We go back and draw off PIV locks occasionally in the EC, based on a few criteria: 1) the patient has poor vascular access and/or won't tolerate an additional stick (peds and geri, I'm looking at you); 2) nothing has been run through the line other than .9; and 3) you get brisk blood return from the line and can draw the sample with minimal manipulation of the site. Generally the call for additional draws will be within 30 minutes of starting the line, making thrombosis slightly less of an issue, and we love our 18-gauges in Emergency
, so it's a little different than going back and drawing off a three-day-old 22ga PIV that was already on its last legs. Some things absolutely can't be drawn this way, for example blood cultures (must be a fresh venipuncture site unless you're culturing an existing central line), and you can't draw from a pre-hospital line (major risk of infection - EMS lines aren't known for their pristine conditions). And, of course, the floor will never do it this way, for all the reasons already mentioned.
We scrub the hub with alcohol or ChloraPrep prior to re-draws, and waste 10 mL in adults and 3 in babies prior to drawing the actual sample. Syringe draws work a lot better than Vacutainer draws when you're working this way; you have control over how much suction you're applying to the line, and you can stop if you meet resistance or get line vibration. Hemolysis generally isn't any more of a problem drawing this way than it is from a butterfly puncture.* Once you have what you need, flush the line with 10 mL .9 and you're done.
* Personally, I think a lot of the hullabaloo over hemolysis is bad lab technique. You don't know how many times I've had to draw and draw and re-draw on patients, a new venipuncture every time, because the lab swears the sample was hemolyzed; and then I'll call the lab phlebotomists to come do it themselves, and lo and behold their samples come up "hemolyzed" too. We've also noticed that the hemolysis monster tends to appear at odd intervals, and when it happens, literally everyone's samples will magically come up "hemolyzed" at the same time. One night you can send the Worst. Draw. EVER. down and it'll be fine, and the next night send a beautiful draw out of a juicy vein and they'll claim "hemolysis." Verrry eeenterestink. But not funny.