What is the role of lab testing in the workup of alloimmunized refractoriness to platelet transfusions?

Updated: Sep 08, 2017
  • Author: Douglas Blackall, MD, MPH; Chief Editor: Michael A Kaliner, MD  more...
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Answer

Refractoriness to platelet transfusions is defined as a repeated failure to achieve the expected increment in platelet count after 2 or more platelet transfusions. The expected increment, also called the corrected count increment (CCI), can be calculated based on the number of platelets transfused and the patient's body surface area (BSA) using the following formula: [31]

CCI = (posttransfusion count – pretransfusion count in platelets/µL) X BSA (in M2) ÷ number of platelets transfused (X 1011)

As an example, if a man with a BSA of 2.0 M2 increases his platelet count from 5,000/ul to 45,000/ul after receiving 4 X 1011 platelets, then his CCI equals 20,000 (ie, (45,000-5,000) X 2.0 ÷ 4.0 = 20,000).

Of note, if the actual number of platelets in a platelet product are unknown, 6 X 1010 platelets can be assumed for each whole blood-derived platelet transfused; an average apheresis unit contains the equivalent of approximately 6 pooled whole blood-derived platelet units or 4 X 1011 platelets.

On the basis of studies in patients receiving prophylactic platelet transfusions and platelet transfusions in healthy volunteers, the expected CCI in a successful transfusion should range between 10,000 and 20,000.

CCIs of less than 7500 at 10-60 minutes posttransfusion indicate a probable platelet transfusion failure. Consistent failure to achieve expected CCIs defines the platelet refractory state.

A simple screen for platelet refractoriness is failure to achieve an increment of >5 X 109/L (5,000/µL) at 20-24 hours after transfusion of a standard platelet dose (1 unit/10 kg of pooled platelets or one single-donor apheresis unit).

Note that many cases of apparent refractoriness result from causes other than alloimmunization (see Differentials).

In general, alloimmunization results in the rapid removal of platelets and in lower counts at 10 minutes to 1 hour posttransfusion, whereas nonimmune causes mostly affect the 4- to 24-hour posttransfusion count. Mild alloimmunization, however, can still be present with 1-hour posttransfusion platelet increments within the expected range.

Microcytotoxicity assays against a panel of 30-60 different lymphocyte cells can demonstrate lymphocytotoxic HLA antibodies. The percentage of cells to which the patient's serum reacts is referred to as the panel-reactive antibody (PRA) level. PRA values greater than 20% indicate significant alloimmunization to HLA antigens and correlate with an increased risk for platelet refractoriness.

The presence of antiplatelet antibodies can be demonstrated by flow cytometry or by immunoassays such as the modified antigen capture enzyme-linked assay, the solid-phase RBC adherence assay, and the monoclonal antibody immobilization of platelet antigens assay. Most of these assays permit screening for HLA and HPA antibodies as well as specific identification of the most commonly involved HPA antigens.

A negative result from platelet antibody screening (HLA and HPA) strongly suggests nonimmune causes of refractoriness.


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