Kidney Infection: Signs, Symptoms, and Complications

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A kidney infection, called pyelonephritis, can sometimes be confused with a urinary tract infection, but the diseases are different. A kidney infection can be a complication of a urinary tract infection.

Some common symptoms of a kidney infection are pain with urination, back pain (particularly on one side), and chills or fever.

This article covers kidney infection symptoms and how they differ from those of urinary tract infections. This article also explores how to recognize a kidney infection in young children and older people, and it will discuss kidney infection complications.

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Frequent Symptoms

Sometimes people confuse a urinary tract infection with a kidney infection. To help understand why, it's helpful to know a little about anatomy.

The kidneys are connected to the bladder through the urinary tubular system. Urinary tract infections (bladder infections) are common, particularly in people with a vagina, since they have a shorter urethra (the tube leading from the bladder to where urine leaves the body) than people with a penis.

Sometimes these infections can spread upward through the urinary tubular system to infect the kidneys. Kidney infections are less common and can happen if a urinary tract infection is not treated appropriately.


About 1 in every 30 cases of urinary tract infection leads to a kidney infection.

It can be challenging to differentiate the two diseases since both are associated with pain with urination. But a kidney infection is more severe than a urinary tract infection. Common symptoms that suggest a person has a kidney infection include:

  • Pain with urination
  • Urinary frequency (urinating often)
  • Urinary urgency (feeling the need to urinate even when little urine is available)
  • Back or flank pain, particularly on one side
  • Chills and fever
  • Nausea and vomiting

People with a urinary tract infection will also have pain with urination, frequency, and urgency. Additionally, they may have lower abdominal discomfort.

However, the back pain associated with a urinary tract infection is usually lower and in the middle of the back. In contrast, the back and flank pain associated with a kidney infection is higher in the back. Fever and associated gastrointestinal symptoms usually suggest a higher infection (in the kidney) than a urinary tract infection. 

Symptoms in Children and Older People

It is challenging to recognize some bacterial infections in infants and older people.

Very young children may only have a fever on presentation. Infants and small children have difficulty describing how they feel, and they may not be able to describe pain with urination. Therefore, an isolated fever without other symptoms can suggest a urinary tract or kidney infection. 

Older people can also present with atypical symptoms like an isolated fever. However, they can show some rare symptoms that will be discussed shortly.

Kidney Infection vs. Kidney Stone

A kidney infection can sometimes be confused with a kidney stone. Kidney stones typically cause acute, one-sided back pain that develops over minutes to an hour. In contrast, the back pain associated with a kidney infection increases slowly and is often associated with a general unwell feeling and chills.


As mentioned, urinary and kidney infections are more common in people with a vagina, and pregnant people are at an even higher risk of kidney infection. Approximately 2% of pregnant people develop a kidney infection.

This can be serious for the pregnancy, so pregnant people must seek medical attention if they suspect a urinary tract or kidney infection.

Rare Symptoms

Rarely a kidney infection can lead to life-threatening sepsis, an extreme, even life-threatening response to overwhelming infection. Symptoms of sepsis include:

  • Fatigue
  • Fever
  • Low blood pressure
  • Fast heart rate
  • Muscle aches
  • Rapid breathing
  • Confusion

Bacterial Cause of Kidney Infection

Most kidney infections are caused by the bacteria Escherichia coli.

Older people are at higher risk for severe infections, such as kidney infections. They can develop sepsis since they usually have weaker immune systems. This is complicated by the fact that older people can also show atypical symptoms of a kidney infection that include:

  • Confusion
  • Hallucinations
  • Unusual fatigue 

A healthcare provider must evaluate these symptoms to ensure an older person is not developing a severe infection or sepsis. This is particularly true if the person also has a fever. 


Complications of a severe kidney infection, particularly one that goes untreated, include:

  • Sepsis
  • Kidney abscess (a pocket of infection)
  • Chronic kidney infection
  • Kidney damage and failure

People who are at higher risk for complications from a kidney infection include those with a weakened immune system, such as: 

  • Older people
  • People with cancer
  • Transplant recipients
  • People with diabetes mellitus
  • People who take immunosuppressive medications 

Those with structural problems in their kidneys or urinary system are also at risk for complications from a kidney infection. Examples of such issues include: 

When to See a Healthcare Provider

As soon as a person develops urinary pain and frequency, it is vital to see a healthcare provider. This is especially important for pregnant people. Urine tests can evaluate for a urinary tract infection and detect it early so it does not become a complicated kidney infection. 

Symptoms in small children that suggest the need for evaluation include an isolated fever without respiratory symptoms. (A fever with respiratory symptoms suggests an upper respiratory tract infection, not a urinary tract infection.

Older people with confusion and different behavior than usual should also be evaluated.


Kidney infections are not as common as urinary tract infections, but they must be recognized and treated to avoid complications. Common symptoms include pain with urination, back pain, fever, nausea, and vomiting.

Symptoms in small children and older adults can be atypical or might only include a fever. It’s important for people to seek medical attention when they have symptoms of a urinary tract or kidney infection.

3 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Urology Care Foundation. Kidney (renal) infection: pyelonephritis.

  2. Herness J, Buttolph A, Hammer NC. Acute pyelonephritis in adults: rapid evidence reviewAm Fam Physician. 2020;102(3):173-180.

  3. MedlinePlus. Sepsis.

By Christine Zink, MD
Dr. Christine Zink, MD, is a board-certified emergency medicine with expertise in the wilderness and global medicine. She completed her medical training at Weill Cornell Medical College and residency in emergency medicine at New York-Presbyterian Hospital. She utilizes 15-years of clinical experience in her medical writing.