Abstract: An apparatus for determining the mean corpuscular haemoglobin concentration (MCHC) in a whole blood sample includes a sample holder including an elongate sample chamber having an open end and a closed end. A holding member is adapted to receive and retain the sample holder. The holding member rotates may rotate about an axis of rotation. When the sample holder is received and retained by the holding member the sample chamber is substantially perpendicular to the axis of rotation. First and second light sources are positioned on one side of the sample holder and are configured to emit light in respective different frequencies. At least one light sensor is positioned on a second side of the sample holder, opposite from the first side, so that light from the light source may pass through the sample chamber, in at least one rotational position of the sample holder, and impinge on the light sensor.