Measurement of platelet-mediated force development during plasma clot formation.

We have developed an instrument that measures force development during clot retraction. Clots are formed between an aluminum plate and cup. The plate is coupled to a strain gauge transducer. Force generated by platelets is transmitted to the upper plate and the resulting voltage change is recorded. Calibration using standard weights allows conversion of the voltage signal to dynes/cm2. Study parameters include lag phase prior to force development, maximum rate of force development (MRFD), and maximum force generated (MFG). When controlled for platelet count, ionic strength (0.15), pH (7.4), calcium concentration (10mM), thrombin concentration (1.5 u/ml), temperature, and gap distance (1 mm), measurements are reproducible. Control of each variable is critical since clot retraction depends on platelet function and fibrin structure. At 37 degrees C and 26 x 10(3) platelets/microL, the lag phase was 300 seconds, the MRFD was 0.69 dynes/(cm2 sec), and the force at 1000 seconds (MFG) was 2450 dynes. At 7.5 x 10(4) platelets/microL, the lag phase was 160 seconds, the MRFD was 1.06 dyne/(cm2 sec), and the MFG was 4400 dynes. Decreasing the sample temperature from 37 degrees C to 27 degrees C (75 x 10(3) platelets/microL) increases the lag phase from 170 to 300 seconds, decreases MRFD from 1.06 to 0.66 dynes/(cm2 sec), and decreases the MFG at 1000 seconds from 4,400 to 2,800 dynes. At 15 degrees C, retraction is totally inhibited. This instrument provides a simple way to quantify the impact of altered fibrin structure and platelet function on clot retraction. Measurement of force development during clot retraction may offer a new approach to the study of qualitative platelet dysfunction.