An acute kidney infection, also called acute pyelonephritis, is a bacterial infection of one or both kidneys. It often starts as a bladder infection and travels upward. E. coli is a common cause.

What to do: seek same-day medical assessment. A clinician may test your urine, sometimes blood, and prescribe antibiotics. Untreated infection can spread to the bloodstream or, if there is a blockage such as a stone, become dangerous. Acute pyelonephritis involves infection and inflammation of the renal pelvis and kidney tissue, not just the bladder. Pye's Surgical Handicraft, urinary tract infections section.

Go to emergency care now if there is confusion, fainting, severe weakness, difficulty breathing, persistent vomiting or inability to keep fluids down, very high fever/rigors, worsening severe pain, very little urine, or if the person is pregnant, immunocompromised, has diabetes, known kidney disease, or a urinary catheter. NHS guidance advises hospital referral when pyelonephritis may involve sepsis or another serious condition, as summarized in this NHS pyelonephritis guidance.

Do not rely on leftover antibiotics. Drink fluids if you can tolerate them, but do not delay care. Recent systematic reviews continue to assess the appropriate antibiotic route and duration, so treatment should be selected based on illness severity, pregnancy status, allergies, kidney function, and urine-culture results (PMID 41222820; PMID 40228579).