Pyelonephritis is a kidney infection, a type of urinary tract infection in which bacteria have spread up into one or both kidneys rather than staying in the bladder. It usually starts as an ordinary bladder infection that, left untreated or in higher-risk situations, travels upward through the ureters to reach the kidney. A kidney infection can feel alarming, with fever, chills, and pain that comes on quickly, but it is familiar to doctors and responds well to prompt antibiotics in most cases. Recognizing the warning signs and getting treated early keeps most people from ever developing serious complications. This guide explains what pyelonephritis is, its symptoms, causes, how it is diagnosed, the treatments available, and the latest research.
What is pyelonephritis?
Pyelonephritis is an infection of the kidney or kidneys, almost always caused by bacteria that travel up the urinary tract. Urine normally flows from the kidneys, where blood is filtered and waste removed, down through two tubes called ureters into the bladder and out through the urethra. Bacteria that enter through the urethra can climb this pathway in reverse, first infecting the bladder and then reaching one or both kidneys, where the infection can cause tissue swelling and, if untreated, lasting damage.
Most cases are acute, meaning they start suddenly and clear up with a course of antibiotics, and doctors call them uncomplicated when they occur in an otherwise healthy, non-pregnant woman, or complicated when pregnancy, diabetes, a urinary blockage, a catheter, or a weakened immune system is involved. A less common chronic or recurrent form, seen mostly in children with a structural problem called vesicoureteral reflux, can scar the kidneys gradually over time. Kidney infections affect people of all ages, but women are diagnosed far more often than men because anatomical differences make it easier for bacteria to reach the bladder.
Symptoms of pyelonephritis
The classic signs of a kidney infection are fever, shaking chills, and pain in the back or side, usually just below the ribs on one side, often with nausea and vomiting. Because most kidney infections begin as a bladder infection, they are frequently accompanied by burning or painful urination, needing to urinate more often or urgently than usual, and urine that looks cloudy, dark, bloody, or foul-smelling. Symptoms tend to appear quickly, over a day or two, rather than gradually.
In infants and children under age 2, a kidney infection can look quite different, showing up as a high fever, poor feeding, irritability, or slow weight gain rather than the classic back pain adults describe, which is why an unexplained fever in a young child always deserves medical evaluation. In adults, symptoms that come with confusion, a very rapid heartbeat, low blood pressure, or a fever that will not break point to a more severe infection and call for prompt medical attention rather than waiting it out at home.
What causes pyelonephritis and its risk factors
The great majority of kidney infections are caused by Escherichia coli, or E. coli, a bacterium normally found in the digestive tract that can spread to the urinary opening and travel upward; other bacteria, including Proteus, Klebsiella, Enterococcus, and Staphylococcus species, cause most remaining cases. Pyelonephritis is not contagious the way a cold or the flu is, since it comes from a person’s own bacteria moving into a part of the body where they do not belong, not from person-to-person spread.
Several factors raise the risk of a kidney infection. Female anatomy plays a large role, since a shorter urethra gives bacteria less distance to travel to reach the bladder. A blocked urinary tract raises risk too, and so does the urinary blockage caused by kidney stones, along with an enlarged prostate, a urinary catheter, pregnancy, poorly controlled diabetes, and a weakened immune system. In children, a structural condition that lets urine flow backward from the bladder toward the kidneys, called vesicoureteral reflux, is a major risk factor for repeated infections and gradual kidney scarring.
How pyelonephritis is diagnosed
Doctors typically suspect a kidney infection from the combination of fever, flank pain, and urinary symptoms, then confirm it with urine and blood testing. A physical exam often finds tenderness over the affected kidney, and in pregnant women or anyone with risk factors for a complicated infection, testing tends to be more thorough from the start.
| Test | What it shows |
|---|---|
| Urinalysis | Checks a urine sample for white blood cells, leukocyte esterase, nitrites, blood, and bacteria |
| Urine culture | Grows the bacteria from urine to identify the exact cause and which antibiotics will work |
| Blood tests | Look for a high white blood cell count, signs of inflammation, and how well the kidneys are working |
| Imaging (CT or ultrasound) | Used for severe illness, symptoms that are not improving, or a suspected blockage, stone, or abscess |
A urinalysis is usually the fastest clue and is often ready within minutes. A dipstick that flags the nitrite marker in urine and the leukocyte esterase marker suggests bacteria and white blood cells are present, and seeing the pyuria, or white blood cells, in the urine confirms the immune system is actively responding; white blood cell casts, when present, point specifically to the kidney rather than the bladder alone. Confirming the exact bacteria and which antibiotics will work takes a urine culture, ideally collected before antibiotics start, with results typically available in one to three days. Because the kidneys themselves are involved, your care team will also check the creatinine blood test, the blood urea nitrogen level, and the eGFR blood test to see how well the kidneys are filtering waste and how serious the infection is.
Treatment options for pyelonephritis
Prompt antibiotics are the main treatment for a kidney infection, and the choice between pills and an IV depends mainly on how sick you are. Most uncomplicated infections, especially in healthy, non-pregnant adults who can keep fluids and medication down, are treated entirely at home with oral antibiotics chosen based on local resistance and, when available, culture results. If a blockage such as a kidney stone or an enlarged prostate is contributing to the infection, relieving it, sometimes with a procedure, matters as much as the antibiotics themselves.
| Approach | Role |
|---|---|
| Oral antibiotics | Standard treatment for most uncomplicated infections, taken entirely at home |
| IV antibiotics | Used for severe illness, sepsis, pregnancy complications, or when oral medication cannot be tolerated, often with hospital admission |
| Relieving obstruction | Clears a blocked ureter from a stone or an enlarged prostate, sometimes with a drainage procedure |
| Supportive care | Fluids, rest, and fever or pain control while antibiotics take effect |
Hospitalization and IV antibiotics are generally reserved for people who appear seriously ill, show signs of sepsis such as a racing heart or low blood pressure, are pregnant, cannot keep down oral medication, or have a complicating condition such as uncontrolled diabetes or a urinary blockage; treatment often switches to an oral antibiotic once symptoms start improving. Certain warning signs call for emergency care rather than a routine appointment, including a high fever that will not respond to medication, confusion, fainting, an inability to keep any fluids down, or pain that is severe and worsening; anyone who is pregnant and develops these symptoms should also be seen promptly. Most people feel noticeably better within a couple of days of starting the right antibiotic, and full courses today are often shorter than the two weeks once routinely prescribed.
Living with pyelonephritis and long-term outlook
With prompt treatment, most people recover from a kidney infection completely, with no lasting effect on kidney function. The main risk comes from delaying care: untreated or severe pyelonephritis can lead to a renal abscess, sepsis, temporary or occasionally permanent loss of kidney function, and, rarely, a severe gas-forming infection that mostly affects people with diabetes. These outcomes are uncommon when treatment starts early, which is why finishing the full antibiotic course matters even after you feel better.
For people who get kidney infections repeatedly, or children with a structural cause such as vesicoureteral reflux, long-term care focuses on finding and addressing the underlying reason, since repeated infections raise the chance of gradual kidney scarring. Staying well hydrated, urinating when you feel the urge rather than holding it, and promptly treating any bladder infection before it travels upward all help prevent a repeat episode. Anyone with frequent kidney infections, a known urinary abnormality, or one severe episode should ask their doctor about follow-up testing to confirm kidney function has stayed normal.
Latest scientific advances in pyelonephritis research
Recent research has focused on how long antibiotic treatment for a kidney infection really needs to last. According to PubMed-indexed research, a 2025 living systematic review and meta-analysis pooling 16 randomized controlled trials and 4,643 adults with pyelonephritis or a complicated urinary tract infection found that 5- to 7-day courses of antibiotics were about as effective as the older 10- to 14-day standard, with similar cure, relapse, and side-effect rates (Zahavi et al., 2025). What this means for you: if your clinician prescribes a shorter course than you expected, this is not a shortcut; current evidence supports it as equally effective, so it is still important to finish every dose prescribed.
Other research is looking at which antibiotics make the best first choice as resistance patterns shift. A 2024 study of 851 adults discharged home from 11 US emergency departments found a 14-day treatment-failure rate of 17.2% with an oral cephalosporin, compared with 22.5% with the fluoroquinolone or trimethoprim-sulfamethoxazole antibiotics guidelines have traditionally preferred, a difference that was not statistically meaningful once other factors were accounted for (Koehl et al., 2024). What this means for you: being prescribed a cephalosporin instead of a traditional first-choice antibiotic does not mean a lesser treatment; it may reflect local resistance and real-world evidence. A related 2025 analysis of 130,514 E. coli urine samples from the community found that resistance to first-line bladder-infection antibiotics was closely linked with resistance to pyelonephritis antibiotics, with predicted resistance to amoxicillin-clavulanate exceeding 50% in some models (Williams et al., 2025). What this means for you: telling your doctor which antibiotics you have taken recently helps them choose one more likely to work, which is exactly why urine culture results matter so much.
Glossary of key pyelonephritis terms
| Term | Definition |
|---|---|
| Urinary tract infection (UTI) | An infection anywhere along the urinary system, from the urethra to the kidneys. |
| Cystitis | An infection limited to the bladder, the most common type of urinary tract infection. |
| Urine culture | A lab test that grows bacteria from urine to identify them and guide antibiotic choice. |
| Pyuria | The presence of white blood cells in the urine, a sign the body is fighting infection or inflammation. |
| Sepsis | A dangerous, body-wide reaction to an infection that has spread into the bloodstream. |
| Vesicoureteral reflux | A condition, mostly in children, in which urine flows backward from the bladder toward the kidneys. |
| Renal abscess | A pocket of pus that can form in or around a severely infected kidney. |
Frequently asked questions about pyelonephritis
What is pyelonephritis and how is it different from a regular bladder infection (UTI)?
Pyelonephritis is a kidney infection, while a regular bladder infection, called cystitis, stays lower in the urinary tract. Both are usually caused by the same bacteria, but a kidney infection develops when bacteria travel further, into one or both kidneys, making it more serious and typically adding fever and back or side pain to the usual urinary symptoms.
What are the first signs and symptoms of a kidney infection?
The first signs are often fever, chills, and pain in the back or side below the ribs, sometimes with nausea or vomiting. Many people also notice burning with urination, needing to go more often or urgently, and urine that looks cloudy, dark, or bloody. Symptoms typically develop quickly, over a day or two.
How is a kidney infection diagnosed, and what tests are used?
Doctors diagnose a kidney infection from symptoms, a physical exam, and lab testing. A urinalysis looks for white blood cells, nitrites, and leukocyte esterase, and a urine culture identifies the exact bacteria and which antibiotics will work. Blood tests, including kidney function tests such as creatinine and eGFR, show whether the infection is affecting the kidneys and how serious it is.
How is pyelonephritis treated, and how many days of antibiotics do you need?
Pyelonephritis is treated with antibiotics chosen based on the likely or confirmed bacteria and local resistance patterns. Most people take oral antibiotics at home, while those who are more seriously ill receive antibiotics through an IV until they improve enough to switch to pills. Many people now need only about 5 to 7 days of treatment rather than the 10 to 14 days once considered standard, though your clinician will tailor the length to your situation.
Can a kidney infection be treated at home, or do you need to go to the hospital?
Most uncomplicated kidney infections in healthy, non-pregnant adults can be treated entirely at home with oral antibiotics, as long as you can keep food and medication down and are not severely ill. Hospitalization is generally needed for people who are pregnant, cannot tolerate oral medication, show signs of sepsis, or have a complicating condition such as a urinary blockage or uncontrolled diabetes. A high fever that will not come down, confusion, or worsening pain should be evaluated urgently.
What happens if pyelonephritis is left untreated, and can it cause permanent kidney damage?
Left untreated, a kidney infection can worsen into complications such as a renal abscess, a bloodstream infection called sepsis, or temporary and sometimes permanent loss of kidney function. Repeated or poorly treated infections, especially in children with a structural problem, can gradually scar the kidneys over time. Prompt treatment makes these outcomes uncommon, which is why it is worth seeking care as soon as symptoms appear.
Sources
- National Institute of Diabetes and Digestive and Kidney Diseases — Kidney Infection (Pyelonephritis): Symptoms & Causes — NIDDK, 2023 — niddk.nih.gov
- Cleveland Clinic — Kidney Infection (Pyelonephritis) — Cleveland Clinic Health Library, 2024 — my.clevelandclinic.org
- Mayo Clinic — Kidney infection — Mayo Clinic, 2023 — mayoclinic.org
- Zahavi I, Kunwar D, Olchowski J, et al. — Short vs. long antibiotic treatment for pyelonephritis and complicated urinary tract infections: a living systematic review and meta-analysis of randomized controlled trials — Clinical Microbiology and Infection, 2025 — doi.org/10.1016/j.cmi.2025.04.008
- Koehl J, Spolsdoff D, Negaard B, et al. — Cephalosporins for Outpatient Pyelonephritis in the Emergency Department: COPY-ED Study — Annals of Emergency Medicine, 2024 — doi.org/10.1016/j.annemergmed.2024.10.013
- Williams P, Barton E, Bhamber R, et al. — Co-resistance between oral antibiotics for pyelonephritis and those for cystitis — applying an escalation antibiogram model to local community data — JAC-Antimicrobial Resistance, 2025 — doi.org/10.1093/jacamr/dlaf204
Further reading
- Explore how repeated or poorly treated infections can affect long-term kidney function in this guide to the kidney damage seen in chronic kidney disease.
- Get more comfortable interpreting flags and reference ranges with this guide to reading your lab results before a doctor’s visit.
Understand your lab results with BloodSense
A kidney infection is confirmed and tracked mostly through lab results rather than symptoms alone. A urinalysis and urine culture identify the infection and the specific bacteria causing it, while kidney function tests such as creatinine, blood urea nitrogen, and eGFR show whether the infection is affecting how well the kidneys are filtering waste. Seeing these values together, and watching how they change as treatment works, makes it much easier to understand how serious an infection is and to know when your kidneys have returned to their usual baseline.
