It is usually pretty hard to confuse Lyme arthritis with osteoarthritis. Generally, Lyme arthritis causes very marked warmth and swelling of a single joint, usually knee or ankle. Rarely it can affect more than one joint. Osteoarthritis, on the other hand, is more insidious pain and usually without a great deal of swelling or warmth. As aspiration of the knee, if swollen, can discern whether the fluid is inflammatory or not and this will help in the diagnosis. A blood test for Lyme disease is most helpful is well. IF in doubt, see a rheumatologist to get the diagnosis straightened out.