Kits and methods for closed-system sampling blood in patients are disclosed. The kit includes a four-port stopcock adapted to allow communication at least between two ports. The stopcock has a first port adapted to receive a syringe, a second port for communicating with an infusion solution reservoir, a third port for receiving a flush solution, and a fourth port for communication with a patient. The kit also includes at least one sampling site adapted to fluidically communicate with the fourth port and to be positioned between the four-port stopcock and the patient. At least one of the sampling sites is adapted to allow extraction of a fluid being communicated between the stopcock and the patient.