Qualitative Platelet Defects

Platelet-function defects produce mucocutaneous bleeding with a normal or near-normal platelet count, so the differential moves from the count to what platelets do: adhere to injured vessel walls, aggregate into a plug, and secrete their granule cargo. VWF and its platelet receptor GPIb/IX/V mediate adhesion, fibrinogen and its receptor GPIIb/IIIa (integrin αIIbβ3) mediate aggregation, and the dense granules recruit additional platelets. The agonist-response pattern in platelet aggregometry follows directly from which of these functions fails.

Von Willebrand disease

VWF is the largest protein in human plasma, a multimeric glycoprotein of 500,000 to 20,000,000 Da that circulates at 0.5 to 1.0 mg/dL with a much larger reserve available on demand. Endothelial cells synthesize it and store it in Weibel-Palade bodies; megakaryocytes store it in platelet α-granules. The mature subunit is organized as D′-D3-A1-A2-A3-D4-C1-C2-C3-C4-C5-C6-CK domains, and each domain carries a specific binding partner.²

Shear forces unfold released ultralarge multimers, which ADAMTS13 cleaves into the smaller plasma forms; ADAMTS13 deficiency instead lets ultralarge multimers accumulate, the mechanism of TTP. Quantitative or qualitative VWF defects impair platelet adhesion and produce mucocutaneous hemorrhage of variable severity that can fluctuate over time and within a kindred sharing one variant. Severe VWF deficiency secondarily lowers factor VIII, and factor VIII below 30 units/dL adds anatomic joint and muscle bleeding to the mucocutaneous pattern.¹

The core panel requires at least three assays: VWF antigen, a platelet-dependent VWF activity assay, and factor VIII activity. The traditional ristocetin cofactor assay has been partially replaced by automated GPIb-based methods (VWF:GPIbM or VWF:GPIbR), which bind the A1 domain without ristocetin and offer a lower detection limit and less lot-to-lot variability. An activity-to-antigen ratio below 0.7 supports a qualitative type 2 defect. Multimer analysis or VWF collagen binding distinguishes 2A and 2B from 2M, and type 2N is suspected when factor VIII activity is markedly reduced relative to VWF antigen.³

The 2021 ASH, ISTH, NHF, and WFH guideline anchors the diagnosis: confirm type 1 VWD at VWF levels below 0.30 IU/mL regardless of bleeding history, or below 0.50 IU/mL when abnormal bleeding is present, using a lower local lower limit if one applies. Test at baseline health, because VWF rises with inflammation, trauma, pregnancy, and hormone exposure, and the second and third trimesters can temporarily normalize even moderately deficient levels, with a rapid postpartum drop that raises hemorrhage risk. Prolonged tourniquet application and refrigerated storage distort results; rheumatoid factor and heterophile antibodies cause false positives. Historically listed ABO-stratified reference intervals have been de-emphasized since low VWF, rather than blood group itself, drives bleeding risk.³

Bernard-Soulier syndrome

A rare autosomal recessive adhesion defect in which the GPIb/IX/V complex is absent or dysfunctional, so platelets cannot bind VWF and therefore adhere poorly to exposed subendothelium. Bleeding begins in infancy or childhood as ecchymoses, epistaxis, and gingival bleeding; hemarthroses and expanding hematomas are rare. Four glycoproteins, GPIbα, GPIbβ, GPIX, and GPV, form the complex in a 2:2:2:1 ratio, and surface expression requires synthesis of the first three. Most mutations involve GPIbα.^1,4

Homozygotes have enlarged platelets with thrombocytopenia typically from 40 × 10⁹/L to near normal and usually decreased platelet survival, producing moderate-to-severe bleeding. Platelet diameter runs 5 to 8 µm against a normal 2 to 3 µm, and the film picture of giant platelets in a count of this depth is the first clue. Electron microscopy shows increased cytoplasmic vacuoles and membrane complexes. Variant forms exist in which the complex reaches the surface in normal amounts but binds VWF poorly, and an acquired anti-GPIb/V antibody can produce a clinically similar pseudo-Bernard-Soulier picture. Aggregometry shows no response to ristocetin with preserved responses to other agonists, the mirror image of Glanzmann thrombasthenia.¹

Glanzmann thrombasthenia

Glanzmann thrombasthenia was first described by abnormal in vitro clot retraction, the actin-myosin-driven contraction of a clot’s incorporated platelets, despite a normal platelet count. Inheritance is autosomal recessive with consanguinity risk; heterozygotes are clinically normal, and homozygotes bleed from early infancy, with circumcision bleeding, epistaxis, and gingival bleeding as typical first presentations. The lesion is a deficiency or abnormality of GPIIb/IIIa, the receptor that binds fibrinogen, VWF, fibronectin, and other ligands, so fibrinogen bridging fails and aggregation and plug formation collapse. Historically type 1 carried 0 to 5% of normal receptor and type 2 carried 10 to 20% with milder clot retraction. Mutations in ITGA2B or ITGB3 cause the defect, and rare mutations also produce thrombocytopenia with large platelets. Acquired thrombasthenia-like states arise with anti-GPIIb/IIIa autoantibodies, with a paraprotein directed against GPIIIa in multiple myeloma, in afibrinogenemia, and with therapeutic antiplatelet drugs.^4,1

The platelet count and morphology are normal. Aggregometry shows absent aggregation to all agonists but normal ristocetin-induced agglutination, the reciprocal of Bernard-Soulier syndrome. Granule secretion still occurs despite absent aggregation, and platelet procoagulant surface activity is often diminished as well. Flow cytometry for GPIIb/IIIa (CD41, CD61) is a useful adjunct where aggregometry is unavailable.⁴

Storage pool diseases

These defects of granule content or release present with mucocutaneous hemorrhage, hematuria, epistaxis, and easy or spontaneous bruising, with petechiae less prominent than in the adhesion or aggregation disorders. The count is usually normal, and the aggregation pattern varies by which granule compartment fails.¹

Dense-granule deficiency. The dense granules store serotonin, ADP and ATP, calcium, and pyrophosphate. In the isolated non-albinoid form, granule membranes form in near-normal numbers but content packaging fails, and bleeding is usually mild. Arachidonic acid fails to generate a response, epinephrine and low-dose ADP give only a primary wave, and low-dose collagen gives a decreased-to-absent response while high-dose collagen is near-normal.

Systemic forms pair dense-granule deficiency with other findings:

• Hermansky-Pudlak syndrome, autosomal recessive, adds tyrosinase-positive oculocutaneous albinism and defective lysosomal function across cell types with ceroid-like reticuloendothelial deposition.
• Chédiak-Higashi syndrome contributes dense-granule deficiency to its bleeding tendency (see the hereditary leukocyte anomalies topic).
• Wiskott-Aldrich syndrome platelets are small microthrombocytes with decreased dense granules and diminished stored adenine nucleotides, and aggregation shows the storage-pool pattern.
• TAR syndrome has structural dense-granule defects on top of the marrow-based thrombocytopenia.

Gray platelet syndrome. α-Granule deficiency is a rare autosomal recessive disorder of NBEAL2, which is critical for α-granule development. Absent recognizable α-granules cause lifelong mild bleeding, moderate thrombocytopenia, marrow fibrosis, and large gray-appearing platelets on Wright stain. By electron microscopy the platelets carry vacuoles and small α-granule precursors with VWF and fibrinogen, and these membranes carry P-selectin and GPIIb/IIIa and can translocate to the surface on thrombin stimulation, suggesting α-granules that fail to retain their cargo. Consistent with that leak, plasma platelet factor 4 and β-thromboglobulin are elevated.¹

Combined and rarer defects. A rare autosomal dominant deficiency affects α and dense granules together. Quebec platelet disorder, from a PLAU-region tandem duplication, floods α-granules with urokinase-type plasminogen activator and proteolytically degrades α-granule proteins; thrombocytopenia occurs in some patients. Scott syndrome is a defect of procoagulant surface exposure: activated platelets fail to externalize phosphatidylserine despite a normal count and normal standard aggregation responses, producing bleeding out of proportion to any aggregometry abnormality.¹

Separating the disorders at the bench

The platelet count and film sort quickly: a normal film with normal count points to VWD, Glanzmann, or a storage-pool defect; giant platelets with thrombocytopenia point to Bernard-Soulier. The aggregometry pattern then separates the rest, and confirmatory studies follow. Test selection and aggregometry procedure detail belong to the hematology testing topics, and transfusion support for severe bleeding follows blood banking guidance.^1,3