Tests and diagnosis. Doctors often diagnose kidney infections based on signs and symptoms, such as fever and back pain. If your doctor suspects you have a kidney infection, you may be asked to provide a urine sample to test for bacteria, blood or pus is in your urine. A urine test can't tell whether the infection – if you have one – is in your kidneys or another part of your urinary system, such as your bladder. For your GP to be confident you have a kidney infection, you need to have a positive urine test plus certain symptoms, such as a fever or a pain in your side.