An instrument and a method for determining a coagulation characteristic of blood, a blood fraction or a control comprise a radiation-reflective surface, a first source for irradiating the surface, and a first detector for detecting radiation reflected from the surface. A cuvette holds a sample of the blood, blood fraction or control the coagulation characteristic of which is to be determined. 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 and the radiation reflective surface is disposed adjacent a second of the two opposed walls.