the method of taking a blood sample utilizes a cannula having a penetrating point at its distal end and an elongate lateral opening through the cannula wall on the side opposite the point . the steps of the method include sealing the lateral opening with a slidable sleeve which has an outwardly - extending base , sealing the base against the patient &# 39 ; s skin , aligning the cannula with a blood vessel and inserting the cannula to a depth sufficient to pierce the blood vessel , and applying a slight vacuum on the cannula to withdraw blood therethrough .