CVP And PowerPICC

There was a recent discussion in our ICU when we received an order for CVP monitoring.
there are reports, although I'm not certain the origin, that the valved structure of the "Bard Purple PowerPICC" will skew the results of your pt's CVP.

Has anyone had first hand experience with this, or know of any ways to minimize that variance.
Does leaving the Positive pressure caps in place help or hinder?

The waveform looked normal once monitoring was established, and demonstrated a CVP around 20 which was expected for how overloaded they appeared.
My Question come down to this; even if the circut is appropratly zeroed and balanced, can this number be trusted?