Acute pyelonephritis is a bacterial infection of the kidney, typically caused when bacteria travel upward from the bladder through the tubes (ureters) that connect the bladder to the kidneys. It’s more serious than a standard urinary tract infection because the infection has moved beyond the bladder and into the kidney tissue itself, triggering inflammation that can cause high fever, severe flank pain, and systemic illness. Most cases respond well to antibiotics, but untreated or complicated cases can lead to dangerous outcomes including kidney abscess and bloodstream infection.
How the Infection Reaches the Kidney
Almost all cases of acute pyelonephritis start as a lower urinary tract infection. Bacteria, most commonly E. coli, colonize the area around the urethra, travel up into the bladder, and then continue ascending through the ureters into one or both kidneys. E. coli causes roughly 80% of cases in women and 70% in men, though its dominance decreases in older age groups where other bacteria become more common.
Once bacteria reach the kidney, they invade the tissue (called the renal parenchyma) and set off an intense inflammatory response. The kidney swells, white blood cells flood the area, and small pockets of infection can form throughout the organ. This is what distinguishes pyelonephritis from a simple bladder infection: the immune system’s fight against the bacteria in kidney tissue produces the high fevers, body aches, and general feeling of being seriously unwell that most people experience.
Symptoms to Recognize
The hallmark signs of acute pyelonephritis are fever, flank pain (pain in the side of your back, just below the ribs), and nausea or vomiting. Unlike a bladder infection, which mostly causes burning with urination and frequent trips to the bathroom, pyelonephritis makes you feel sick throughout your whole body. Chills, muscle aches, and fatigue are common. Some people also have the typical lower UTI symptoms like painful urination and urgency, but others don’t, which can make the diagnosis less obvious at first.
The flank pain is usually on one side and can range from a dull ache to sharp, intense discomfort. Tapping on the affected area during a physical exam typically produces noticeable tenderness. Fever often runs high, sometimes above 102°F (38.9°C), and can come in waves with shaking chills. If you’ve had bladder infections before, the difference in how sick you feel is usually unmistakable.
Who Is Most at Risk
Women develop pyelonephritis far more often than men, largely because of anatomy. A shorter urethra means bacteria have a shorter path to the bladder, and from there to the kidneys. Sexual activity, pregnancy, and a history of recurrent UTIs all increase the risk. In men, pyelonephritis is less common but more likely to signal an underlying structural issue like an enlarged prostate or kidney stones that block normal urine flow.
Several other factors raise your risk significantly:
- Kidney stones or urinary obstruction: Anything that blocks or slows the flow of urine gives bacteria time to multiply and ascend.
- Diabetes: High blood sugar impairs the immune response and makes infections harder to clear.
- Weakened immune system: Conditions or medications that suppress immunity make it easier for bacteria to take hold in kidney tissue.
- Urinary catheters: These provide a direct path for bacteria into the urinary tract.
- Structural abnormalities: Conditions where urine flows backward from the bladder toward the kidneys (vesicoureteral reflux) are a well-known risk factor, especially in children.
Pyelonephritis During Pregnancy
Pregnancy creates a perfect setup for kidney infections. Hormonal changes cause the ureters to relax and widen, while the growing uterus physically compresses them. Together, these changes cause urine to pool and flow backward toward the kidneys, making it much easier for bacteria to ascend. A bladder infection during pregnancy carries a real risk of progressing to pyelonephritis, which is why pregnant individuals are routinely screened for bacteria in the urine even without symptoms.
Not all antibiotics are appropriate during pregnancy. Some commonly used UTI medications can’t reach effective levels in kidney tissue and are therefore unsuitable for treating pyelonephritis specifically. Treatment typically lasts 5 to 7 days, as studies show this duration is more effective than single-dose approaches and is associated with better birth outcomes, including lower rates of low birth weight.
How It’s Diagnosed
Diagnosis usually starts with a urine sample. The most common finding is white blood cells in the urine, a sign your body is actively fighting infection in the urinary tract. The sample may also show bacteria, protein, and small amounts of blood. A urine culture confirms which specific bacteria is causing the infection and, critically, which antibiotics will work against it.
One common point of confusion: the nitrite test on a urine dipstick. A positive result suggests certain types of bacteria are present, but only about 25% of people with a urinary tract infection will test positive for nitrites. A negative result does not rule out pyelonephritis.
When Imaging Is Needed
Most people with straightforward acute pyelonephritis don’t need any imaging. The diagnosis is made based on symptoms, physical exam, and urine tests. Imaging becomes important when there’s concern about complications like an abscess, gas-forming infection within the kidney, or a stone blocking urine flow.
The primary candidates for early imaging are people with diabetes, a compromised immune system, a history of kidney stones, or those who’ve had a recent urologic procedure. For everyone else, imaging is typically reserved for cases where fever or elevated white blood cell counts persist beyond 72 hours of appropriate antibiotic treatment. When imaging is done, a CT scan of the abdomen and pelvis with contrast is the preferred study. Research suggests that if you have no history of kidney stones, normal kidney function, and a urine pH below 7.0, the chance of finding an urgent problem on imaging is very low.
Treatment and Recovery
Antibiotics are the cornerstone of treatment. For uncomplicated cases where you can keep fluids and pills down, oral antibiotics at home are often sufficient. Current guidelines recommend 5 to 7 days of treatment for most people who are improving, which is shorter than the 10 to 14 day courses that were standard in the past. The specific antibiotic chosen depends on local resistance patterns and your urine culture results, but common classes include fluoroquinolones, cephalosporins, and a combination antibiotic called trimethoprim-sulfamethoxazole.
Hospitalization is necessary when you can’t keep oral medications or fluids down due to vomiting, when there are signs of severe infection or sepsis (very high fever, rapid heart rate, low blood pressure, confusion), or when the infection involves a complicating factor like an obstructing kidney stone. In these situations, antibiotics are given intravenously until you stabilize enough to switch to oral medication.
Most people start feeling noticeably better within 48 to 72 hours of starting effective antibiotics. Fever usually breaks within the first couple of days. If it doesn’t, that’s a signal that something else may be going on, such as a resistant organism, an abscess forming, or an obstruction that needs to be addressed.
Potential Complications
When treated promptly, acute pyelonephritis rarely causes lasting kidney damage. The serious complications arise when treatment is delayed, when the bacteria are resistant to the chosen antibiotic, or when underlying conditions make the infection harder to control.
The most concerning complications include kidney abscess (a walled-off pocket of pus within or around the kidney), emphysematous pyelonephritis (a gas-producing infection that destroys kidney tissue, seen almost exclusively in people with diabetes), and sepsis, where the infection spreads into the bloodstream and triggers a dangerous whole-body inflammatory response. Sepsis from pyelonephritis is a medical emergency. Even with bloodstream involvement, however, current guidelines suggest that 7 days of effective antibiotics is sufficient for patients who are improving clinically.
Repeated episodes of pyelonephritis, especially in children with structural urinary tract abnormalities, can lead to scarring of the kidney over time. This scarring can eventually affect kidney function and, in severe cases, contribute to high blood pressure or chronic kidney disease. For this reason, recurrent infections in children typically prompt further evaluation to look for correctable anatomical causes.