Dr. T explains why:

... This week I was asked to replace an I.V. in a patient prior to surgery. The bag of I.V. solution had been connected to the I.V. tubing by a nurse, as was customary at this hospital and in fact all the hospitals served by my anesthesia group. Most of the I.V.'s, in fact, are placed by the nurses unless they have trouble with them, in which case they call us to help.

I inserted the I.V. fairly easily and hooked up the tubing. Before directing the nurse to open the line and let the fluid start dripping into the patient's vein, I noticed something about the tubing. A subtle difference in the color of it (though it's colorless), or in the way light was being reflected through it (or not), or SOMETHING - I don't know exactly what - made me stop in my tracks. I disconnected the tubing. ...

And a good thing she did.