A urinary tract infection that stays in the bladder is uncomfortable. One that travels up to the kidneys can become dangerous. The symptoms of kidney infection — medically called pyelonephritis — overlap with a standard UTI at first but escalate quickly into fever, flank pain, and systemic illness that often requires aggressive antibiotic treatment. According to the National Institute of Diabetes and Digestive and Kidney Diseases, kidney infections send approximately 250,000 Americans to the hospital each year. Knowing how to distinguish a kidney infection from a simple bladder infection can help you get the right treatment before complications develop. For more on medical conditions, visit our medical conditions guide.
What Is a Kidney Infection?
A kidney infection is a type of urinary tract infection that develops when bacteria — usually E. coli from the intestinal tract — travel from the bladder up through the ureters and into one or both kidneys. While lower UTIs (cystitis) affect the bladder and urethra, pyelonephritis involves the kidney tissue itself, creating a more serious and potentially dangerous infection.
Most kidney infections start as bladder infections that aren’t treated promptly or completely. Less commonly, bacteria can reach the kidneys through the bloodstream from an infection elsewhere in the body. Women are significantly more likely to develop kidney infections than men, largely due to anatomical differences that make UTIs more common in women overall. The Mayo Clinic identifies pregnancy, urinary tract abnormalities, kidney stones, catheter use, and weakened immune systems as additional risk factors.
Common Symptoms of Kidney Infection
The symptoms of kidney infection typically build on existing lower UTI symptoms but add several distinctive features that signal the infection has moved to the kidneys.
Flank Pain
Pain in the side of the back, just below the rib cage — on the side of the affected kidney — is the signature symptom of pyelonephritis. The pain may be dull and constant or sharp and throbbing. It’s sometimes mistaken for a back muscle strain, but unlike muscular pain, kidney infection pain doesn’t change with movement and is often accompanied by fever. Doctors check for “costovertebral angle tenderness” by tapping on the flank area — sharp pain with this maneuver strongly suggests kidney involvement.
High Fever and Chills
A simple bladder infection rarely causes a fever above 100.4 degrees Fahrenheit. A kidney infection frequently pushes the temperature to 101 to 103 degrees or higher, often accompanied by shaking chills and rigors. Fever with urinary symptoms is one of the clearest signals that the infection has reached the kidneys.
Nausea and Vomiting
Unlike a bladder infection, pyelonephritis commonly causes significant nausea and sometimes vomiting. This systemic response reflects the body’s reaction to the kidney tissue infection and the inflammatory chemicals it releases into the bloodstream.
Persistent UTI Symptoms
The lower urinary tract symptoms of a bladder infection typically persist: frequent and urgent urination, burning or pain during urination (dysuria), and cloudy or foul-smelling urine. Blood in the urine (hematuria) may also be present and can make the urine appear pink, red, or brown.
Fatigue and General Malaise
Kidney infections produce a feeling of being profoundly unwell that goes beyond the discomfort of a bladder infection. Patients often describe feeling wiped out, achy, and unable to function normally — similar to having a bad flu alongside urinary symptoms.
How to Tell a Kidney Infection From a UTI
Bladder infections and kidney infections exist on the same spectrum, so the line between them isn’t always sharp. However, certain features suggest the infection has moved beyond the bladder.
Location of pain matters. Bladder infections cause lower abdominal pressure and burning during urination. Kidney infections add flank pain — pain in the side or back, typically on one side, at or just below the rib cage.
Fever is the key differentiator. A temperature of 101 degrees Fahrenheit or higher with urinary symptoms strongly suggests pyelonephritis. Simple bladder infections almost never cause significant fever.
Systemic symptoms signal kidney involvement. Nausea, vomiting, chills, and profound fatigue suggest the infection has become systemic — meaning it’s affecting the body beyond the urinary tract. For more on how UTI symptoms present, see our guide on what a UTI feels like.
Who Is at Higher Risk?
Certain groups face elevated risk for developing kidney infections.
Women. The shorter female urethra makes it easier for bacteria to reach the bladder, and from there, the kidneys. Sexually active women, pregnant women, and postmenopausal women are at particular risk.
People with urinary tract obstructions. Kidney stones, enlarged prostate, and anatomical abnormalities can prevent complete bladder emptying, allowing bacteria to multiply and ascend to the kidneys.
Catheter users. Indwelling urinary catheters provide a direct pathway for bacteria to enter the urinary tract.
Immunocompromised individuals. Diabetes, HIV, immunosuppressive medications, and other conditions that weaken immune defenses increase susceptibility to kidney infections and make them harder to clear.
People with vesicoureteral reflux. This condition causes urine to flow backward from the bladder toward the kidneys, carrying bacteria with it. It’s more common in children but can persist into adulthood.
Kidney Infection During Pregnancy
Kidney infections during pregnancy deserve special attention because they are more common and carry greater risks than in non-pregnant women. Hormonal changes during pregnancy cause the ureters to dilate and urine flow to slow, making it easier for bacteria to ascend from the bladder to the kidneys. According to the American College of Obstetricians and Gynecologists, approximately 2% of pregnant women develop pyelonephritis, and the condition is associated with increased risk of preterm labor, low birth weight, and maternal sepsis.
Pregnant women with asymptomatic bacteriuria (bacteria in the urine without symptoms) are routinely screened and treated with antibiotics because untreated bacteriuria progresses to pyelonephritis in 20% to 40% of pregnant women — a rate much higher than in the general population. If you’re pregnant and experience any urinary symptoms or flank pain, contact your obstetrician immediately rather than waiting to see if symptoms resolve.
Potential Complications
Untreated or inadequately treated kidney infections can lead to serious complications. The NIDDK identifies several major risks:
Kidney abscess. A pocket of pus can form within or around the kidney, requiring drainage and prolonged antibiotic treatment. Sepsis. Bacteria entering the bloodstream from an infected kidney can trigger a life-threatening systemic inflammatory response. Permanent kidney scarring. Repeated or severe infections can scar kidney tissue, potentially leading to chronic kidney disease, high blood pressure, and reduced kidney function over time. Complications in pregnancy. Kidney infections during pregnancy increase the risk of preterm labor and low birth weight, making prompt treatment especially critical.
When to See a Doctor
See your doctor promptly if you have UTI symptoms that aren’t improving after two to three days of treatment, or if you develop new symptoms like flank pain, fever, nausea, or chills. A urine culture and blood tests can confirm whether the infection has reached the kidneys and guide antibiotic selection.
When to seek emergency care: Call 911 or go to the nearest emergency room if you experience a high fever (103 degrees or above) with urinary symptoms, inability to keep fluids or oral medications down due to vomiting, severe flank pain that doesn’t respond to over-the-counter pain relievers, confusion or altered mental status, or signs of sepsis such as rapid heart rate, rapid breathing, and feeling extremely ill.
How Kidney Infections Are Diagnosed
Doctors diagnose kidney infections through a combination of clinical evaluation and laboratory testing. A physical exam typically reveals tenderness when the doctor taps on the flank (costovertebral angle tenderness). A urinalysis shows white blood cells, bacteria, and often blood in the urine. A urine culture — which takes 24 to 48 hours — identifies the specific bacteria and which antibiotics will be effective against it.
Blood tests may show elevated white blood cell count and markers of inflammation (elevated C-reactive protein or procalcitonin). Blood cultures are drawn if the doctor suspects the infection has entered the bloodstream (bacteremia). In some cases — particularly recurrent infections or suspected complications — imaging studies such as a CT scan or renal ultrasound may be ordered to look for kidney stones, abscesses, or anatomical abnormalities that predispose to infection.
Treatment Overview
Kidney infections are treated with antibiotics — typically a 7 to 14 day course of oral antibiotics for mild to moderate cases. Common first-line antibiotics include ciprofloxacin, levofloxacin, and trimethoprim-sulfamethoxazole, though your doctor will choose based on local resistance patterns and your urine culture results. According to the Infectious Diseases Society of America, fluoroquinolones remain a standard treatment for uncomplicated pyelonephritis.
Severe kidney infections — particularly those with high fever, inability to tolerate oral medications, or signs of sepsis — require hospitalization for IV antibiotics and fluid support. Most patients improve within 48 to 72 hours of starting appropriate antibiotics, but completing the full course is essential to prevent recurrence and antibiotic resistance. Your doctor will typically schedule a follow-up urine culture after treatment to confirm the infection has fully cleared. If symptoms recur shortly after completing antibiotics, a repeat culture with sensitivity testing helps identify whether resistant bacteria require a different antibiotic regimen.
Recurrent Kidney Infections
Some people experience repeated kidney infections, which can cause cumulative kidney scarring and long-term damage. Recurrent infections are defined as two or more episodes within six months or three or more within a year. Common underlying causes include incomplete treatment of previous infections, anatomical abnormalities such as vesicoureteral reflux or kidney stones, chronic catheter use, and uncontrolled diabetes.
If you’ve had more than one kidney infection, your doctor should investigate potential contributing factors. This workup may include imaging of the urinary tract (CT urogram or renal ultrasound), voiding cystourethrogram to check for reflux, and a thorough metabolic evaluation. Preventive strategies for recurrent infections may include low-dose prophylactic antibiotics, post-coital antibiotics for sexually active women, vaginal estrogen therapy for postmenopausal women, and treatment of any identified anatomical or functional abnormalities.
Frequently Asked Questions
Can a kidney infection go away on its own?
No. Kidney infections require antibiotic treatment. Unlike some mild bladder infections that occasionally resolve without medication, pyelonephritis carries significant risks of complications — including sepsis and permanent kidney damage — if left untreated. Always seek medical care for suspected kidney infection symptoms.
How quickly do kidney infection symptoms develop?
Symptoms can develop within hours to days after a bladder infection starts. Some people notice a rapid escalation from burning urination to fever and flank pain over the course of a single day. Others experience a more gradual progression over several days. The speed of onset depends on the bacteria involved and your immune response.
Can men get kidney infections?
Yes, though kidney infections are less common in men than women. Men with enlarged prostates, kidney stones, or catheter use are at higher risk. Kidney infections in men should prompt further evaluation to identify underlying anatomical or functional abnormalities.
Is a kidney infection the same as a kidney stone?
No, they are different conditions, though they can coexist. Kidney stones are mineral deposits that form in the kidney. Kidney infections are bacterial infections of the kidney tissue. Kidney stones can block urine flow and increase infection risk, creating a situation where both conditions are present simultaneously. Learn more in our article on what a kidney stone feels like.
How can I prevent kidney infections?
Treat bladder infections promptly and completely. Stay well hydrated, urinate frequently rather than holding urine, and urinate after sexual intercourse. If you experience recurrent UTIs, talk to your doctor about preventive strategies, which may include low-dose prophylactic antibiotics or post-coital antibiotics.
The Bottom Line
The symptoms of kidney infection — particularly flank pain, high fever, and nausea on top of urinary symptoms — tell you the infection has moved beyond the bladder and needs aggressive treatment. Don’t wait for symptoms to resolve on their own. Prompt antibiotic therapy clears most kidney infections without lasting damage, but delays can lead to hospitalization, sepsis, or permanent kidney injury. If your UTI symptoms are getting worse instead of better, that’s your signal to escalate care. For related topics, explore our articles on when to go to the ER for dehydration and symptoms of liver damage.