1. A method of using a disposable reagent pack with a blood analytical instrument having a red blood cell probe to analyze a red blood cell sample and a white blood cell probe to analyze a white blood cell sample, the method comprising the steps of: (a) providing a disposable reagent pack comprising a red blood cell sample well and a white blood cell sample well, the white blood cell sample well being disposed substantially within a lysing solution well such that a blood sample in the white blood cell sample well and a lysing solution in the lysing solution well may be selectively transferred between the white blood cell sample well and the lysing solution well; (b) placing a cover on the disposable reagent pack; (c) inverting the disposable reagent pack to selectively transfer and to mix the blood sample in the white blood cell sample well and the lysing solution; (d) removing the cover from the disposable reagent pack; (e) placing the disposable reagent pack on the blood analytical instrument; (f) simultaneously inserting the white blood cell probe into the lysing solution well and the red blood cell probe into the red blood cell sample well; (g) simultaneously analyzing the red blood cell sample with the red blood cell probe and the white blood cell sample with the white blood cell probe; and (h) removing the disposable reagent pack from the analytical instrument.