Qualitative Platelet Defects

VWF is a large multimeric glycoprotein made by endothelial cells and megakaryocytes. Endothelial cells store it in Weibel-Palade bodies, and megakaryocytes package it in platelet α-granules. Each subunit contains D′-D3-A1-A2-A3-D4-C1-C2-C3-C4-C5-C6-CK domains with distinct binding functions.^{33 Springer TA. Von Willebrand factor, Jedi knight of the bloodstream. 2014.}

Shear stress unfolds newly released ultralarge multimers and exposes the A2 cleavage site to ADAMTS13. Severe ADAMTS13 deficiency allows ultralarge multimers to persist, which is central to the pathophysiology of thrombotic thrombocytopenic purpura. In VWD, reduced VWF concentration or impaired VWF function weakens platelet adhesion and produces bleeding that can vary over time and among relatives with the same variant. Severe VWF deficiency also lowers factor VIII. When factor VIII is markedly reduced, joint and muscle bleeding can accompany the usual mucocutaneous pattern.^{4,54 James PD, et al. ASH ISTH NHF WFH 2021 guidelines on the diagnosis of von… 2021.5 Keohane EM, et al. Rodak's Hematology. 2025.}

The initial laboratory panel includes VWF antigen, platelet-dependent VWF activity, and factor VIII activity. VWF ristocetin cofactor activity (VWF:RCo) measures the agglutination of reagent platelets in the presence of ristocetin. Newer GPIb-binding assays use recombinant receptor fragments. VWF:GPIbR uses ristocetin and recombinant wild-type GPIbα. VWF:GPIbM uses a gain-of-function GPIbα fragment and does not require ristocetin.^{4,6,74 James PD, et al. ASH ISTH NHF WFH 2021 guidelines on the diagnosis of von… 2021.6 Bodó I, et al. Platelet-dependent von Willebrand factor activity. 2015.7 Platton S, et al. Guideline for the laboratory diagnosis and monitoring of von… 2024.}

An activity-to-antigen ratio below 0.7 supports a qualitative type 2 defect. Classify the type from the full pattern. Multimer analysis or VWF collagen-binding activity helps separate types 2A and 2B from 2M. When factor VIII is disproportionately low, the next step is a VWF-factor VIII binding assay or molecular testing for type 2N.^{4,74 James PD, et al. ASH ISTH NHF WFH 2021 guidelines on the diagnosis of von… 2021.7 Platton S, et al. Guideline for the laboratory diagnosis and monitoring of von… 2024.}

Ristocetin binding can also create an assay discrepancy. The VWF p.D1472H variant can reduce VWF:RCo without a corresponding loss of physiologic function or increased bleeding risk. When a ristocetin-dependent result is low and the ristocetin-independent VWF:GPIbM activity is preserved, review the reagent interaction, bleeding history, and full panel before reporting a pattern consistent with a functional VWF disorder. Normal multimers describe size distribution only and do not resolve the activity discrepancy.^{44 James PD, et al. ASH ISTH NHF WFH 2021 guidelines on the diagnosis of von… 2021.}

Platelet-dependent VWF activity assays test patient plasma with reagent platelets or recombinant GPIb. Ristocetin-induced platelet agglutination (RIPA), by comparison, tests the patient's own platelets in platelet-rich plasma. A mixing study can localize an absent high-dose ristocetin response. Normal plasma corrects VWF deficiency. A platelet GPIb defect stays uncorrected, because the defect is in the patient's own platelets. An increased response to low-dose ristocetin occurs in type 2B and platelet-type VWD, so molecular testing is usually needed to distinguish them.^{1,41 Gomez K, et al. Clinical and laboratory diagnosis of heritable platelet… 2021.4 James PD, et al. ASH ISTH NHF WFH 2021 guidelines on the diagnosis of von… 2021.}

The 2021 ASH, ISTH, NHF, and WFH guideline supports type 1 VWD when VWF is below 0.30 IU/mL regardless of bleeding history, or below 0.50 IU/mL in a patient with abnormal bleeding. If your laboratory's lower reference limit is below 0.50 IU/mL, use that limit.^{44 James PD, et al. ASH ISTH NHF WFH 2021 guidelines on the diagnosis of von… 2021.} Collect the specimen when the patient is at baseline health because VWF rises with inflammation, trauma, pregnancy, and estrogen exposure. Pregnancy can temporarily normalize a low result, followed by a rapid postpartum decrease. Prolonged tourniquet application and delays in processing can also affect results. Transport whole blood at room temperature, prepare platelet-poor plasma promptly, and freeze separated plasma only according to the laboratory's validated procedure when testing is delayed. Rheumatoid factor, heterophile antibodies, and other method-specific interferences can affect antibody-based assays. The direction and size of the error depend on the assay.^{4,74 James PD, et al. ASH ISTH NHF WFH 2021 guidelines on the diagnosis of von… 2021.7 Platton S, et al. Guideline for the laboratory diagnosis and monitoring of von… 2024.}

At the bench, Bernard-Soulier syndrome appears as large platelets, a low to near-normal count, and an absent response to high-dose ristocetin. It is a rare inherited adhesion defect caused by absent or dysfunctional GPIb-IX-V. Without a functioning receptor, platelets cannot bind VWF normally at sites of vascular injury. Bleeding often begins in infancy or childhood with ecchymoses, epistaxis, gingival bleeding, or excessive bleeding after a procedure. The platelet count ranges from low to near normal, and the platelets are characteristically large.^{1,51 Gomez K, et al. Clinical and laboratory diagnosis of heritable platelet… 2021.5 Keohane EM, et al. Rodak's Hematology. 2025.}

The GPIb-IX core contains one GPIbα, two GPIbβ subunits, and one GPIX. GPV associates with this core at the platelet surface. Pathogenic variants most often affect GP1BA, GP1BB, or GP9 and can reduce surface expression or leave a receptor that binds VWF poorly.^{88 McEwan PA, et al. Quaternary organization of GPIb-IX complex and insights into… 2011.}

Giant platelets may fall outside an impedance analyzer's platelet-size threshold, causing a falsely low automated count. Review analyzer flags and the platelet histogram, examine the blood film, and use an optical or fluorescence platelet count when the reported value does not fit the film. LTA or RIPA shows an absent response to high-dose ristocetin with preserved responses to standard aggregation agonists. Flow cytometry can demonstrate reduced or absent GPIbα (CD42b) and GPIX (CD42a), and molecular testing confirms the inherited defect.^{11 Gomez K, et al. Clinical and laboratory diagnosis of heritable platelet… 2021.}

At the bench, Glanzmann thrombasthenia shows a normal count and film, with absent aggregation to every standard agonist except ristocetin. It is an autosomal recessive aggregation defect caused by pathogenic variants in ITGA2B or ITGB3. These genes encode the GPIIb/IIIa receptor, also called integrin αIIbβ3. After platelet activation, the receptor binds fibrinogen and other adhesive proteins that bridge adjacent platelets. Absent or dysfunctional GPIIb/IIIa therefore prevents normal aggregation and impairs clot retraction. Bleeding usually appears early in life as epistaxis, gingival bleeding, purpura, or excessive bleeding after injury or a procedure.^{99 Nurden AT. Glanzmann thrombasthenia. 2006.}

The platelet count and morphology are usually normal. LTA shows absent or severely reduced aggregation with ADP, epinephrine, collagen, arachidonic acid, and other standard aggregation agonists, and ristocetin-induced agglutination remains present. Flow cytometry for GPIIb (CD41) and GPIIIa (CD61) identifies most quantitative deficiencies. Some variant receptors are present in near-normal amounts and still fail to activate or bind ligand. When flow cytometry shows near-normal receptor, test receptor function with agonist-induced fibrinogen binding or an activated-receptor assay such as PAC-1. Molecular testing confirms the diagnosis. Autoantibodies, paraproteins, afibrinogenemia, and GPIIb/IIIa-inhibiting drugs can produce an acquired thrombasthenia-like pattern.^{1,9,101 Gomez K, et al. Clinical and laboratory diagnosis of heritable platelet… 2021.9 Nurden AT. Glanzmann thrombasthenia. 2006.10 Frelinger AL III, et al. Consensus recommendations on flow cytometry for the assessment… 2021.}

Dense granules store ADP, ATP, serotonin, calcium, and pyrophosphate. A dense-granule disorder can reduce granule number, reduce granule content, or impair release. The LTA pattern therefore varies. Low-dose ADP and epinephrine may produce a primary wave without the expected secondary wave, and low-dose collagen may have a prolonged lag with a reduced maximum. Arachidonic acid can be normal or reduced, and some affected patients have normal LTA.^{22 Dupuis A, et al. Platelet δ-storage pool disease. 2020.}

Systemic disorders can combine a dense-granule defect with other findings:

• Hermansky-Pudlak syndrome causes oculocutaneous albinism and lysosomal trafficking abnormalities.
• Chédiak-Higashi syndrome combines a dense-granule defect with giant leukocyte granules, immunodeficiency, and partial albinism. It is discussed in the hereditary leukocyte anomalies lesson.
• Wiskott-Aldrich syndrome causes small platelets, thrombocytopenia, eczema, and immunodeficiency.
• Thrombocytopenia-absent radius syndrome combines thrombocytopenia with bilateral absence of the radii and preserved thumbs.

Gray platelet syndrome is an α-granule disorder caused by biallelic NBEAL2 variants. Platelets are large and pale gray on a Wright-stained blood film because recognizable α-granules are markedly reduced. Patients may also have thrombocytopenia, splenomegaly, and progressive marrow fibrosis. Quebec platelet disorder results from a PLAU duplication that increases urokinase-type plasminogen activator within α-granules and promotes proteolysis of stored proteins. Scott syndrome affects platelet procoagulant activity: activated platelets fail to expose phosphatidylserine even though standard aggregation results may be normal.^{55 Keohane EM, et al. Rodak's Hematology. 2025.}

Confirm a suspected dense-granule defect with testing beyond LTA, because some affected patients have normal LTA. Useful confirmatory studies include ATP release by lumiaggregometry, measurement of platelet ADP and ATP content, and whole-mount electron microscopy to count dense granules. Selected flow-cytometric secretion assays can support the assessment when the laboratory has validated them.^{1,21 Gomez K, et al. Clinical and laboratory diagnosis of heritable platelet… 2021.2 Dupuis A, et al. Platelet δ-storage pool disease. 2020.}

Inherited qualitative platelet disorders can coexist with thrombocytopenia, so a low count does not rule them out. Before interpreting platelet-function studies, review medications and recent drug exposure. Aspirin, P2Y12 inhibitors, nonsteroidal anti-inflammatory drugs, and high-dose penicillins can alter aggregation or secretion. Uremia, myeloid neoplasms, liver disease, and paraproteinemia can also cause acquired dysfunction. Test a fresh specimen collected and handled under your laboratory's validated conditions. Repeat an unexpected or borderline result on a separate specimen once reversible acquired effects, such as a recent drug, are addressed.^{1,51 Gomez K, et al. Clinical and laboratory diagnosis of heritable platelet… 2021.5 Keohane EM, et al. Rodak's Hematology. 2025.}