When ordering serum platelet antibody testing, it is important to understand the type of antibody that will be informative for the specific situation. When assessing a patient for an autoantibody, an antibody that recognizes self-antigens, it is important to order the cell-bound antibody test. This testing is appropriate for autoimmune causes of thrombocytopenia. When assessing a patient for an alloantibody, an antibody that binds to non-self-antigens (foreign antigens), it is important to order the platelet antibody test. This testing is appropriate for cases of alloimmune thrombocytopenia.
Presenters and Credentials:
Justin Kreuter, M.D., is an Instructor in Laboratory Medicine and Pathology and a Consultant in the Division of Transfusion Medicine at Mayo Clinic in Rochester, Minnesota.
Today our presenter is Dr. Justin Kreuter, associate director of the histocompatibility laboratory in the division of Transfusion Medicine at Mayo Clinic in Rochester, Minnesota. Dr. Kreuter discusses platelet antibody testing.Welcome to Mayo Medical Laboratories Hot Topics. These presentations provide short discussion of current topics and may be helpful to you in your practice.
Dr. Kreuter, Thank you for presenting with us today.
Thank you for that introduction. I have nothing to disclose.
As you view this presentation, consider the following important points regarding testing. How is the testing going to be used in your practice? When should the test be used? And how will these results impact patient management?
Immune Destruction: Platelets
Antibodies come in 2 varieties. Alloantibodies, which are antibodies that bind to non-self-antigens. For example, in pregnancy, we understand that a fetus receives half of its DNA from its mom and half from its dad. If the dad’s DNA encodes a platelet antigen 1a, and the mom’s does not, then, if mom is exposed to platelets that express human platelet antigen 1a during pregnancy, she may make an antibody that binds to human platelet antigen 1a. This antibody, once made, may cross the placenta to cause critical thrombocytopenia in the fetus. This condition is known as NAIT–neonatal alloimmune thrombocytopenia. A similar situation may occur following transfusion with a platelet that expresses an antigen that the patient does not express. The patient may make an antibody that recognizes the foreign antigen, which may result in the patient rapidly clearing future platelet transfusions when those platelets also express the foreign antigen. These patients are commonly referred to as refractory to platelet transfusion. In contrast, autoantibodies are antibodies that bind to self-antigens. In the normal, healthy individual, we do not make antibodies that recognize self-antigens. However, this is not the case in certain diseases. For example, in ITP–immune thrombocytopenic purpura–the body seems to make antibodies that recognize glycoproteins on the platelet surface, which results in rapid clearance of platelets and potentially critical thrombocytopenia. There are several disease states, such as systemic lupus erythematosus, HIV, and hepatitis C, that are associated with secondary ITP. In these cases dysfunction of the immune system results in the similar process of antibodies that bind to glycoproteins on the platelet surface, to cause thrombocytopenia.
To better understand platelet antibody testing, let’s take a closer look at these antigens on the platelet surface.
Several glycoproteins on the platelet surface play a functional and potentially pathologic role.
Glycoprotein Ib/IX: binds to von Willebrand factor and collagen. This receptor is essential for platelet adhesion under high sheer. Patients with a congenital absence of this glycoprotein are diagnosed with the platelet adhesion deficit of Bernard Soulier syndrome.
Glycoprotein IIb/IIIa: binds to fibrinogen and von Willebrand factor. Patients with a congenital absence of this glycoprotein are diagnosed with the platelet aggregation defect of Glansmann’s thrombasthenia.
Glycoprotein Ia/IIa: binds to collagen, a component of subendothelial matrix, this further strengthens a platelet plug.
When talking about, HPA–the human platelet antigens–these are point mutations at specific locations on the various platelet glycoproteins.
Platelet Antibody Testing (alloantibody)
When we perform our serum platelet antibody test, we take the patient’s serum and expose it to specific platelet antigens, that is, specific subcomponents of the glycoprotein. Then we detect if the patient has alloantibodies that bind to these specific human platelet antigens.
Platelet Antibody Testing (autoantibody)
When we perform our cell-bound platelet antibody test, we first wash away the patient’s serum, because we want to test only antibodies that are bound to circulating platelets. Once any antibodies are removed from the platelet surface, we then expose them to the full glycoproteins and detect any autoantibodies that bind to these glycoproteins.
Platelet Antibody testing (together)
In this way, our serum test is optimized to identify alloantibodies that bind to specific human platelet antigens; whereas, the cell-bound assay is optimized to identify autoantibodies that bind broadly to platelet glycoproteins.
Alloantibodies—Platelet Antibody, Serum
When ordering the serum platelet antibody test, when you’re considering alloimmune cases of thrombocytopenia, such as a patient that is refractory to platelet transfusion or neonatal alloimmune thrombocytopenia or posttransfusion purpura, the first check point we ask ourselves is, is the patient a neonate, less than 31 days old? If yes, then we should consider sending a maternal sample instead of the neonate, looking for unbound platelet antibodies that may not be detected in the neonate serum. If the patient is not a neonate, then order that platelet serum antibody test on the patient.
Autoantibodies—Cell-Bound Platelet Autoantibody
When ordering the cell-bound antibody test looking for autoantibodies, we’re looking for autoimmune causes of thrombocytopenia such as ITP or those conditions that are considered secondary ITP. We first ask “has the patient received a platelet transfusion in the last 72 hours?” If yes, we want to delay collecting a specimen until 72 hours posttransfusion because transfused platelets may interfere with the assay. If it has been longer than 72 hours since the platelet transfusion, we then ask “is the patient a neonate, less than 31 days old?” If the answer is yes, then we want to order the platelet serum antibody test on the mother. If the patient is not a neonate, then we order the cell-bound platelet autoantibody test on the patient.
Patient-Centered Platelet Antibody Testing
We have reviewed the common clinical situations in which platelet antibody testing may be performed.
If considering NAIT or the platelet refractory state because of an antibody that recognizes a human platelet antigen, then order our serum test. This report will inform you regarding the specific antibody, which will inform platelet support for the critically thrombocytopenic neonate or adult.
If considering ITP or a secondary ITP cause, then order our cell-bound test, as this report will help confirm your clinical impression.
Importantly, ordering the serum test to evaluate for an autoantibody or ordering the cell-bound test to evaluate for an alloantibody will yield a report that is noninformative and potentially misleading. For optimal patient care, please order platelet antibody testing with these issues in mind, and call our support line with questions.