1. A sample acquiring device for volumetric enumeration of white blood cells 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, which is a uniform thickness of 50-170 micrometers, a reagent, which is arranged in a dried form on a surface defining the measurement cavity, said reagent comprising a hemolysing agent for lysing red blood cells in the blood sample, and a staining agent for selectively staining white blood cells in the blood sample.