Key points

Introduction

Platelets play an integral role in the maintenance of hemostasis by adhering to sites of vascular injury and forming a platelet plug. Platelets are anucleate discoid-shaped cells, 3 to 5 μm in diameter by 0.5 μm in depth, derived from bone marrow megakaryocytes. In healthy individuals, they circulate at a level of 150 × 10 ⁹ to 400 × 10 ⁹ platelets per liter. Thrombocytopenia can lead to bleeding symptoms ranging from petechiae and simple bruising to intracranial hemorrhage, pulmonary hemorrhage, and death. Low platelet counts are the result of either decreased production of platelets from factors adversely affecting megakaryocyte production in the bone marrow or increased destruction of platelets.

Platelet transfusions became routinely available in the 1970s when Murphy and Gardner showed that platelets could be stored at 22°C, for up to 3 days and still maintain their function. Platelet transfusions are now commonly used for both qualitative and quantitative platelet defects. Current guidelines recommend prophylactic platelet transfusions at a threshold of less than or equal to 10 × 10 ⁹ platelets per liter. With current advances in oncologic therapies as well as the use of hematopoietic stem cell transplantation, more patients develop severe hypoproliferative thrombocytopenia for longer durations than were previously seen and this often requires platelet transfusion support. The availability of effective platelet transfusions has decreased mortality from bleeding complications in these patient populations. In some patients, the observed platelet increment following transfusion is significantly less than the expected increment. Although these inadequate responses to platelet transfusions (ie, platelet refractoriness) have decreased with increased usage of leukoreduced products, they still occur and are of serious concern to both clinicians and the transfusion services supporting the special transfusion needs of these patients.

Introduction

Platelets play an integral role in the maintenance of hemostasis by adhering to sites of vascular injury and forming a platelet plug. Platelets are anucleate discoid-shaped cells, 3 to 5 μm in diameter by 0.5 μm in depth, derived from bone marrow megakaryocytes. In healthy individuals, they circulate at a level of 150 × 10 ⁹ to 400 × 10 ⁹ platelets per liter. Thrombocytopenia can lead to bleeding symptoms ranging from petechiae and simple bruising to intracranial hemorrhage, pulmonary hemorrhage, and death. Low platelet counts are the result of either decreased production of platelets from factors adversely affecting megakaryocyte production in the bone marrow or increased destruction of platelets.

Platelet transfusions became routinely available in the 1970s when Murphy and Gardner showed that platelets could be stored at 22°C, for up to 3 days and still maintain their function. Platelet transfusions are now commonly used for both qualitative and quantitative platelet defects. Current guidelines recommend prophylactic platelet transfusions at a threshold of less than or equal to 10 × 10 ⁹ platelets per liter. With current advances in oncologic therapies as well as the use of hematopoietic stem cell transplantation, more patients develop severe hypoproliferative thrombocytopenia for longer durations than were previously seen and this often requires platelet transfusion support. The availability of effective platelet transfusions has decreased mortality from bleeding complications in these patient populations. In some patients, the observed platelet increment following transfusion is significantly less than the expected increment. Although these inadequate responses to platelet transfusions (ie, platelet refractoriness) have decreased with increased usage of leukoreduced products, they still occur and are of serious concern to both clinicians and the transfusion services supporting the special transfusion needs of these patients.

Definition of platelet refractoriness

Platelet refractoriness can be simply defined as a posttransfusion platelet increment that is less than expected. The Trial to Reduce Alloimmunization to Platelets (TRAP) study defined platelet refractoriness as a corrected count increment (CCI) ( Box 1 ) of less than 5 × 10 ⁹ /L after 2 sequential transfusions, using ABO-compatible platelets, at least 1 of which had been stored for no more than 48 hours, with a posttransfusion platelet count obtained within an hour after transfusion. This definition has been generally accepted by the American Society of Clinical Oncology. The CCI formula takes both the number of platelets transfused and the patient’s body surface area into account. Furthermore, the timing of the posttransfusion platelet count is important for the interpretation of the CCI. A 1-hour posttransfusion count is preferred because it requires at least 1 hour to reach intravascular equilibrium after transfusion. However, in a busy hospital, a 1-hour posttransfusion platelet count can be difficult to obtain, and therefore some clinicians have advocated using a 10-minute posttransfusion count. The timing of the posttransfusion platelet count can be suggestive of the cause of refractoriness; a low CCI before 1 hour after transfusion is suggestive of immune causes, whereas a reduced CCI at 18 to 24 hours following a normal CCI at 1 hour is more suggestive of increased platelet consumption from other nonimmune clinical factors. It is currently recommended that the following criteria be used to determine platelet refractoriness : A 10-minute to 1-hour posttransfusion CCI of less than 5 × 10 ⁹ /L, observed on at least 2 sequential occasions, using ABO-identical freshest available platelets.

Cause

Platelet refractoriness can have both immune and nonimmune causes ( Boxes 2 and 3 ). Although the nonimmune causes are more likely to be responsible for a poor response to a platelet transfusion, there is little a transfusion service can do, other than recommending treating the underlying disorder, to improve the outcome of transfusion in these cases. Thus, the discussion focuses on the immune causes of platelet refractoriness.

The most common immune causes of platelet refractoriness are antibodies to human leukocyte antigens (HLAs) and/or human platelet antigens (HPAs), with anti-HLA antibodies more commonly responsible. The presence of these antibodies can be caused by prior exposure from pregnancy, transfusions, and/or transplantation.

The HLA system arises from the major histocompatibility complex that encodes polymorphic cell-surface proteins important for antigen presentation. Platelets express HLA class I antigens. HLA class I antigens consist of HLA-A, HLA-B, and HLA-C; HLA-A and HLA-B antigens predominate on platelets and are considered most relevant for platelet refractoriness. However, HLA-C antigens have also been reported to cause platelet refractoriness. Primary immunization against HLAs is caused by contaminating leukocytes in the platelet product and reducing contaminating leukocytes in blood products by filtration or ultraviolet B irradiation reduces the development of lymphocytotoxic antibodies and alloimmune platelet refractoriness to transfusions compared with untreated pooled platelet concentrates from random donors.

There are several platelet-specific antigens that have been characterized; however, only 5 of them are known to be polymorphic, leading to alloimmunization and platelet refractoriness: GPIa, GPIb, GPIIb, GPIIIA, and CD109. There are significant differences in the prevalence of the HPA polymorphisms in various populations, and patients become alloimmunized to these antigens through transfusions and pregnancy. Leukoreduction does not affect the incidence of platelet-specific antibodies, which varies from 2% to 11%. Furthermore, platelet-specific antibodies are generally not associated with a statistically significant reduction in CCI. Although case reports describe platelet-specific antibodies causing refractoriness to transfusion, these cases are usually confounded by concurrent anti-HLA antibodies.

Most patients who have HLA antibodies do not develop platelet refractoriness. In the TRAP study, 45% of the control group developed anti-HLA antibodies, but only 13% developed platelet refractoriness. A dose-response relationship between the number of platelets transfused and the incidence of alloimmunization is also not observed. In the TRAP trial, there was no difference in lymphocytotoxic antibodies or platelet refractoriness following transfusion of platelets obtained by apheresis from a single random donor compared with pooled platelet concentrates from random donors, which is surprising considering there is exposure to more donor HLA and HPA in pooled platelet concentrates. This finding suggests that alloimmunization and refractoriness are complicated processes with unknown modifying factors determining whether an individual will become alloimmunized and, subsequently, whether this alloimmunization will cause refractoriness to transfusion.

Transfusion service factors

In addition to the immune causes of platelet refractoriness, transfusion service factors, such as the storage of the platelet product and ABO compatibility, also affect platelet increments. Platelets show variable expression of ABH antigens on their surface; these antigens are both intrinsic to the platelet membrane and passively adsorbed from plasma. Thus, recipient ABO antibodies can increase the clearance of transfused incompatible platelets. Furthermore, platelets can be stored at room temperature for 5 days before transfusion. In vitro markers of platelet quality decline by day 5 of storage and platelet age significantly affects CCI, with platelets stored for less than 48 hours resulting in a significantly improved platelet increment at both 1 hour and 18 to 24 hours following transfusion. Thus, the CCI can be improved by using ABO-compatible and fresh platelets (ie, stored for less than 48 hours) and these factors must be considered when assessing patients for platelet refractoriness and for providing optimal care.

Incidence

Based on the TRAP study, in a study population of patients with acute myeloid leukemia (AML) receiving standard induction chemotherapy, 16% of patients receiving nonleukodepleted products met the criteria for refractoriness, as opposed to 7% to 10% of patients receiving leukodepleted products. Depending on the study population and the definition of refractoriness, the incidence of refractoriness in hematology/oncology patients varies from 7% to 34%. It is estimated that approximately two-thirds of refractory episodes have nonalloimmune causes, with another 20% having a combination of both alloimmune and nonalloimmune causes.

Laboratory diagnosis of immune-mediated platelet refractoriness

There are several laboratory tests available to determine whether a patient with platelet refractoriness has anti-HLA and/or anti-HPA antibodies. However, there is no consensus regarding which test is ideal for diagnosing refractoriness. Some assays may be too sensitive, identifying weak HLA antibodies that do not predict platelet refractoriness. Cell-based cytotoxicity assays may better predict platelet refractoriness; however, these tests are more cumbersome than the more automated techniques. Thus, the results of platelet refractoriness testing should be interpreted in conjunction with the clinical picture and, in the absence of a gold standard test, a change in transfusion management should only be pursued if both clinical and laboratory evidence suggest the presence of true immune-mediated platelet refractoriness.