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 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.
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]||Bernard Soulier Syndrome [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
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
Defects in G protein activation [defective intracellular signalling]
|Abnormalities of Arachadonic Acid metabolism||COX deficiency
Thromboxane synthase deficiency
|Disorders of the platelet procoagulant mechanism||Scott Syndrome|
|Platelet cytoskeletal defects||MYH9-related disorders
Click HERE to see a more comprehensive summary.
Click HERE to go to the Data Interpretation Exercises.