Turbidity curves. Representative turbidity curves of fibrinogen Philadelphia treated with thrombin in 0.05 M Tris HCl, 0.15 M NaCl, pH 7.4 either with 0.002 M CaCl₂ or 0.02 M citrate. The presence of calcium ion (Ca⁺⁺) has a profound effect on lateral aggregation, with a more pronounced difference for fibrinogen Philadelphia than for the control. In the presence of calcium, the lag period is shorter and the final turbidity is lower (Table 1).