Continuous optical tracking of contracting blood clots in vitro. After the injection of 1 U/mL of thrombin into recalcified PRP, 80-μL aliquots of PRP were added to transparent cuvettes, which were then placed in the optical Thrombodynamic Analyzer System. Clot images were then recorded by a charge-coupled device (CCD) camera every 15 seconds based on the light scattering properties of the clot vs the expelled serum. Data on relative clot size were converted to original kinetic curves that could be analyzed for parameters such as the extent of clot contraction at a 30-minute end point, the lag time from the addition of thrombin until the clot reached 95% of its initial size, and the area under the curve, as described in “Methods.” The instantaneous first derivative of the kinetic curve was used to find a local maximum that determined the borderline between phases 1 and 2 of contraction.