Heparin Drips

I've always wondered why so many nurses think that it is necessary to pause a heparin drip for 10-15min before drawing a pTT. I've talked to multiple doctors, pharmacists, and other IV therapists (not to mention a bit of Googling) and have yet to hear any real rationale for it aside from "it's what I was taught", yet I've heard this myth many times over my years of nursing and seen many nurses do it. My girlfriend is a new nurse and was also told to do it, even though no rationale was given. If anything it will give you an artificially low pTT, given heparin's short half-life and goal to keep the pTT at an elevated therapeutic level. Obviously if you're drawing from the same arm as the drip, or a different lumen of the same PICC, etc, then you would pause the drip before the draw to avoid contamination, but why pause for 10 minutes as a general rule?

Do any of you practice this? If so, what is your rationale? I wonder where this myth originated and how it became so widespread.