Why is so much blood required for HLA testing? 
HLA typings are now performed using molecular methods, which mean that a typing can be completed using less blood from the patient.  However, crossmatch techniques still require that live lymphocytes, a specific type of white blood cells, be isolated from the peripheral blood.  The ratio of red blood cells to white blood cells is about 1000:1.  Therefore, we need to be able to start with more whole blood to isolate enough lymphocytes to complete the testing.  Lymphocytes are also used for preliminary and final crossmatches.  If extra cells can be isolated, they will be frozen and saved in case they are needed in the future.

What is a PRA? 
PRA stands for Percent (or Panel) Reactive Antibody.  The PRA is expressed as a percentage and is used as a rough estimate of what percent of donors a person will NOT be compatible with.  In other words, a person with a PRA of 40% will NOT be compatible with roughly 40% of possible donors.

Can my PRA change?
Yes.  Typically a PRA is highest following a sensitizing event (pregnancy, transfusion, or transplant) and decreases with time.  Patients who are not sensitized should consistently have a PRA of 0%.