I would say (which mostly agrees with previous posts) according to protocol I have run into:
1.) Turn off fluids
2.) Clamp the line to avoid air getting through the selected port.
3.) Remove the port cap if it has one and flush and draw directly off the line (less chance for hemolysis of the cells as they pass through any type of cap). (Bring a new cap with you to replace it with)
4.) Swab port with alcohol swipe.
5.) Attach a prefilled 10mL syringe, unclamp and flush.
6.) Using same syringe pull back 10mL, clamp line, remove and waste.
7.) Attach empty syringe, unclamp and draw sample.
8.) Clamp line, remove sample syringe, apply clean, new flushed cap.
9.) Unclamp, flush line and restart fluids/TPN etc.
That is a mix of my logic and protocol I have worked with.