HLA/HPA Selected Platelets, Part of the Specialized Cells Program

Human leukocyte antigens (or HLA) are expressed on almost every cell in the body.1 They play a key role in our ability to recognize our own cells from the outside world like bacteria. The two main classes of antigens are HLA Class I and Class II. Each HLA Class has three main genes (e.g. HLA Class I A, B, C) and within each gene, there are many alleles (Figure 1). Every year, new HLA alleles are identified with over 9,000 HLA Class I alleles currently identified. This is most similar to red blood cells in the context of blood groups.  Biologically, HLA Class I A and B are the only meaningfully expressed antigens on platelets.2

What is platelet alloimmunization?

Platelet alloimmunization occurs when an individual has HLA and or HPA antibodies that cause the immune system to destroy incompatible platelets. The severity of platelet alloimmunization can be measured by the calculated Panel Reactive Antibody or cPRA? 1 The cPRA estimates the percentage of the general population with whom the patient would be incompatible. A cPRA of 20% means that approximately 80% of Canadian donors could match this patient. The HLA/HPA Selected Platelet program considers HLA/HPA matched platelets for a patient when the cPRA is more than 20% (1/5 donors). Patients not responding to platelet transfusions without another underlying medical condition are considered to be platelet refractory. 

Before a physician requests HLA/HPA Selected Platelets they must first determine if the patient is refractory due to infection, drugs, and fever which are the most common non-immune causes. Less common conditions include disseminated intravascular coagulation and splenomegaly, or Alloimmunized. 

Why does the patient have thrombocytopenia?

Is the patient thrombocytopenic because of an underlying condition such as bleeding, sepsis, or medication?

If your answer is Yes,

  • Treat the underlying condition
  • Support with random donor platelets
  • Consider other hemostatic options

If your answer is No,

  • Does the recipient get appropriate increase in platelet count, are they refractory?
    • Assess the platelet response measured between 10 and 60 minutes after completion of a platelet transfusion on two separate occasions. It is commonly stated that one unit of apheresis platelets should achieve an increment of at least 30 x 109 cells/Liter to 50 x 109 cells/Liter.
    • There are two common ways to determine refractoriness:
      • The platelet count did not increase by at least 10x 109  cells/litre within 60 min of a transfusion of one unit of apheresis platelets. This is called the platelet increment (PI) which is calculated.
      • PI= P2 – P1 , where P1 is the pre-transfusion platelet count and P2 is the post-transfusion platelet count.
      • The corrected count increment (CCI) is below 7.5x 109  cells/litre one hour post transfusion4.
      • CCI= [(PI x BSA)/n] x 100, where BSA represents the body surface area in meters2,and n is the number of platelets transfused.
      • The percentage platelet recovery (PPR)= [(PI x body weight (kg) x 0.075 (l/kg))/n] x 100. Body weight (kg) x 0.075 (l/kg) is an estimate of blood volume and n is the number of platelets transfused.

Does the patient have any HLA antibodies?

If the answer is Yes, 

  • If the patient has HLA Class 1 platelet antibodies, then you can consider requesting HLA/HPA Selected platelets from Canadian Blood Services.  

If the HLA antibody testing is negative or a patient continues to be refractory with HLA-matched platelets, then ordering HPA antibody testing may be warranted.  

Below is a decision tree that was developed based on the work from the International Collaboration for Transfusion Medicine Guidelines in 2015.3