Hemophilia A is a genetic bleeding disorder in which the blood doesn’t clot properly because the body produces too little of a protein called factor VIII. It affects an estimated 33,000 males in the United States and is the most common severe form of hemophilia. People with hemophilia A bleed longer than normal, not necessarily faster, and the most dangerous bleeding often happens internally, particularly inside joints.
How Factor VIII Fits Into Clotting
When you cut yourself or bruise tissue, your body launches a chain reaction involving about 20 different clotting proteins. These proteins activate one another in sequence, ultimately forming a stable clot that plugs the wound. Factor VIII is one link in that chain. Without enough of it, the cascade stalls partway through, and the blood can’t build a firm clot. Small cuts may eventually stop on their own, but deeper injuries and internal bleeding become much harder for the body to manage.
Why It Mostly Affects Males
Hemophilia A follows an X-linked recessive inheritance pattern. The gene responsible for making factor VIII sits on the X chromosome. Males have only one X chromosome, so a single defective copy of the gene is enough to cause the disorder. Females have two X chromosomes, so a working copy on the second X usually compensates. That makes most women with one altered gene carriers rather than patients. They can pass the gene to their children without experiencing significant symptoms themselves.
In rare cases, a female carrier can develop symptoms. This happens when the body randomly shuts down the X chromosome carrying the normal gene through a natural process called X-inactivation, leaving cells reliant on the defective copy. The result can range from mildly low factor VIII levels to, occasionally, a full clinical picture of hemophilia.
Severity Levels
Hemophilia A is classified by how much factor VIII your body actually produces compared to normal levels:
- Mild: Factor VIII activity between 6% and 40% of normal. Bleeding problems typically surface only after surgery, dental work, or significant injury.
- Moderate: Factor VIII activity between 1% and 5% of normal. Bleeding can follow minor trauma and occasionally happens without an obvious trigger.
- Severe: Factor VIII activity below 1% of normal. Spontaneous bleeding episodes, especially into joints and muscles, are common and can begin in early childhood.
Severity stays consistent throughout a person’s life because it’s determined by genetics, not by age or lifestyle.
What Symptoms Look Like
The hallmark of hemophilia A is bleeding into joints, a problem called hemarthrosis. It’s the single most common musculoskeletal complication of the disorder. In severe hemophilia, joint bleeding affects 75% to 90% of patients, with the first episode usually occurring between ages 2 and 3. The knees, elbows, and ankles are the most frequently affected.
Older children and adults often notice a warning phase: a tingling sensation or stiffness in the joint before pain and swelling set in. In infants, the signs are less obvious. A baby might become irritable or stop using one arm or leg without a visible injury. Typically only one joint is affected at a time, though bilateral involvement is possible.
Beyond joints, hemophilia A can cause deep muscle bleeds, prolonged bleeding after cuts or dental procedures, easy bruising, and in serious cases, bleeding inside the skull. Repeated joint bleeds without adequate treatment eventually damage cartilage and bone, leading to chronic pain and limited mobility.
How It’s Diagnosed
Diagnosis usually starts with a blood test called the activated partial thromboplastin time (aPTT), which measures how long it takes blood to form a clot. The aPTT checks the function of several clotting factors, including factor VIII. In people with hemophilia A, the clotting time comes back longer than normal.
A prolonged aPTT points toward a problem but doesn’t pinpoint the exact cause. The next step is a factor assay, a test that measures the specific activity level of factor VIII in the blood. This confirms the diagnosis and establishes whether the condition is mild, moderate, or severe. Because hemophilia A runs in families, genetic testing and family history also play a role, and many children are tested shortly after birth when a parent is a known carrier.
Standard Treatment: Replacing the Missing Protein
The core treatment for hemophilia A is straightforward in concept: replace the factor VIII the body isn’t making. Commercially prepared factor VIII concentrates are infused into a vein, either from donated plasma or produced synthetically through recombinant technology. There are two main approaches to when these infusions happen.
Episodic (on-demand) treatment means infusing factor VIII after a bleed has started. The goal is to stop the bleeding episode as quickly as possible. For joint bleeds, prompt treatment within two hours of the first warning signs (that tingling or stiffness phase) significantly reduces joint damage.
Prophylactic treatment means infusing factor VIII on a regular schedule, typically several times a week, to keep levels high enough that bleeds are prevented in the first place. Prophylaxis is the standard of care for children and adults with severe hemophilia A because it protects joints from the cumulative damage of repeated bleeds.
The Inhibitor Problem
One of the biggest challenges in treating hemophilia A is that the immune system sometimes fights back. Between 20% and 35% of patients develop what are called inhibitors: antibodies that recognize infused factor VIII as foreign and neutralize it before it can do its job. When this happens, standard factor VIII replacement becomes less effective or stops working entirely, and bleeding episodes become much harder to control.
Inhibitor development is more common in people with severe hemophilia and tends to occur within the first 50 treatments. Managing inhibitors often requires alternative clotting agents that bypass factor VIII in the clotting cascade, along with immune tolerance therapy designed to train the body to accept factor VIII over time.
Newer Treatment Options
A medication called emicizumab (brand name Hemlibra) has changed the treatment landscape significantly. Rather than replacing factor VIII directly, emicizumab is a lab-engineered antibody that mimics what factor VIII does. It bridges two other clotting proteins, activated factor IX and factor X, to keep the clotting cascade moving forward. Because it works independently of factor VIII levels, it’s effective even in patients who have developed inhibitors. It’s approved for prevention of bleeding episodes in people with hemophilia A regardless of inhibitor status, and it’s given as an injection under the skin rather than into a vein, which is simpler for patients to manage at home.
Gene therapy has also reached a milestone. The FDA approved the first gene therapy for severe hemophilia A, based on results from the GENEr8-1 study, the longest and largest gene therapy trial conducted for hemophilia. The treatment delivers a working copy of the factor VIII gene into the body, with the goal of enabling patients to produce their own factor VIII and potentially reduce or eliminate the need for regular infusions. It’s currently approved for adults with severe disease and represents a fundamentally different approach: treating the genetic root of the condition rather than managing its symptoms.
Living With Hemophilia A
Day-to-day life with hemophilia A revolves around bleed prevention. People on prophylaxis learn to infuse factor VIII or administer emicizumab on schedule. Physical activity is encouraged because strong muscles help protect joints, but contact sports and activities with high injury risk require careful consideration. Many people with hemophilia work with specialized treatment centers that coordinate hematology, physical therapy, and orthopedic care.
Joint health is the long game. Each bleed that goes into a joint causes a small amount of damage, and over years, untreated or undertreated bleeds lead to arthritis-like deterioration. Early and consistent prophylaxis, starting in childhood, has dramatically improved joint outcomes compared to previous generations who relied solely on on-demand treatment. With current therapies, many people with hemophilia A maintain active, largely normal lives, though the condition requires ongoing management and awareness.