PTT blood samples with heparin infusions

[Shocked]

When a patient has a central line with heparin infusing, is it ok to draw the blood sample for the next PTT/INR from the central line? There has been much discussion at our facility. Some say "no" cause it will effect the PTT result. Others say it is ok as long as you discard 10cc instead of the usual 4-6cc for other blood samples. (Also, both would require a 10 cc saline flush prior to withdrawing the blood to be discarded-right?) Many nurses have been told to get the PTT from a peripheral stick which sounds crazy since the patient is a difficult stick anyway. Is there a clear protocol that anyone is currently using?