Joint taps are often required to collect a sample of fluid within the joints.  It is usually recommended in pets with sudden or chronic lameness.  Samples are collected, in most cases, without anesthesia, or under a short period of general anesthesia, dependent on the location of the joint in question.  Samples are then submitted to a laboratory for bacterial culture which looks for the presence of a bacterial infection and for cytology which lets the doctor know what kinds of cells are in the joint fluid, i.e. inflammatory or infectious.  A small needle will be steriley placed into the joint(s)  and the sample is collected.  Usually multiple joints will be tapped.