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The standards of practice are not written as procedures. The standard is to know the risks of drawing a sample with skewed data and work to reduce those risk. If you have heparin infusing through one lumen and the other lumen has not been exposed to heparin, think about where the heparin could get into the sample you are collecting. That would be from the blood flow at the catheter tip. All you would need to do is stop the heparin infusion, then follow you usual practice for drawing from the opposite lumen and restart the heparin. There is no need to flush the heparin out of the lumen where it is infusing. There is no evidence about a specific length of time to wait. For the tip in the SVC, blood flow is very fast so a few seconds should be all that is needed so you will not be collected the infusing heparin. If the other lumen has been locked with heparin, you can try flushing that lumen with 10-20 of saline after you stop the heparin infusion. Heparin can adsorb to the catheter wall and the intraluminal biofilm. Anytime you are drawing from a lumen where the medication can effect the lab results, someone must be following up on those results to see if they are as expected or skewed in any way. If there is a question about those results, the only thing you can do to get accurate results is from a peripheral venipuncture for drawing blood.