The Contribution of Pin End-Cup Interactions to Clot Strength Assessed with Thrombelastography

Viscoelastic methods have been developed to assess the contribution of plasma proteins and platelets to coagulation in vitro to guide clinical transfusion therapy. One of the cardinal precepts of determining clot strength is making sure that the viscoelastic technique includes complete exposure of the plastic pin in the testing chamber with the fluid analyzed so as to assure maximal interaction of the cup wall with the pin surface. However, the various contributions of the pin surface area to final clot strength have not been investigated. That is, it is not clear what is more important in the in vitro determination of clot strength, the surface area shared between the cup and pin filled with fluid or the final viscoelastic resistance of the gel matrix formed. Thus, the purpose of this investigation was to determine the clot strength when only the tip of the pin was engaged with plasma thrombus and to compare these values with clot strength values obtained when the pin was completely in plasma. After determining the minimal amount of plasma required to cover a pin tip in a thrombelastographic system (30 μL), clot strength (elastic modulus, G) was determined in plasma samples of 30 or 360 μL final volume (n = 12 per condition) after tissue factor activation. The G value with 30 μL volume was 1057 ± 601 dynes/cm² (mean ± SD; 95% confidence interval, 675-1439 dynes/cm²), which was (P = 0.0015) smaller than the G value associated with 360-μL sample volumes, that was 1712 ± 48 dynes/cm² (confidence interval, 1681-1742 dynes/cm²). In conclusion, these data demonstrate that clot strength is not determined by a simple ratio of surface area of pin and cup to volume of sample, but rather strength is importantly influenced by the viscoelastic resistance of the fluid assessed.