The present application provides apparatus and methods for determining the density of a fluid sample. In particular, it provides a sensor device which can be loaded with a fluid sample such as blood, and which further comprises at least one oscillating beam member or resonator. Exposure of the blood sample to clotting agents allows a clotting reaction to commence. The device allows the density of the sample fluid to be monitored with reference to the oscillation of the vibrating beam member, thus allowing the monitoring of the clotting of the fluid sample.