The erythrocyte sedimentation rate of anticoagulated blood is determined for diagnostic purposes by positioning a sample (2) of the blood in contact with and vertically above or below an essentially horizontally disposed detection surface (12) of an optothermal sensor and permitting erythrocytes in the samples to sediment under gravity away from or towards the detection surface (12), the sample being periodically irradiated through the detection surface (12) with radiation such that heat produced by absorption of the radiation by the sample is detected by a detector (13) and provides signals indicative of the erythrocyte sedimentation rate.
