A device for monitoring a skin surface for leakage of blood at a monitoring position comprising a patient unit , a connection unit and a monitor unit , wherein the patient unit comprises a patch , including an adhesive layer to be attached to the skin surface adjacent the monitoring position, whereby the adhesive layer is arranged to attach the patch to the skin so that a predetermined removal force is required for removing the patch from the skin surface; the patient unit further comprising a patient optical fiber having an exposed distal end arranged at the patch and a proximal end for connection to the connection unit; the connection unit comprises a connection optical fiber , a distal end of which comprises a connector device having an opening passing through the connector device from one end to the other, whereby the connection optical fiber is arranged in a proximal end of the opening and the patient optical fiber is insertable in a distal end of the opening for being arranged with a proximal end of the patient optical fiber in contact with and in register with the distal end of the connection optical fiber, wherein, the connector device comprises a radial spring member arranged so that it normally covers an entrance to said distal end of said opening, whereby the radial spring member further comprises a hole , which is moveable into register with said distal end of said opening against a spring action by said radial spring member, thereby permitting entrance of the patient optical fiber into said distal end of said opening, and wherein side surfaces of the hole exerts a force on the patient optical fiber when the radial spring member is released for preventing withdrawal of the patient optical fiber , until a predetermined withdrawal force is exerted on the patient optical fiber, whereby the predetermined withdrawal force is smaller than said removal force and is smaller than 30 N but larger than 10 N. the patient unit comprises a patch , including an adhesive layer to be attached to the skin surface adjacent the monitoring position, whereby the adhesive layer is arranged to attach the patch to the skin so that a predetermined removal force is required for removing the patch from the skin surface; the patient unit further comprising a patient optical fiber having an exposed distal end arranged at the patch and a proximal end for connection to the connection unit; the connection unit comprises a connection optical fiber , a distal end of which comprises a connector device having an opening passing through the connector device from one end to the other, whereby the connection optical fiber is arranged in a proximal end of the opening and the patient optical fiber is insertable in a distal end of the opening for being arranged with a proximal end of the patient optical fiber in contact with and in register with the distal end of the connection optical fiber, wherein, the connector device comprises a radial spring member arranged so that it normally covers an entrance to said distal end of said opening, whereby the radial spring member further comprises a hole , which is moveable into register with said distal end of said opening against a spring action by said radial spring member, thereby permitting entrance of the patient optical fiber into said distal end of said opening, and wherein side surfaces of the hole exerts a force on the patient optical fiber when the radial spring member is released for preventing withdrawal of the patient optical fiber , until a predetermined withdrawal force is exerted on the patient optical fiber, whereby the predetermined withdrawal force is smaller than said removal force and is smaller than 30 N but larger than 10 N. <EOS>