Some organisms directly invade the normal kidney, leading to acute bacterial pyelonephritis.
Other organisms invade the kidney when obstruction or urinary reflux is present, contributing to tubulointerstitial damage in the form of chronic pyelonephritis. Some systemic infections lead to an acute interstitial reaction without direct invasion of the renal parenchyma. It may be associated with dysuria, frequency, urgency, and other symptoms related to lower urinary tract infection. Pyuria and leukocyte casts in the urine sediment are often present. Most cases of acute bacterial pyelonephritis that occur in previously normal kidneys resolve after therapy without residual damage.