Abstract: An isolation assembly that employs a pack-off device at an annular space between a landing string and a riser. The assembly is configured to ensure sealing off of an annular space below the pack-off device such that potentially hazardous well testing applications may safely proceed early on in completions operations. That is, in advance of any significant rig-level pressure control equipment hook-up, the pack-off device may seal off the annulus from any potential hydrocarbon leaks up the riser toward the rig. Further, where such seal holds back pressures exceeding a predetermined level, a burst element may be incorporated into the wall of the riser to allow for controlled venting therefrom.