In combination, an instrument for determining a characteristic of a biological fluid or a control, and a cuvette for holding a sample of the biological fluid or control, the characteristic of which is to be determined. The instrument comprises a radiation-reflective surface, a first source for irradiating the surface, and a first detector for detecting radiation reflected from the surface. The cuvette has two opposed walls substantially transparent to the source radiation and reflected radiation. The first source and first detector are disposed adjacent a first one of the two opposed walls. The radiation reflective surface is disposed adjacent a second of the two opposed walls. A second source is provided for irradiating the surface. The first detector detects radiation from the second source reflected from the surface. The second source is positioned to transmit radiation through the two opposed walls for reflection by the surface and transmission back through the two opposed walls to the first detector to indicate whether a sample has reached a first point in the cuvette.