A blood count was a medical test run to determine the number of each component cell type in blood.
In 2155, Elizabeth had an elevated white blood cell count. (ENT: "Terra Prime")
After encountering the dikironium cloud creature, Ensign Rizzo's blood count was 60% less than normal. (TOS: "Obsession")
While acting as a temporary host for a Trill symbiont, William T. Riker's white blood cell count was elevated as his body rejected the foreign entity. (TNG: "The Host")