Kathy,According to the AAMT BOS, they say to NOT use a hyphen with most disease-entity modifiers, and urinary tract infection is one example on their list. Others are pelvic inflammatory disease, cervical disk disease, and so on. It also says to check appropriate references for guidance.
They go on to say that some compound modifiers are used as a unit so often, that they are automatically read as a unit. Examples are deep tendon reflexes, low back pain, left lower quadrant, etc. I think left arm pain would fit in this category,therefore, no hyphen necessary.
Again, this is from AAMT BOS under Compound Modifiers.
Patty