People with hemophilia
have a higher quality of life today than ever before, but complications can
still occur. Approximately 15-20% of people with hemophilia will develop an
antibody—called an inhibitor—to the product used to treat or prevent bleeding
episodes. Developing an inhibitor is one of the most serious and costly
complications of hemophilia.
About Inhibitors
People with hemophilia
use treatment products called clotting factor concentrates. This treatment
improves blood clotting and is used to stop or prevent a bleeding episode.
Inhibitors develop when the body’s immune system stops accepting the factor
(factor VIII for hemophilia A and factor IX for hemophilia B) as a normal part
of blood. The body thinks the factor is a foreign substance and tries to
destroy it using inhibitors. The inhibitors stop the factor from working. This
makes it more difficult to stop a bleeding episode. People with hemophilia who
develop an inhibitor do not respond as well to treatment. Inhibitors most often
appear during the first year of treatment but they can appear at any time.
Cost of Care
Caring for people with
inhibitors poses a special challenge. The health care costs associated with
inhibitors can be staggering because of the cost and amount of treatment
product required to stop bleeding. Also, people with hemophilia who develop an
inhibitor are twice as likely to be hospitalized for a bleeding complication.
Risk Factors and Causes
Scientists do not know
exactly what causes inhibitors. Risk factors that have been shown in some
studies to possibly play a role include:
·
Age
·
Race/ethnicity
·
Type of hemophilia gene
defect
·
Frequency and amount of
treatment (inhibitors typically occur within the first 50 times factor is used)
·
Family history of
inhibitors
·
Type of factor treatment
product
·
Presence of other immune
disorders
Diagnosis
A blood test is used to
diagnose inhibitors. The blood test measures inhibitor levels (called inhibitor
titers) in the blood. The amount of inhibitor titers is measured in Bethesda
units (BU). The higher the number of Bethesda units, the more inhibitor is
present. “Low titer” inhibitor has a very low measurement, usually less than 5
BU. “High titer” inhibitor has a very high measurement, usually much higher
than 5 BU.
Inhibitors are also
labeled “low responding” or “high responding” based on how strongly a person’s
immune system reacts or responds to repeated exposure to factor concentrate.
When people with high-responding inhibitors receive factor concentrates, the
inhibitor titer measurement increases quickly. The increased inhibitor titer
prevents the clotting factor concentrates from stopping or preventing a
bleeding episode. Repeated exposure to clotting factor concentrates will cause
more inhibitors to develop.
When people with
low-responding inhibitors receive factor concentrates, the inhibitor titers do
not rise. Therefore, people with low-responding inhibitors can usually still
use clotting factor concentrates to stop or prevent a bleeding episode.
Treatment
Treating people who have
inhibitors is complex and remains one of the biggest challenges in hemophilia
care today. If possible, a person with inhibitors should be cared for at a hemophilia treatment center (HTC). HTCs are
specialized health care centers that bring together a team of doctors, nurses,
and other health professionals experienced in treating people with hemophilia.
Some treatments for
people with inhibitors include the following:
·
High-Dose Clotting Factor Concentrates: People who have low responding inhibitors may be
treated with higher amounts of factor concentrate to overcome the inhibitor and
yet have enough left over to form a clot. It is important to test the blood and
measure the factor level after this new treatment schedule is established to
see if the inhibitor is gone.
·
Bypassing Agents: Special blood products are used to treat bleeding in people with
high titer inhibitors. They are called bypassing agents. Instead of replacing
the missing factor, they go around (or bypass) the factors that are blocked by
the inhibitor to help the body form a normal clot. People taking bypassing
agents should be monitored closely to make sure the blood is not clotting too
much or clotting in the wrong place in the body.
·
Immune Tolerance Induction (ITI) Therapy: The goal of ITI therapy is to stop the inhibitor
reaction from happening in the blood and to teach the body to accept clotting
factor concentrate treatments. With ITI therapy, people receive large amounts
of clotting factor concentrates every day for many weeks or months.
ITI therapy requires
specialized medical expertise, is costly, and may take a long time to work. In
many cases, ITI gets rid of the inhibitor. However, patients may need to
continue taking frequent, large amounts of factor concentrates for many years
to keep the inhibitor from coming back. HTCs can serve a vital role in supporting patients
who undergo a treatment regimen as intensive as ITI.
CDC Research
CDC is interested in
learning more about why some people develop inhibitors and how they could be
prevented. The Inhibitor Project began in 2005 and was implemented to explore
the following questions:
·
Does a change in
treatment products (from one type of factor product to another) lead to an
inhibitor?
·
Are people with specific
gene mutations more likely to develop an inhibitor?
·
What characteristics
make some people more likely to develop an inhibitor than others?
·
Why do some people
develop inhibitors and others do not?
·
How often do inhibitors
occur?
In the Inhibitor Project, a limited number of federally funded hemophilia treatment centers across the United States enroll study participants who have hemophilia A or hemophilia B. Detailed information about their hemophilia, their complications, and their treatment is collected and studied over time. The CDC laboratory tests each participant’s blood and determines if he or she has an inhibitor. Testing all of the blood at the CDC laboratory, rather than using several different labs throughout the country, ensures that the testing procedures are consistent and reliable for analysis. The blood samples are also used to study hemophilia-related genes and gene mutations to learn more about who is more likely to develop an inhibitor even before treatment is started. The laboratory results are shared with the study participant’s physician.
Through additional
research, we hope to increase our understanding of inhibitors. Knowing more
about why some people develop inhibitors and others do not may help us predict
who will develop an inhibitor before treatment is started. This may lead to a
decreased rate of inhibitors, decreased health care costs, and the licensure of
safe and more effective treatment products for people with hemophilia.
Did You Know?
When people are being treated for an inhibitor, they receive
frequent, large amounts of clotting factor concentrates and therefore may need
a central venous access device (CVAD). A CVAD is a small tube placed in a vein.
It can stay in the vein for a long time. Clotting factor concentrates can be
given through the CVAD instead of by injection or infusion through a painful
needle stick. A downside of having a CVAD is that people with CVADs are more
likely to develop infections, blood clots, and other complications than those
without CVADs. It is important for people with hemophilia and their caregivers
to learn how to care for the device.
Source : http://www.cdc.gov/ncbddd/hemophilia/inhibitors.html
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