What ever is the cause of the pyelonephritis.
Drain and treat for whatever is causing, can I hear you say it, the pyelonephritis.
Usually it manifests with fevers greater than 5 days on appropriate therapy for, say it again, pyelonephritis. CT or US makes the diagnosis. Then drain it.
ICD9 Codes (Soon to be supplanted by ICD10)
perinephric abscess 590.2.