Focused instruction

VWF is a domain-specific bridge rather than a generic adhesive coat. Its A1 domain binds platelet GPIbα, while its A3 domain and other regions bind fibrillar collagen. ADAMTS13 cleaves the VWF A2 domain under shear-induced tension, limiting multimer size and adhesive activity. Firm platelet-collagen signaling then uses GPVI and α2β1 in addition to the initial VWF tether.

Dense granules release ADP, ATP, serotonin, and calcium, while alpha granules release adhesive and coagulation-modulating proteins. ADP and thromboxane A2 act synergistically in recruitment. Activated αIIbβ3 is the receptor required for fibrinogen-mediated platelet-platelet aggregation. A defect can therefore leave the count intact while changing an agonist response pattern.

Platelet count begins with a routine impedance or optical analyzer method, while a peripheral smear is a plausibility check for clumps, satellitism, and giant forms. The ICSH/ISLH reference method is CD41/CD61 immunofluorescence flow cytometry using a platelet to red-cell ratio, not a light-microscope smear estimate. Light transmission aggregometry, abbreviated LTA, is one function method that assesses agonist-driven platelet aggregation against a parallel normal control.

LTA does not assess every platelet pathway. Ristocetin-induced agglutination is especially useful for localizing VWF-GPIb-related adhesion, whereas other agonists probe aggregation responses in the stated assay context. Agonist source, lot, concentration panel, instrument and software version, reference intervals, and acceptance criteria are local policy. Report the method and its limits with any platelet-function interpretation.