to last a lifetime.
A 2026 final analysis in Haemophilia reported seven-year outcomes for the hemophilia A gene therapy valoctocogene roxaparvovec, showing an 87 percent reduction in bleeding and far less factor use, while noting that factor VIII levels gradually decline over time (Raheja et al., 2026). What this means for you: gene therapy for hemophilia A offers substantial, lasting benefit, but the effect may wane, which is important for setting expectations. On the prevention side, a 2026 real-world study in the British Journal of Haematology followed 132 people, many of them young children, on emicizumab prophylaxis and found a very low annual bleeding rate and resolution of most problem joints, with no dangerous clots (Xu et al., 2026). What this means for you: newer subcutaneous prophylaxis can protect joints and cut bleeds even in very young patients, reinforcing its role as a convenient first-line option.
Glossary of key hemophilia terms
| Term | Definition |
|---|---|
| Clotting factor | One of the proteins that work in sequence to form a stable blood clot. |
| Factor VIII / factor IX | The clotting proteins missing in hemophilia A and hemophilia B. |
| Activated partial thromboplastin time | A test of clotting speed that is prolonged in hemophilia A and B. |
| Factor activity assay | A blood test that measures how much working clotting factor a person has. |
| Inhibitor | An antibody that blocks replacement factor and complicates treatment. |
| Prophylaxis | Regular preventive treatment given to stop bleeds before they start. |
| Hemarthrosis | Bleeding into a joint, a common and damaging problem in hemophilia. |
Frequently asked questions about hemophilia
What is hemophilia and what causes it?
Hemophilia is a bleeding disorder in which the blood cannot clot properly because a clotting factor is missing or low. It is caused by changes in the genes for those factors, most often factor VIII in hemophilia A or factor IX in hemophilia B. It is usually inherited and is present from birth.
What is the difference between hemophilia A and hemophilia B?
Hemophilia A is caused by a deficiency of clotting factor VIII and is the most common type, while hemophilia B is caused by a deficiency of factor IX and is less common. Their symptoms are similar, but the treatments differ because each replaces or targets a different factor.
Can women and girls have hemophilia?
Hemophilia A and B are X-linked, so they mainly affect males, but females can be carriers and some do have bleeding symptoms, occasionally as severe as in males. Hemophilia C affects both sexes equally. Carrier testing can clarify a woman’s status and bleeding risk.
How is hemophilia diagnosed, and what blood tests are used?
Diagnosis uses clotting tests, including the activated partial thromboplastin time, which is prolonged, alongside a normal prothrombin time, followed by factor activity assays that confirm the type and severity. A complete blood count and platelet count help rule out other causes, and genetic testing can identify the exact gene change.
Is there a cure for hemophilia?
There is no routine cure, but treatment is highly effective, and one-time gene therapies for hemophilia A and B can greatly reduce or eliminate the need for infusions in eligible adults for years. Long-term data show meaningful, though not always permanent, benefit.
What is the life expectancy for someone with hemophilia?
With modern preventive treatment, life expectancy for people with hemophilia now approaches that of the general population. The biggest long-term issue is joint damage from repeated bleeds, which prophylaxis and prompt treatment help prevent.
Sources
- Centers for Disease Control and Prevention — About Hemophilia — CDC, 2024 — cdc.gov
- Centers for Disease Control and Prevention — Hemophilia Diagnosis — CDC, 2024 — cdc.gov
- MedlinePlus, National Library of Medicine — Hemophilia — MedlinePlus Genetics, 2024 — medlineplus.gov
- Pipe SW, Miesbach W, Recht M, et al. — Final Analysis of a Study of Etranacogene Dezaparvovec for Hemophilia B — New England Journal of Medicine, 2025 — doi.org/10.1056/NEJMoa2514332
- Raheja P, Rangarajan S, Lester W, et al. — Final Analysis of the Phase 1/2 Trial of Valoctocogene Roxaparvovec for Severe Haemophilia A — Haemophilia, 2026 — doi.org/10.1111/hae.70284
- Xu Y, Wang Y, Wang N, et al. — Long-term outcomes with emicizumab prophylaxis for haemophilia A in China: A multicentre, large-cohort retrospective study — British Journal of Haematology, 2026 — doi.org/10.1111/bjh.70623
Further reading
- See how the main hemophilia clotting test works in this guide to the partial thromboplastin time and what it measures.
- Compare it with the other side of the clotting cascade in this guide to the prothrombin time blood test.
- Understand the cells that hemophilia does not affect in this guide to the platelet count on a blood test.
- Learn about a complication that repeated bleeding can cause in this guide to the low red blood cell counts of anemia.
- Build confidence reading test results with this guide to reference ranges, flags, and next steps on a lab report.
Understand your lab results with BloodSense
Hemophilia is a clear example of how much clotting tests can reveal, from a prolonged partial thromboplastin time to the factor assays that pin down the type and severity. Whenever you receive results, seeing where each value falls against its reference range, and how the clotting numbers fit together, makes them far easier to understand. BloodSense translates a full lab report into plain language, showing what each marker means and helping you follow changes over time rather than interpreting one result at a time.
