We used a yellow plastic device that accessed the catheter--I believe it was a leur-lock screw on.. Then the blood tubes are pushed down into the device and onto a needle that punctures the tube top. Vacuum draws in blood. I always used these when I worked in CCU. Many of the nurses weren't accustomed to them so did not use the.
I can't remember the brand, but it our Pyxxis it was a catheter blood collection adapter.
http://www.vanishpoint.com/Simple4.aspx?PageID=137
If link works--it looks like the big blue thing on far right, but instead of needle, there was a leur-lock to connect to CVC port. There was a short needle inside the adapter that pierced the vaccutainer tube when you pushed it down into the adapter.