How to know when a uti has spread and become a more serious problem?

Pyelonephritis: If a UTI is able to cause a kidney infection called pyelonephritis, you usually will have fever, pain in your back at the level or your low rib cage or around to the flank area, and sometimes nausea and vomitting.

Fevers, back pain...: If the infection spreads to the kidneys, you may experience back or flank pain, fevers, chills, drenching sweats, malaise, nausea, blood in urine. If bacteria spreads to the blood, symptoms may be much worse and will be dangerous.... Confusion, lightheadedness due to low blood pressure, collapse... Treatment is antibiotics, even intravenously depending on severity, may need hospitalization.

UTI: If you have a uti, you may have pain and pressure as the bladder fills and pain with urination. If it travels to the kidneys, that becomes more serious. It may cause fever, chills, nausea, vomiting and back pain. If you think you have a kidney infection, you should see a provider asap.

Fever, Back pain: A UTI becomes more severe when it "ascends" and moves up toward the kidney and causes infection beyond the bladder. When the kidney becomes infected (pyelonephritis), high fever, flank pain, nausea and vomiting occur, and even more severe and possibly life-threatening with decreasing blood pressure, increased pulse, and signs of infection in the blood (sepsis). See md emergently for uti+fever.