Congenital Platelet Function Disorders
General Diagnostic Evaluation
Within a routine diagnostic laboratory, the diagnosis of platelet dysfunction may be limited to an analysis of platelet aggregation in vitro, an evaluation of platelet morphology and size on peripheral blood smear, and in some cases, an analysis of platelet antigen expression by flow cytometry or platelet ultrastructure by electron microscopy.
Reference centers can provide additional and more specialized analyses, including measurement of platelet content and release of ATP/ADP, characterization of granule content, or an assessment of platelet signal transduction. Utilizing newer technologies, a more precise diagnosis is now possible based on specific genomic and proteomic characteristics, allowing hematologists to counsel patients and family members based on the finding of specific and well-characterized gene variants. Most bleeding disorder centers can send out samples for analysis if these assays are not performed on site.
Platelet Function Evaluation
Family history is very important, particularly in children, since they normally have not experienced many hemostatic challenges. In addition, specific attention should be paid to the occurrence and severity of umbilical stump bleeding, bleeding after circumcision, cephalohematoma, recurrent petechiae and purpura that are unusual for developmental age, conjunctival bleeding, and hematuria. Several bleeding scores (eg, BAT-ISTH, Pediatric Bleeding Questionnaire) have been developed that can serve as templates to assess severity of bleeding history in cases of platelet function disorders.11,12,13
Von Willebrand factor antigen level and activity
An important initial laboratory test is the quantitation of plasma von Willebrand Factor (VWF) antigen and activity. Abnormal VWF activity (assessed as ristocetin cofactor) typical of von Willebrand disease (VWD) type 2B and platelet-type VWD or quantitative deficiencies of VWF antigen, even if at the low end of the normal range, may confound the diagnosis of a primary platelet function disorder.
Platelet count and size
The next and most common laboratory tests utilized are assessments of platelet count and size. Careful evaluation of platelet count and morphology should be performed most simply by assessment of a peripheral blood smear and can guide subsequent evaluations.
In addition, automated platelet count and mean platelet volume (MPV) can be obtained via an automated CBC, but cell counters can often underestimate the platelet count and may render inaccurate MPV values in the setting of either macrothrombocytopenia or microthrombocytopenia.14 The peripheral smear can also provide other clues toward a diagnosis, such as pale, large platelets in gray platelet syndrome (GPS); Dohle-like inclusions in the leukocytes in MYH9-related disorders; and abnormal red blood cell morphology consistent with GATA1 mutations.
Normal platelet counts
If a normal platelet count is found in the setting of a history of bleeding, the next step is to analyze platelet function. If an evaluation of VWD has not yet been performed, it should be conducted at this stage as the clinical manifestations of mild platelet function defects can be similar to those observed in VWD.15
Light transmission aggregometry (LTA)
This is the most common and a very effective method to evaluate platelet function. Guidelines for standardization have been formulated by a number of international groups.16,17 In the case of children, the most serious limiting factor to performing LTA assessments is the volume of blood required to complete informative panels using several platelet agonists, and to validate initial findings. This often may necessitate repeat phlebotomies on several occasions to establish the presence of a platelet function disorder and rule out minor variances in aggregation. The latter are often observed in the general population, particularly in response to epinephrine or ADP, and represent a cumulative effect of common gene variants.18,19
For an overview of the patterns of platelet aggregation responses that are used to establish a diagnosis, please refer to the “Differential Diagnosis” section.
If a patient is thrombocytopenic (platelet count <100,000 per mm3), the diagnosis is largely dependent on platelet size. For a summary of platelet defects and the associated thrombocytopenia, please see the section on Platelet Defects.