How do doctors differentiate between a bladder infection and a kidney infection?

By symptoms: Bladder infections usually cause painful, frequent urination with or without low back pain. Patients with kidney may have the above but will typically be quite ill with fever, nausea/vomitting and flank (upper back toward the side) pain. Both will usually have an abnormal urine test.

Pain locale/ fever: Bladder: rarely fever, chills, or ill; causes burning, frequency, lower abdominal pain, bleeding, etc. Kidney: flank pain, fever, chills, ill.