Platelet Function Testing: Introduction

An evaluation of patients with abnormal bleeding symptoms requires an objective assessment of the bleeding history, a physical examination, a careful drug history and followed by laboratory investigation.  

Numerical and functional platelet disorders are common amongst patients with abnormal bleeding and may be clinically indistinguishable from other haemostatic disorders.  It is probable that because of the complexity of platelets and the difficulty in investigating them that we under diagnosed disorders of platelet function.

Laboratory tests for platelet disorders may include:

  • Assessing platelet number and size [MPV]
  • Assessing platelet morphology – blood film
  • Screening tests of platelet function e.g. ACT, BT and PFA-100
  • Light Transmission Aggregometry e.g. classical Born aggregometry
  • Assessment of platelet nucleotides
  • Flow cytometry e.g. to quantitate the presence or absence of platelet membrane glycoproteins
  • More specialised investigations which are primarily the province of research laboratories

The aim of this section is introduce you to the complexities of investigating platelet function and to provide some help in how to investigate a patient with a suspected platelet function abnormality. 

There are some excellent reviews on the subject of platelets and we have referenced these in the relevant sections.

 

Classification of Platelet Function Disorders

This is not a comprehensive list of platelet function disorders but it is useful to have a scaffold that you can fit the various platelet function disorders into:

  • Abnormalities of the platelet receptors for adhesive proteins [Disorders of platelet adhesion]
    • BSS
      • BSS
      • VCF syndrome
    • GTT
      • Congenital Afibrinogenaemia [although not a primary platelet disorder – fibrinogen is required for platelet-platelet interaction
    • VWS
      • VWD or Platelet-type VWD
    • Collagen receptor defects
  • Abnormalities of the platelet receptors for soluble agonists [released during platelet activation]
    • TxA2 receptor defects
    • a-adrenergic defects
    • P2Y12 receptor defects
      • Inherited deficiencies
      • Drugs e.g. Clopidogrel
  • Abnormalities of platelet granules
    • Dense-granule deficiency
      • Inherited or acquired
        alpha-granule deficiency
      • GPS
      • Quebec platelet disorder
  • Abnormalities of Platelet Secretion or the Signal Transduction Pathways [Impaired secretion of granule contents]
    • Primary Secretion Defects
      • Defects in aggregation which are similar to those seen in SPD but normal granule contents and normal TxA2 generation
    • Defects in the agonist receptors on the surface of platelets
      • Epinephrine
      • Tx
      • ADP
      • Collagen
    • Defects in G protein activation [defective intracellular signaling]
    • Miscellaneous
  • Abnormalities of Arachadonic acid metabolism
    • COX deficiency
    • Drugs
    • Tx synthase deficiency
  • Disorders of the platelet procoagulant mechanism
    • Scott Syndrome
  • Platelet cytoskeletal defects
    • MYH9-related disorders
    • Wiskott-Aldrich


Click HERE to see a more comprehensive summary.