What Is a Kidney Infection? Symptoms, Causes, Diagnosis, Treatment, and Prevention

A kidney infection, also known as pyelonephritis, occurs when harmful bacteria reach your kidneys as part of a urinary tract infection (UTI).

Most UTIs don’t involve the kidneys. They affect only your lower urinary tract, meaning your urethra and bladder. But sometimes an infection that begins there moves into your upper urinary tract, affecting one or both kidneys.

It’s also possible to get a kidney infection following surgery, if bacteria enter your body during the procedure and travel through your bloodstream to your kidneys. In this case, your lower urinary tract may not be affected.

If you have symptoms of a UTI or bladder infection (also known as cystitis) — such as pain with urination, smelly urine, low back pain, or discolored urine — it’s important to seek medical treatment to prevent the infection from spreading to your kidneys.

Kidney infections can be quite painful and require prompt evaluation and treatment. If your infection isn’t treated soon enough, it may permanently damage your kidneys or spread to your bloodstream, possibly leading to sepsis and the impairment of other vital organs. (1,2,3)

Signs and symptoms of a kidney infection include those of a lower urinary tract infection — including painful and frequent urination — as well as:

• Fever and chills
• Pain in the lower back or sides
• Nausea and vomiting
• Abdominal or pelvic pain
• Cloudy or smelly urine
• Dark or bloody urine

In people ages 65 and older, the above signs and symptoms of kidney infection may not occur. Instead, these individuals may experience confusion, hallucinations, or difficulty speaking.

In newborns and very young children, the only sign of a kidney infection may be a high fever. (1)

Learn More About Signs and Symptoms of a Kidney Infection

Kidney infections can be caused by bacteria or viruses that spread in your urinary tract, but bacterial infections are much more common.

In fact, about 90 percent of kidney infections are caused by a single type of bacteria, Eschericia coli, or E. coli. (2)

The infection is thought to occur from bacteria that travel from your gastrointestinal tract to your skin surface, through your urethra to your bladder, then through the tubes (known as ureters) that connect your bladder and kidneys.

Normally, urine flushes any potentially harmful bacteria out of your ureters before they can cause an infection. But sometimes this doesn’t happen due to the following conditions:

• Structural abnormalities in your ureters or kidneys
• Kidney stones
• Enlarged prostate (benign prostatic hyperplasia)
• Urine backflow (reflux) from your bladder to your kidneys

It’s also possible for bacteria from your bloodstream to enter your kidneys and cause an infection. This happens most commonly with staphylococcus, or staph, bacteria. (2)

Kidney infections that spread from your bloodstream tend to develop after surgery or as a consequence of another infection elsewhere in the body. For example, if an artificial joint or heart valve becomes infected, the kidneys can become secondarily infected.

It’s very uncommon for kidney surgery to lead to a kidney infection. (3)

A number of factors can increase your risk of developing a kidney infection:

Being a Woman In women, the urethral opening — where urine exits the body — is close to the vagina and anus, which makes it easy for bacteria from these orifices to enter the urethra. In men, the urethral opening is further away from the anus.

Women’s urethras are also shorter than men’s, which makes it easier for bacteria to travel to the bladder and cause an infection. (3)

Being Sexually Active Especially for women, having sex can make it easier for bacteria in your genital area or anus to enter your urethra.

Using condoms with spermicidal coating also raises the risk that a woman will develop a UTI. (4)

Being Pregnant When you’re pregnant, your enlarged uterus can press against and squeeze your ureters, reducing the flow of urine from your kidneys to your bladder.

This reduced urine flow can make it easier for bacteria in your bladder to migrate into your kidneys. (2)

Having Impaired Urine Flow The flow of urine throughout your urinary tract is an important barrier against infection. This flow can be slowed by a narrowed urethra, enlarged prostate, or kidney stone. (1)

Having a Weakened Immune System Your immune system can be weakened by health conditions such as diabetes, HIV, and cancer, or by certain drugs.

Wearing a Catheter Long-term use of a urinary catheter to drain urine from the bladder, such as during a surgical procedure or hospital stay, raises the risk of developing a UTI.

Having Nerve Damage If you have a spinal cord injury or nerve damage around your bladder, you may not notice when you develop a bladder infection, which can allow it to progress to your kidneys more easily.

Having Urine Reflux Problems A medical condition called vesicoureteral reflux results in small amounts of urine flowing backward from your bladder into your kidneys, potentially carrying bacteria with it.

Vesicoureteral reflux is most likely to be diagnosed during childhood.

Having Trouble Emptying Your Bladder If your bladder doesn’t empty fully, known as urinary retention, the remaining urine can contribute to developing an infection. (3)

A kidney infection is usually diagnosed based on your symptoms, a physical exam, and the results of urine tests, indicating bacteria in your urinary tract.

Your doctor will likely ask about your health history and any conditions that might place you at higher risk of a kidney infection, such as having an enlarged prostate gland or a medical condition that can cause urinary retention, such as multiple sclerosis. (1)

Sometimes a doctor will order imaging tests, such as an ultrasound or computerized tomography (CT) scan, to look for signs of swelling or other abnormalities in your kidneys or bladder. (2)

Learn More About Diagnosing a Kidney Infection: Tests, Imaging, and Early Diagnosis

A kidney infection usually starts out as a urinary tract infection (UTI) that affects the bladder. There’s no rule for how long it takes a UTI to spread from your bladder to your kidneys.

If left untreated, a kidney infection may not resolve on its own, potentially becoming a severe infection (which can cause sepsis) or leading to a chronic or recurrent infection. (5)

When treated with antibiotics, you may start to feel better two to three days after starting the drug. It’s important, though, to continue your treatment for the entire course that’s prescribed. (2)

For a mild kidney infection, treatment can last 7 to 14 days. It may take a week or longer for your symptoms to resolve with treatment.

A severe or complicated kidney infection may take much longer to treat, depending on how far the infection has spread and what other complications it has caused. (6)

Kidney infections are treated with antibiotics, either oral or intravenous (IV), depending on the severity of your infection and its symptoms. (1)

People with severe illness may need to be hospitalized for several days or longer for initial treatment. During this time, you may receive IV antibiotics in addition to fluids while your medical team monitors you closely. (2,3)

The oral antibiotics prescribed for kidney infection generally must be taken for 7 to 14 days, to make sure all of the bacteria causing the infection have been killed. It’s most common for your doctor to prescribe a 14-day course of antibiotics. (2,6)

For reasons that aren’t fully understood, kidney infections in men are often resistant to treatment and may require as long as six weeks of antibiotic therapy.

Your doctor may order follow-up tests after antibiotic treatment to make sure you no longer have an infection. (2)

Medication Options for Kidney Infection

Your doctor will decide what antibiotic to prescribe, and for how long, based on your symptoms and the results of your urine tests. Your doctor may start you on one medication right away, but change it later if your urine test results indicate that a different antibiotic will be more effective. (2)

Drugs taken by mouth for kidney infection include:

• ciprofloxacin (Cipro)
• levofloxacin (Levaquin)
• trimethoprim-sulfamethoxazole (Bactrim)

If additional therapies are needed, you may be given the following drugs intravenously (by IV) in your doctor’s office:

• ceftriaxone (Rocephin)
• gentamicin (Garamycin)
• ciprofloxacin

All of these drugs and others may be considered for IV treatment if you need to be hospitalized for a severe kidney infection. (4)

Alternative and Complementary Therapies

There are no effective alternative or complementary methods or therapies for curing a kidney infection.

But there may be ways you can increase your comfort while being treated with antibiotics, such as placing a heating pad on your abdomen, back, or side to ease any pain in those areas.

Drinking plenty of fluids can also help flush the bacteria causing the infection from your system. (3)

And getting plenty of rest will help in your recovery from the infection.

Learn More About Treatment for Kidney Infection: Medication, Surgery, and More

Prevention of a Kidney Infection

Preventing kidney infections usually starts with preventing UTIs in the lower urinary tract. Steps you can take to help prevent UTIs include:

• Drink lots of water throughout the day.
• Urinate when you feel the urge to do so; don’t wait.
• After using the toilet, wipe from front to back so you don't spread bacteria from your anus to your urethra.
• Urinate after having sex.
• Don’t use unnecessary cleansing or deodorizing products in the genital area.
• Steer clear of birth control methods that can raise the risk of a UTI. (2,3)

If you do develop a UTI, get it treated as soon as possible. See your primary healthcare provider for treatment, or visit an urgent care facility for diagnosis and a prescription for antibiotics.

Treating a UTI promptly may prevent it from spreading from your bladder to your kidneys. (5)

Learn More About Preventing Kidney Infections

If it isn’t treated promptly, a kidney infection can lead to a number of potentially serious complications.

You’re more likely to develop a complication from a kidney infection if you have:

• Prior kidney disease
• A history of kidney infections
• A structural abnormality in your urinary tract (1)

The following complications can result from a kidney infection:

High Blood Pressure Your blood pressure can rise to unhealthy levels as your immune system tries to fight your infection.

Kidney Failure If it’s serious enough, an infection can stop your kidneys from effectively removing waste products from your blood.

Scarring If you develop scars in your kidneys from an infection, your kidneys may not be able to function properly, potentially leading to chronic kidney disease.

Blood Infection One of the main jobs of your kidneys is to filter waste products out of your blood. A kidney infection can spread to your entire bloodstream in this process.

A bloodstream infection can, in turn, lead to sepsis, which is a severe inflammatory reaction to bacteria. Sepsis can be life-threatening.

Renal or Perinephric Abscess If the infection in your kidney is not treated promptly, the bacteria may create an abscess, or pocket of pus, inside or next to your kidney.

Pregnancy Complications A kidney infection during pregnancy raises the risk of low birth weight in your baby. (1,3,4)

The Cleveland Clinic estimates that each year between 3 and 7 out of 10,000 people in the United States develop a kidney infection. Kidney infections are much more common among pregnant women, occurring in about 2 percent of pregnancies. (2)

There are about 100,000 hospital visits for UTIs each year in the United States, most of which are related to kidney infections. (1)

One important area of research for kidney infection is antibiotic resistance — when bacteria contain a way to block the effect of the drugs designed to kill them. Currently, guidelines for treating kidney infections recommend adding IV antibiotics to oral drugs when a certain percentage of bacteria from a urine sample show signs of resistance. (4)

“Treatment for kidney infections has largely stayed the same over the last several years,” says Amin S. Herati, MD, an assistant professor of urology at Johns Hopkins Medicine in Baltimore. But, Dr. Herati says, “there are some new therapies in the pipeline that are very exciting.”

One promising potential treatment on the horizon, says Herati, is bacteriophage therapy, which involves injecting a virus that targets specific bacteria, causing them to burst and die. This approach could be useful for UTIs and kidney infections in which antibiotic resistance is a problem.

While many diseases affect different populations in the United States at different rates, there isn’t much information available on the relative risk of kidney infection among different racial and ethnic groups.

Black Americans are at higher risk for kidney failure than any other group — 4 times as likely as white Americans. (7) But kidney infection rarely leads to kidney failure, accounting for a tiny sliver of cases. (1)

There is some evidence that the risk of getting a UTI during pregnancy — which can develop into a kidney infection — is different for different groups. (8)

Looking at records of 24,000 births, researchers found that rates of UTI during pregnancy were 28.7 percent for white and Asian women, 30.1 percent for Black women, and 41.1 percent for Hispanic women. However, once the researchers controlled for socioeconomic differences, there were no significant differences between the groups. (8)

Kidney infection can develop from, or lead to, certain other health conditions, including the following:

Urinary Tract Infection (UTI) Most UTIs affect the lower urinary tract — the urethra and bladder. But bacteria can migrate from the bladder to the kidneys, causing infection there.

Benign Prostatic Hyperplasia Having an enlarged prostate can reduce the flow of urine, making it more likely that you’ll develop a UTI or kidney infection.

Sepsis One of the most serious potential complications of a kidney infection, sepsis is an extreme immune system response to infection, resulting in widespread inflammation and sometimes vital organ failure and death.

The following organizations and websites offer more information on kidney infection and related conditions:

National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)

A research arm of the U.S. government and part of the National Institutes of Health, the NIDDK offers a detailed overview of kidney infection and other conditions of the urinary system.

National Kidney Foundation

One of the leading research and support organizations for people with kidney disease, the National Kidney Foundation offers information on a variety of kidney conditions.

American Kidney Fund

This group advocates on behalf of people with kidney disease, provides financial support, and offers information on conditions and procedures.

Urology Care Foundation

This website from the American Urological Association offers information on kidney infection and many other urologic conditions. It also offers a podcast that covers a wide variety of health conditions and topics related to urology.