A device (1) for formation testing while drilling. The device includes a drill string (2) with a packer (3) arranged on the drill-string (2) above a drilling-bit (7). The packer can be expanded and thereby isolate a lower open part (6) of a well-bore, distinguished in that the packer is fastened sealingly but slideably over a distance (4) of the well-bore, such that the drill-string can be moved up and down, with or without rotation, within the distance while the packer remains stationary due to being expanded in the well-bore. Also, the device includes at least one pressure transmitter for measuring and transferring in real-time to the surface measured pressure in the isolated lower open part of the well-bore. Methods for formation testing utilizing the device and a device for formation testing of a production well.