A blood count is a type of blood test where the types of cells in a given volume of blood are actually counted to determine whether they are within the normal range. Nowadays, the count is determined by a piece of lab aparatus, but in relatively modern times, the test required measuring a small volume of blood, placing it on a slide, and making an actual count of the cells within a given sample on the slide.

Both red blood cells and white blood cells can be counted in this manner. If all blood elements including Hemoglobin and hematocrit  are required, it is generally referred to as a complete blood count, blood panel or CBC. The blood count is an important test for anemia, infection, immunosuppression and many other diseases of the blood and bone marrow.

Complete blood count at Wikipedia

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