Question
For suspected arthitis caused by bacteria or infection what are the most reliable tests, and common tests.

Answer
Terry,

There are two main tests to diagnose infectious or septic arthritis.

If your doctor suspects that you have septic arthritis they will usually run a CBC or Complete Blood Count to see if you have an infection in your body. IF this shows an elevated white blood cell count or if your history shows a recent infection in your body then they will usually do a synovial fluid analysis.

A synovial fluid analysis is the best way of determining if you have some form of septic arthritis. This is where the doctor uses a needle to remove some of the fluid form the effected joint. There are two things they can do with this fluid. They can test it for signs of infection and they can then do a culture on it to determine the type of bacteria that is causing the infection.

Rusty
http://arthritis-symptom.com/s-z/septic-arthritis.htm