Platelet Function Testing
Platelet function testing assesses how well platelets respond and aggregate, as distinct from how many platelets are present. Light transmission aggregometry, the long-standing reference method, measures the change in light passing through platelet-rich plasma as platelets clump in response to agonists, while other assays probe secretion, surface receptors, and adhesion. These tests are central to investigating bleeding tendencies that the platelet count alone cannot explain.
Definition
Platelet function testing comprises laboratory assays that evaluate platelet responsiveness, including aggregation (notably light transmission aggregometry), secretion, surface receptor expression, and adhesion, used to characterise qualitative platelet disorders independently of the platelet count.
Scope
The entry covers the distinction between platelet number and platelet function, the principle of aggregometry and its standardisation, and the range of complementary assays. It is an educational reference on platelet function testing and does not provide diagnostic thresholds or antiplatelet management advice.
Core questions
- How does platelet function testing differ from measuring the platelet count?
- What is the principle of light transmission aggregometry?
- Why does aggregometry require standardisation to be comparable between laboratories?
- What complementary assays probe platelet secretion, receptors, and adhesion?
Key concepts
- Platelet count versus platelet function
- Light transmission aggregometry (LTA)
- Platelet agonists (e.g. ADP, collagen, arachidonic acid)
- Platelet-rich plasma preparation
- Secretion and dense-granule assays
- Surface receptor and adhesion testing
- Standardisation and preanalytical control
Mechanisms
Light transmission aggregometry exposes platelet-rich plasma to agonists such as ADP, collagen, or arachidonic acid and measures the increase in light transmission as platelets aggregate and the suspension clears; the shape and extent of the response characterise platelet function. Because the result depends heavily on sample preparation, agonist choice, and instrument settings, the SSC/ISTH has issued consensus recommendations to standardise the method so that results are comparable across laboratories (Cattaneo, 2013). Complementary assays probe granule secretion, surface receptor expression, and adhesion, extending the functional assessment beyond aggregation (Michelson, 2018).
Clinical relevance
Platelet function testing helps the laboratory investigate bleeding tendencies that the platelet count cannot explain, such as qualitative platelet disorders, and is used in specialised settings to assess platelet responsiveness. The entry describes how these tests work; it is educational and is not a basis for individual diagnostic or treatment decisions, and it gives no antiplatelet therapy guidance.
Evidence & guidelines
Light transmission aggregometry, the reference functional assay, is standardised through consensus recommendations of the platelet physiology subcommittee of the SSC/ISTH (Cattaneo, 2013); broader functional methods are described in the reference literature on platelets (Michelson, 2018).
Debates
- How should light transmission aggregometry be standardised?
- Aggregometry results vary with sample preparation, agonist concentration, and instrument settings, which limits comparability between laboratories; consensus recommendations were developed to standardise the method, though uniform adoption remains an ongoing effort.
Related topics
Seminal works
- cattaneo-2013
Frequently asked questions
- How is platelet function testing different from a platelet count?
- The platelet count tells you how many platelets are present, whereas platelet function testing assesses how well those platelets respond and aggregate. A person can have a normal count but impaired platelet function, which functional testing is designed to detect.
- What is light transmission aggregometry?
- It is the long-standing reference method for assessing platelet function: platelet-rich plasma is exposed to agonists, and the instrument measures the increase in light passing through the sample as platelets aggregate. Standardised protocols are used so results can be compared between laboratories.