9. Flank pain and high fever
When a lower urinary tract infection progresses, it may extend to the upper urinary tract and reach the kidney. In this case, it is no longer called cystitis but pyelonephritis. Another way E. coli can take to reach the kidneys is through the bloodstream, but it is less likely. When a urinary infection is severe enough to cause pyelonephritis, the most important symptoms to take in consideration are flank pain and high fever.
Flank pain results from inflammation in the kidneys and the surrounding tissue, which makes the organ more susceptible to activate pain nerve terminals. It is usually a dull pain and may be felt as back pain instead of flank pain, but usually made worse when your doctor performs a maneuver in a physical exam. Fever is usually higher than 102°F and may be associated with other symptoms such as headache and nausea.
