1. A sample acquiring device for volumetric enumeration of thrombocytes in a blood sample, said sample acquiring device comprising: a measurement cavity for receiving a blood sample, said measurement cavity having a predetermined fixed thickness, and a reagent, which is arranged in a dried form on a surface defining the measurement cavity, said reagent comprising a haemolysing agent for lysing red blood cells in the blood sample and said reagent further comprising a staining agent for selectively staining thrombocytes in said blood sample.