How to ground a chandelier in a receptacle wired with BX wire?

I have a chandelier receptacle that the previous owner left with no fixture. It has BX wiring to the box. I can see the white and black wires coming out and a small white one wrapped back around the BX metal sheilding. The chandelier I want to put in must be grounded according to the directions. Can I just run a ground wire to the receptacle box and screw it into the box and have a proper ground. When I attach a circuit tester to the black and white wires and then touch the third prong of the tester to the box, it says it is connected correctly. As expected it says the ground is open when the third terminal is not in contact with the metal receptacle. This leads me to believe the receptacle is grounded through the BX wire. Am I correct and can I use this method to ground the chandelier.