Pyelonephritis is the medical term for a kidney infection. It’s classified as an upper urinary tract infection, meaning bacteria have traveled beyond the bladder and urethra (the lower urinary tract) and reached one or both kidneys. When most people say “UTI,” they’re referring to a bladder infection, or cystitis. Pyelonephritis is the more serious version of that same process, with bacteria ascending deeper into the urinary system.
How Bacteria Reach the Kidneys
In most cases, pyelonephritis starts the same way a bladder infection does. Bacteria from the digestive tract migrate through the urethra into the bladder. If the infection isn’t cleared, those bacteria can continue upward through the ureters, the narrow tubes connecting the bladder to the kidneys, and colonize kidney tissue that is normally sterile.
E. coli is responsible for roughly 95% of uncomplicated cases. In more complex situations, such as infections in people with urinary catheters or structural abnormalities, other bacteria play a larger role, though E. coli still accounts for about 77% of those cases. In children, a condition called vesicoureteral reflux, where urine flows backward from the bladder toward the kidneys, is a recognized risk factor.
Uncomplicated vs. Complicated Pyelonephritis
Clinicians divide pyelonephritis into two categories. Uncomplicated pyelonephritis occurs in otherwise healthy adults (most often women) with normal urinary tract anatomy. Complicated pyelonephritis involves an underlying factor that makes the infection harder to treat: a kidney stone blocking urine flow, diabetes, pregnancy, a weakened immune system, or a structural abnormality in the urinary tract. The distinction matters because complicated cases carry higher risks of serious outcomes and often require more aggressive treatment.
Symptoms and How It Differs From a Bladder Infection
A bladder infection typically causes burning with urination, frequent urges to go, and lower abdominal discomfort. Pyelonephritis can include all of those symptoms, but it adds a distinct set of its own: flank pain (in your side or mid-back, usually on one side), fever, chills, nausea, and vomiting. The combination of fever, chills, flank pain, and tenderness when pressure is applied to the back near the affected kidney is what points clinicians toward a kidney infection rather than a simple bladder infection. Notably, pyelonephritis sometimes occurs without any of the classic bladder symptoms at all.
Symptoms usually develop over hours to about a day. Fever, when present, can exceed 103°F (39.4°C). Visible blood in the urine appears in 30 to 40% of cases in women. Loss of appetite is common, and nausea and vomiting range from absent to severe.
How Pyelonephritis Is Diagnosed
Diagnosis relies on a combination of symptoms and lab work. A urinalysis checks for white blood cells, red blood cells, and nitrites in the urine. Nearly all patients with pyelonephritis show pyuria (white blood cells in the urine), which can be detected quickly with a dipstick test. White blood cell casts, clumps of immune cells that form within the kidney’s tiny tubules, are particularly telling. While they can appear in a few other conditions, in the context of urinary symptoms they point specifically to a kidney-level infection rather than one limited to the bladder.
A urine culture confirms the diagnosis and identifies the exact bacterium involved. The formal threshold established by the Infectious Diseases Society of America is at least 10,000 colony-forming units per cubic millimeter of urine, combined with symptoms consistent with the infection. Urine cultures come back positive in about 90% of pyelonephritis cases.
Who Is Most at Risk
Women develop pyelonephritis far more often than men, largely because of a shorter urethra that gives bacteria easier access to the bladder. Beyond anatomy, several factors raise risk:
- Diabetes. People with diabetes who develop pyelonephritis are nearly three times as likely to have bacteria spread into the bloodstream (about 31% vs. 11% in people without diabetes). They also tend to have longer fevers, longer hospital stays, and higher mortality.
- Pregnancy. Pregnant individuals with pyelonephritis face a significant risk of premature labor.
- Urinary obstruction. Kidney stones or anything else blocking urine flow creates a breeding ground for bacteria and makes the infection harder to resolve.
- Age over 65. Older adults have higher mortality rates, especially when combined with limited mobility or a weakened immune system.
Potential Complications
Most uncomplicated cases resolve with antibiotics. When the infection isn’t treated promptly or when complications are missed, the consequences can be severe. Bacteria can enter the bloodstream, leading to urosepsis, a life-threatening condition where the body’s response to infection begins damaging its own organs. In one study of elderly patients with diabetes, mortality from pyelonephritis reached 12.5%, compared to 2.5% in those without diabetes.
Obstruction combined with infection can cause the kidney to fill with pus (pyonephrosis) or lead to abscess formation around the kidney. Repeated or severe infections can cause permanent scarring of kidney tissue, gradually reducing kidney function over time. A rare but dangerous variant called emphysematous pyelonephritis, where gas-forming bacteria destroy kidney tissue, is most often seen in people with diabetes and urinary obstruction.
What Treatment Looks Like
Uncomplicated pyelonephritis is typically treated with oral antibiotics for 5 to 14 days, depending on the specific medication chosen. Your doctor will usually order a urine culture before starting treatment so the antibiotic can be adjusted if the bacteria turn out to be resistant to the initial choice.
Complicated cases, or those where you can’t keep fluids down due to vomiting, often require a hospital stay for intravenous antibiotics and fluids. If a kidney stone or other blockage is trapping infected urine, that obstruction needs to be relieved, sometimes urgently, before antibiotics alone can work. Most people with uncomplicated pyelonephritis start feeling better within 48 to 72 hours of starting antibiotics, though it’s important to finish the full course even after symptoms improve.