Functional Platelet Disorders

Functional platelet disorders affect the adhesion, activation, or aggregation of platelets and are frequently associated with excessive bleeding, often following trauma or surgery. These disorders can be either acquired or inherited. Because mucocutaneous bleeding can result from a variety of defects or from an illness such as von Willebrand disease (VWD), a number of possibilities must be considered during initial patient evaluation. Laboratory testing for these disorders involves CBC with platelet count and peripheral blood smear, basic coagulation tests, and tests to rule out VWD; further testing may include platelet aggregation tests, platelet flow cytometry, and in some cases, genetic testing.

Quick Answers for Clinicians

How do functional platelet disorders manifest clinically?

The clinical features of functional platelet disorders vary. Some patients may present with symptoms such as a history of easy bruising, menorrhagia, or excessive or protracted bleeding in response to minor injuries, after dental work, or after surgical procedures. It is unusual for patients with functional platelet disorders to demonstrate bleeding into muscle or joints (this bleeding pattern is more typical for factor deficiencies; rarely, it may be observed in functional platelet disorders). Functional platelet disorders are generally associated with immediate bleeding in response to surgery or trauma, whereas factor deficiencies are associated with delayed bleeding.

What is the general diagnostic testing approach to a patient with a platelet-type bleeding pattern?

The workup of a patient with a platelet-type bleeding pattern typically includes correlation with platelet count (thrombocytopenia), evaluation of platelet morphology on peripheral smear, exclusion of acquired causes of platelet dysfunction, and testing for von Willebrand disease (VWD). Some patients may require a more extensive workup to exclude other bleeding disorders as well.

What are some causes of acquired functional platelet disorders?

Acquired disorders of platelet function can arise from a number of causes, including antiplatelet medications (eg, aspirin, ibuprofen, thienopyridines, and beta-lactam antibiotics, among others), herbal supplements and foods, renal failure, cardiopulmonary bypass, myeloproliferative disorders, myelodysplastic syndromes, paraproteinemia, and platelet antibody-induced platelet dysfunction. It is important to rule out acquired conditions during an evaluation for a functional platelet disorder. A thorough medication history is important; aspirin and nonsteroidal anti-inflammatory drugs (NSAIDs) are common causes of abnormal platelet function, and platelet function testing may need to be deferred if patients have been taking these medications.

Which functional platelet disorders are inherited?

Bernard-Soulier syndrome (an adhesion disorder) and Glanzmann thrombasthenia (an aggregation disorder) are among the most well-recognized inherited disorders of platelet function and are severe clinical conditions. Refer to the table, Laboratory Findings in Selected Inherited Functional Platelet Disorders, for additional information and a more comprehensive list of inherited functional platelet disorders.

Indications for Testing

Laboratory testing for functional platelet disorders is appropriate in individuals with bleeding manifestations that suggest a functional platelet disorder (eg, easy bruising, prolonged bleeding following dental work or surgery, menorrhagia), particularly those with a family history of a bleeding disorder and in whom VWD has been previously excluded. Testing is also warranted in infants and children with unexplained thrombocytopenia.

Laboratory Testing

Diagnosis

Initial Testing

Initial laboratory testing for a functional platelet disorder includes a CBC with platelet count and a peripheral smear. These tests are useful to assess platelet morphology and size and to guide further testing. Thrombocytopenia is associated with some platelet function disorders; therefore, a finding of thrombocytopenia does not rule out such disorders.

Automated platelet counting methods are less precise in cases of very low platelet counts (<50 x 10⁹/L) or particularly large platelets. Peripheral blood smear morphologic review can evaluate for platelet clumping and granularity, as well as platelet number and size.

Basic coagulation tests, including prothrombin time (PT), partial thromboplastin time (PTT), and fibrinogen, are also part of the initial evaluation of a bleeding disorder. Bleeding time and point-of-care platelet function tests are not recommended as part of the assessment for functional platelet disorders; they lack sufficient sensitivity and specificity to determine whether the patient needs additional testing.

Exclusion of VWD, the most common inherited bleeding disorder, is recommended before further evaluation for a functional platelet disorder. Testing for VWD involves evaluation of von Willebrand factor quantity and function, as well as factor VIII activity. Refer to the ARUP Consult von Willebrand Disease topic for additional information about testing for this disease.

Platelet Aggregation Testing

Further testing for a functional platelet disorder involves platelet aggregation testing. Light transmission aggregometry (LTA) is the most commonly used method to evaluate platelet function. This testing must be performed locally because it requires fresh patient platelets, which are stable for only a limited period (~4 hours). If clinically feasible, patients should discontinue the use of medications that can affect platelet function for 7-10 days before testing. (The decision to discontinue medications should be made by the patient’s treating physician.) Such medications include aspirin, nonsteroidal anti-inflammatory drugs (NSAIDs), and antibiotics, among others. If medications cannot be safely discontinued, a medication list should be included to facilitate interpretation of test results.

Abnormal aggregation response to multiple agonists, as detected by LTA, is strongly associated with a bleeding disorder. Refer to the Laboratory Findings in Selected Inherited Functional Platelet Disorders table below for typical laboratory results for various conditions.

Platelet aggregation testing in samples with platelet counts <150,000 /µL should be interpreted with caution, and platelet aggregation studies are not recommended in patients with significant thrombocytopenia (platelet counts <100,000/µL). When aggregation test results are abnormal, repeat testing is often necessary to confirm the results.

Platelet Dense Granule Evaluation

Another test often used in conjunction with LTA in evaluation for functional platelet disorders involves the measurement of adenosine triphosphate (ATP) released from platelet dense granules, which is generally performed using lumiaggregometry. (Testing not performed at ARUP Laboratories.) Low levels of ATP release suggest that platelet dense granules may be abnormal in number, content, or capacity for secretion. In addition to ATP release tests, electron microscopy can be used to quantitate platelet dense granules to detect dense granule deficiency. (Testing not performed at ARUP Laboratories.)

Platelet Flow Cytometry

Flow cytometry can be helpful to evaluate and quantify platelet membrane glycoproteins (GPs). This testing is particularly indicated if Bernard-Soulier syndrome or Glanzmann thrombasthenia is suspected because flow cytometry quantification of the membrane GP receptor density can confirm diagnosis of some subtypes of these disorders. Platelet flow cytometry is also useful in patients who have thrombocytopenia (because functional platelet aggregation studies are less reliable in these patients) and could be helpful if platelet aggregation testing is not available to a patient locally. Platelet flow cytometry will not detect rare subtypes of Glanzmann thrombasthenia or Bernard-Soulier syndrome that are characterized by receptor GP dysfunction rather than decreased platelet membrane GPs.

Laboratory Findings in Selected Inherited Functional Platelet Disorders
Disorder	Initial Testing	Platelet Aggregation Studies	Platelet Dense Granule Analysis	Platelet Flow Cytometry
Bernard-Soulier syndrome	Decreased platelet count Giant platelets on peripheral blood smear	Failure to aggregate with ristocetin but normal aggregation with other agonists^a	n/a	Absent or significantly reduced expression of GP Ib/IX/V; normal expression of GP IIb/IIIa^b
Chediak-Higashi syndrome (storage pool disorder)	Normal platelet count and morphology Peripheral blood/marrow smears demonstrate abnormal granules in blood and marrow leukocytes	Findings will be similar to those seen in delta storage pool deficiency (see below)	Decreased or absent dense granules shown by electron microscopy	n/a
Delta storage pool deficiency (dense granule deficiency)	Normal platelet count and morphology	Absent secondary wave in response to epinephrine and ADP, unless high concentrations of ADP are used, and possible decreased aggregation to collagen and ristocetin; aggregation studies may be normal	Decreased or absent dense granules shown by electron microscopy	n/a
Glanzmann thrombasthenia	Normal platelet count and morphology	Absent response to all agonists other than ristocetin (normal response to ristocetin)^c	n/a	Absent or significantly reduced expression of GP IIb/IIIa; normal expression of GP Ib/IX/V^b
Gray platelet syndrome (storage pool disorder)	Decreased platelet number Peripheral smear shows large pale, agranular platelets (macrothrombocytopenia)	May demonstrate decreased response to thrombin and/or collagen, but aggregation results are variable	n/a	n/a
Hermansky-Pudlak syndrome (storage pool disorder)	Normal platelet count and morphology	Findings will be similar to those seen in delta storage pool deficiency (see above)	Decreased or absent dense granules shown by electron microscopy	n/a
Wiskott-Aldrich syndrome (cytoskeletal protein defect)	Decreased platelet number and abnormally small platelets, neutropenia	Decreased aggregation with agonists such as epinephrine, ADP, and collagen	n/a	n/a
^aPatients with severe VWD may have an aggregation profile similar to those with Bernard-Soulier syndrome. ^bA mild decrease in the expression of GP Ib or GP IIb/IIIa suggests a variant or heterozygous state of Bernard-Soulier syndrome or Glanzmann thrombasthenia, respectively. A mild decrease in platelet GP expression could also be a laboratory artifact due to a suboptimal specimen condition. Qualitative subtypes of Bernard-Soulier and Glanzmann thrombasthenia may not be detected by flow cytometry. Correlation with clinical findings and other platelet function or sequence analysis is recommended. ^cPatients with afibrinogenemia will have an aggregation profile similar to those with Glanzmann thrombasthenia. ADP, adenosine diphosphate; n/a, not applicable Sources: Israels, 2015; Mezzano, 2009; Gresele, 2015; Freson, 2014; Nurden, 2014

Genetic Testing

Molecular genetic testing can be useful to confirm diagnosis of suspected inherited functional platelet disorders for which causative variants are known. Examples include variants in GP1BA, GP1BB, and GP9, known to be associated with Bernard-Soulier syndrome. Variants in LYST (CHS1) are associated with Chediak-Higashi syndrome, and variants in PLAU are associated with Quebec platelet disorder. (Testing not performed at ARUP Laboratories.)