Tests in use

Platelet count, morphology, aggregation, and function at high shear rate. Platelet count

Normal range 150-450 x 109/L. Adequate function is maintained even when the count is <M normal level, but progressively deteriorates as it 370 drops. With platelet counts <20 x 109/L there is usually easy bruising, petechial haemorrhages (although more serious bleeding can occur).

Morphology

Large platelets are often biochemically more active; high mean platelet volume is associated with less bleeding in patients with severe thrombo-cytopenia. Reticulated platelets can be counted by new analysers and may prove to be useful in assessing platelet regeneration. Altered platelet size is seen in inherited platelet disorders.

Platelet adhesion

Rarely performed in routine lab practice. Platelet aggregation

Performed on fresh sample using aggregometer but poor correlation with bleeding tendency except in specific circumstances, e.g. Glanzmann's thrombasthenia, Bernard-Soulier syndrome.

Aggregants

• Adenosine 5-diphosphate (ADP) at low and high concentrations. Induces 2 aggregation waves: primary wave may disaggregate at low conc. ADP; the second is irreversible.

• Collagen has a short lag phase followed by a single wave and is particularly affected by aspirin.

• Ristocetin induced platelet aggregation (RIPA) is carried out at a high (1.2mg/mL) and lower concentrations (0.5mg/dL)and is mainly used to diagnose type 2B vWD.

• Arachidonic acid.

• Adrenaline, not uncommonly reduced in normal people. For aggregation patterns in the various platelet disorders ffl p372.

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