Dual PFD's with an Altitude Miscompare message (see photo), requires the aircraft Pitot system to be checked out by an Avionics Shop with a calibrated Pitot system. 

In a single PFD plane; ATC informs you the plane is at XXXX feet and the PFD indicates 75+ feet difference, or the PFD and your internal gauges are off by 75+ feet,  you must take your aircraft to an Avionics Shop with a calibrated Pitot system. 

Always refer to the applicable Product Operating Handbook on www.avidyne.com; Product Documentation is found in the Support tab at the top of the page.