An upper UTI is a urinary tract infection that has reached the kidneys or the tubes (ureters) connecting the kidneys to the bladder. The medical term is pyelonephritis. Unlike the more common lower UTI, which stays in the bladder and urethra, an upper UTI causes whole-body symptoms like fever and back pain and carries a higher risk of serious complications. There are roughly 250,000 cases in the United States each year, with women affected far more often than men.
How Infection Reaches the Kidneys
Most upper UTIs start the same way a bladder infection does. Bacteria from the rectal area colonize the skin around the urethra, travel up into the bladder, and multiply. In a lower UTI the infection stops there. In an upper UTI, the bacteria overcome the body’s defenses and continue climbing up the ureters into one or both kidneys, where they trigger inflammation in the kidney tissue itself.
The vast majority of these infections are caused by E. coli, the same bacterium behind most bladder infections. Less commonly, other gut bacteria are responsible. In rare cases, bacteria reach the kidneys through the bloodstream rather than traveling up from the bladder, typically in people who already have an infection elsewhere in the body.
Symptoms That Set It Apart
An upper UTI usually produces the familiar burning and frequent urination of a bladder infection, plus a distinct set of more intense, systemic symptoms:
- Fever of 38°C (100.4°F) or higher
- Pain in the sides or lower back, often on one side, over the affected kidney
- Shivering and chills
- Nausea or vomiting
- Confusion or restlessness, especially in older adults
The key distinguishing feature is fever combined with flank pain. A straightforward bladder infection almost never causes a high temperature or pain in the back. If you develop these symptoms on top of urinary discomfort, the infection has likely moved beyond the bladder.
Who Is Most at Risk
Women between 18 and 49 have the highest incidence of upper UTIs, at roughly 28 per 10,000 people per year. The short female urethra makes it easier for bacteria to reach the bladder in the first place, and from there, the path to the kidneys is the same for everyone.
Several factors raise the odds that a bladder infection will progress upward. Anything that blocks normal urine flow, like kidney stones or an enlarged prostate, gives bacteria more time to multiply and travel. Pregnancy changes both the anatomy and immune function of the urinary tract, increasing susceptibility. Diabetes and other conditions that suppress the immune system also make it harder for the body to contain an infection at the bladder level.
In children, a condition called vesicoureteral reflux, where urine flows backward from the bladder toward the kidneys, is a major risk factor. Between 5% and 10% of children with this reflux can develop ongoing kidney damage from repeated infections.
Diagnosis and When Imaging Is Needed
Diagnosis typically involves a urine test to confirm infection and a clinical assessment of your symptoms. Fever, flank pain, and lab markers of infection together point strongly toward an upper UTI.
Most straightforward cases don’t require imaging. However, a CT scan or ultrasound is recommended early on if you have diabetes, a weakened immune system, a history of kidney stones, or a known structural abnormality of the urinary tract. These are the patients most likely to develop complications that imaging can catch. If you’re still running a fever after 72 hours of antibiotic treatment, imaging is also recommended to check for a blockage or abscess that’s preventing recovery.
One important distinction doctors make is between a kidney infection and a kidney stone, since both cause flank pain. Kidney stones rarely cause fever on their own. When fever, back pain, and signs of infection appear together, the concern is either a kidney infection alone or, more urgently, a kidney stone that is also infected, which requires immediate treatment.
How Upper UTIs Are Treated
Antibiotics are the cornerstone of treatment, and most people can recover at home. The 2025 guidelines from the Infectious Diseases Society of America recommend 7 days of antibiotic therapy for most cases, a shift from the older standard of 10 to 14 days. Shorter courses of 5 to 7 days may be sufficient with certain antibiotic classes, depending on the specific bacteria involved and how quickly symptoms improve.
About 7% of women with an upper UTI need to be admitted to a hospital. Hospitalization is typically necessary when you can’t keep oral medications down due to vomiting, when the infection has spread to the bloodstream, or when there’s an underlying condition like kidney dysfunction that raises the stakes. In the hospital, antibiotics are given intravenously until symptoms stabilize enough to switch to oral medication.
Most people start feeling noticeably better within 48 to 72 hours of starting antibiotics. Fever is usually the first symptom to resolve. If it hasn’t come down in that window, your treatment team will want to investigate further with imaging or adjusted medication.
Potential Complications
When treated promptly, upper UTIs resolve without lasting damage. The concern is with delayed treatment or infections in people with underlying health conditions. Untreated, the infection can form a pocket of pus in the kidney called a renal abscess. This occurs in a meaningful minority of complicated cases and requires more aggressive treatment, sometimes including drainage.
The most dangerous complication is sepsis, where the infection triggers a severe whole-body inflammatory response. This is uncommon in otherwise healthy people who seek timely care, but it’s the reason upper UTIs are taken more seriously than bladder infections. Repeated kidney infections, particularly in children or people with structural abnormalities, can cause permanent scarring of kidney tissue, which over time may contribute to high blood pressure or reduced kidney function.
Upper vs. Lower UTI at a Glance
A lower UTI is an inconvenience. It burns, it’s uncomfortable, and it usually clears with a short course of antibiotics. An upper UTI is a more significant infection that affects your whole body. The presence of fever, chills, and flank pain marks the dividing line. Both start the same way, but an upper UTI means bacteria have reached the kidneys, and the treatment window and stakes are different. If your bladder infection symptoms suddenly come with a fever and back pain, that’s the signal to seek care promptly rather than waiting it out.