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HLA Typing
Special blood tests, called Human Leukocyte Antigen (HLA) typing,
determine whether a patient has a suitable donor for stem cell
transplant. These tests are performed on several tubes of blood
drawn from the arm. The matching for stem cell transplant is much
more complicated than matching for red cell blood types. HLA typing
is increasingly done using DNA techniques and can take several
days to complete. (For information on the HLA Lab at St. Francis
please refer to their on-line information at http://www.stfrancishospitals.org/).
The most likely place to find a matched donor is within the patient's
own family. Siblings (brothers or sisters) will more likely match than
other relatives, such as parents, children, or cousins. All humans inherit
1/2 of their entire genetic make-up, and thus of their HLA-type, from their
mother and the other 1/2 from their father. For each full sibling, a
patient has a one in four (25%) chance of a full match. If you have full
siblings, the chances of having a completely matched donor are:
| One sibling: |
25% |
| Two siblings: |
44% |
| Three siblings: |
58% |
| Four siblings: |
68% |
| Five siblings: |
76% |
| Six siblings: |
82% |
| Seven siblings: |
87% |
| Eight siblings: |
90% |
| Nine siblings: |
92% |
| Ten siblings: |
94% |
(The chance of having a donor is 1-(3/4)n, where
n is the number of siblings).
With the current size of the average American family, only about 30% of
patients will be found to have an acceptable HLA-matched donor within their
family. Therefore, search for an unrelated donor may be necessary.
What About Unrelated Donors?
The National Marrow Donor Program (NMDP) is a federally supported
organization that assists in locating unrelated donors. Currently, the NMDP
has computer access to over 7 million volunteer donors from throughout the United
States and other countries. How easily
an unrelated bone marrow donor can be found, is dependent on the patient's
HLA type. The more unique, or unusual, the HLA
type, the more difficult it will be to find a sufficiently matched
unrelated donor. Patients from mixed ethnic background may have very
uncommon HLA- types. In addition, although the NMDP has made an enormous
effort to recruit minority donors, the chances for African-Americans and
Asian-Americans of finding a fully matched unrelated donor are still
smaller than for Caucasian patients. Even in cases where the patient's HLA
type is quite common, it can take a minimum of 4 to 6 weeks to obtain the
donor's stem cells. For more information about unrelated stem cell donors,
and how you can become a donor, you may directly contact the NMDP (http://www.marrow.org/).
What If No Unrelated Donor Can Be Found?
In some instances, the HLA type of the patient may be so unique that
finding a matched unrelated donor is impossible. If a patient cannot find a
fully matched related or unrelated donor, it may be necessary to look at
other options, such as using a patient's own bone marrow, matched umbilical
cord blood or a partially matched family donor. The results of transplants
with umbilical cord cells or with partially matched related have improved
considerably over the last few years. Such transplants are feasible, but
the risks are still higher than for HLA-identical sibling transplants or
transplants from matched unrelated donors.
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