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A Practical Guide to Laboratory Haemostasis

 

Platelet Function Testing: Introduction



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 diagnose 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. Activated Clotting Time [ACT], Bleeding Time [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.

Principles

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:

Disorder Possible Diagnoses
Abnormalities of the platelet receptors for adhesive proteins [Disorders of platelet adhesion] Bernard Soulier Syndrome [BSS]
  - BSS
  - Velo-Cardio-Facial [VCF] syndrome
Glanzmann's Thrombasthenia [GTT]
  - Congenital Afibrinogenaemia [although not a primary platelet disorder – fibrinogen is required for platelet-platelet interaction
Von Willebrand Disease
  - VWD or Platelet-type VWD
Collagen receptor defects
Abnormalities of the platelet receptors for soluble agonists [released during platelet activation] TxA2 receptor defects
α-adrenergic defects
P2Y12 receptor defects
  - Inherited deficiencies
  - Drugs e.g. Clopidogrel
Abnormalities of platelet granules Dense-granule deficiency
  - Inherited or acquired α-granule deficiency
  - Gray Platelet Syndrome
  - 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 Storage Pool Disorders [SPD] but normal granule contents and normal TxA2 generation
Defects in the agonist receptors on the surface of platelets
  - Epinephrine
  - Thromboxane
  - ADP
  - Collagen
Defects in G protein activation [defective intracellular signalling]
Miscellaneous
Abnormalities of Arachadonic Acid metabolism COX deficiency
Drugs
Thromboxane synthase deficiency
Disorders of the platelet procoagulant mechanism Scott Syndrome
Platelet cytoskeletal defects MYH9-related disorders
Wiskott-Aldrich Syndrome

 

Click HERE to see a more comprehensive summary.

Data Interpretation

Click HERE to go to the Data Interpretation Exercises.