Abstract: A subsea test assembly securable in a subsea blowout preventer stack is used in methods for performing wellbore tests and for disconnecting a tubular string from a subsea wellhead. The test valve assembly includes an upper subassembly releasably latched to a lower subassembly. Dog members are latched and unlatched in a corkscrew manner requiring only a slight rotation, and are held in the locked position by an axially movable sleeve. Nominal rotation of the latch within the upper subassembly is required to latch and unlatch the dogs, and a relatively short stroke of the movable sleeve locks and unlocks the dog members. The latch may be hydraulically or mechanically deactivated. The valving of the test assembly preferably comprises a ball member having both a metallic and elastomeric seal for sealing the passageway through the assembly. The elastomeric seal is carried by the ball and engages a metallic sleeve axially movable below the ball for sealing engagement against the ball.