A biological fluid dilution device can include a syringe body including a diluent chamber at a front end of the syringe body and a piston tube at a rear end of the syringe body. The diluent chamber can be partially filled with a diluent fluid. A moveable piston can be slidably engaged in the piston tube and form a fluid-tight seal with an interior surface of the piston tube. The moveable piston can include a metering groove to contain a precise volume of a biological fluid between the metering groove and the interior surface of the piston tube. A biological fluid inlet on the piston tube can be capable of delivering the biological fluid to the metering groove. The moveable piston can be slidable toward to diluent chamber to introduce the biological fluid in the metering groove into the diluent chamber.