Abstract: An apparatus and a method for performing a fibrinogen assay are disclosed. The reaction slide bears a sample well for receiving a liquid sample and a reaction chamber in fluid communication with the sample well. The reaction chamber contains a dry reagent matrix in which is embedded a plurality of magnetic particles. A whole blood or blood-derived sample added to the sample well is introduced simultaneously into the reaction chamber where it solubilizes the reagent, freeing the magnetic particles and allowing them to move in an oscillating pattern. This oscillating pattern is optically monitored to measure the concentration of clottable fibrinogen in the sample.