Platelet Aggregation Measurement by Platelet Aggregometer
Platelet aggregation results in increased light transmittance (increased transmittance) as measured by a platelet aggregometer. 1
Principles of Light Transmission Aggregometry
Light transmission aggregometry (LTA) is the gold standard method for assessing platelet function. The fundamental principle behind this technique is:
- When platelets are in suspension (platelet-rich plasma), the solution is turbid and blocks light
- Upon addition of an agonist (such as ADP, collagen, arachidonic acid, or ristocetin), platelets aggregate
- As platelets clump together, the solution becomes clearer
- This clarity allows more light to pass through the sample
- The aggregometer detects this as increased light transmission 1
Technical Explanation of Optical Measurements
The relationship between the optical measurements can be understood as:
Transmittance: The amount of light passing through the sample
- Increases as platelets aggregate
- This is the primary measurement in platelet aggregometry
Absorbance: The amount of light absorbed by the sample
- Decreases as platelets aggregate
- Inversely related to transmittance
Light scatter: The dispersion of light in multiple directions
- Decreases as platelets aggregate
- Not the primary measurement in standard optical aggregometry
Clinical Applications
Platelet aggregometry has several important clinical applications:
- Diagnosis of inherited platelet function disorders
- Monitoring effectiveness of antiplatelet therapy
- Normal platelet aggregation with ADP shows <50% aggregation with 5 μM ADP
- High on-treatment platelet reactivity shows >50-70% aggregation 1
- Quality monitoring of platelet concentrates
- Identification of patients at risk for bleeding or thrombosis
Pre-analytical Considerations
For accurate platelet aggregometry results:
- Use 0.106 M trisodium citrate as anticoagulant
- Discard first 2-5 mL of blood during collection
- Allow blood to rest 15-30 minutes before centrifugation
- Centrifuge at 120-200g for 10-15 minutes to prepare platelet-rich plasma
- Allow an additional 30 minutes resting time before testing 1
Common Pitfalls and Caveats
- Pre-analytical variables can significantly affect results:
- Medication use (especially antiplatelet drugs)
- Improper sample collection or processing
- Insufficient resting time for platelets
- No single function test can address all distinct steps of platelet activation or reliably predict platelet behavior in vivo 2
- Results must be interpreted in the clinical context of the patient