In science: examining samples of animal and human blood. Counting and comparing different types of blood cells in a sample. In medicine: basic blood count, or estimating percentage of certain types of blood cells. Healthy person's blood count is ALWAYS within a certain range. That is very important in diagnosing many diseases, such as lymphomas, anemias, cancers, and many, many others. DNA, RNA, sugar or any other chemical cannot be tested on blood smear, because this is entirely visual technique.