Published: 18 Jun 2025

ICD9: 590.80      ICD10: N12      ICD11: GB51

Pyelonephritis, commonly known as a kidney infection, is a type of urinary tract infection (UTI) that specifically affects one or both kidneys.
It's a more serious condition than a bladder infection (cystitis) because the infection has traveled further up the urinary tract and can potentially cause permanent kidney damage or spread to the bloodstream (sepsis) if left untreated.

Here's a more detailed breakdown:

What it is:

An infection of the kidneys: Bacteria, usually from the lower urinary tract (bladder and urethra), ascend to the kidneys and infect them.
Upper UTI: Classified as an upper urinary tract infection, meaning it affects the kidneys and ureters (the tubes connecting the kidneys to the bladder).
Potentially serious: Can lead to kidney scarring, impaired kidney function, high blood pressure, and sepsis (a life-threatening response to infection).

Causes:

Bacteria: Most commonly caused by *Escherichia coli* (E. coli), which is normally found in the digestive system.
Ascending infection: Bacteria travel up the urethra, through the bladder, and then up the ureters to the kidneys.
Other bacteria: Other bacteria, such as *Klebsiella* and *Proteus*, can also cause pyelonephritis, especially in people with urinary catheters or structural abnormalities in the urinary tract.
Less common causes: Rarely, pyelonephritis can be caused by a bloodstream infection that spreads to the kidneys.

Risk Factors:

Female anatomy: Women are more susceptible because they have shorter urethras, making it easier for bacteria to reach the bladder.
Previous UTIs: A history of UTIs increases the risk.
Urinary tract abnormalities: Structural problems or blockages in the urinary tract can impede urine flow and increase the risk of infection. Examples include:
Kidney stones
Enlarged prostate (in men)
Vesicoureteral reflux (urine flows backward from the bladder to the kidneys)
Urinary catheter use: Catheters can introduce bacteria into the urinary tract.
Weakened immune system: Conditions like diabetes, HIV/AIDS, or medications that suppress the immune system increase the risk of infection.
Pregnancy: Hormonal changes and pressure on the urinary tract during pregnancy can increase the risk.
Enlarged prostate: In men, an enlarged prostate can obstruct urine flow, increasing the risk of infection.
Age: Both very young children and older adults are at higher risk.

Symptoms:

Fever: Often high (101°F or higher) with chills.
Flank pain: Pain in the back or side, often on one side.
Abdominal pain: Sometimes present.
Urinary symptoms:
Frequent urination
Urgent need to urinate
Pain or burning during urination (dysuria)
Cloudy or bloody urine
Strong-smelling urine
Nausea and vomiting:
General malaise: Feeling unwell, tired, and achy.
Mental status changes: Especially in older adults, confusion or altered mental state may occur.
In children: Bedwetting, irritability, poor feeding.

Diagnosis:

Medical history and physical exam: The doctor will ask about symptoms and perform a physical exam, including checking for tenderness in the back and abdomen.
Urinalysis: A urine sample is examined for bacteria, white blood cells (indicating infection), and red blood cells.
Urine culture: A urine sample is cultured to identify the specific type of bacteria causing the infection and determine which antibiotics will be most effective.
Blood tests: May be performed to check for signs of infection in the bloodstream (sepsis) and to assess kidney function.
Imaging tests: In some cases, imaging tests like a CT scan, ultrasound, or X-ray may be needed to look for kidney stones, blockages, or other abnormalities in the urinary tract, especially if the infection is severe or recurrent.

Treatment:

Antibiotics: The main treatment is antibiotics to kill the bacteria causing the infection. The specific antibiotic and duration of treatment depend on the type of bacteria identified and the severity of the infection.
Pain relievers: Over-the-counter or prescription pain relievers can help manage pain and fever.
Hospitalization: Severe cases of pyelonephritis, especially those with complications like sepsis, dehydration, or inability to tolerate oral medications, may require hospitalization for intravenous antibiotics and fluids.
Follow-up: After treatment, a follow-up urine culture may be recommended to ensure that the infection has cleared.
Addressing underlying conditions: If a structural abnormality or kidney stone is contributing to the infection, it may need to be treated to prevent future infections.

Complications:

Kidney scarring: Chronic or recurrent pyelonephritis can lead to scarring of the kidneys, which can impair kidney function.
Chronic kidney disease: Severe kidney scarring can eventually lead to chronic kidney disease (CKD).
High blood pressure: Pyelonephritis can sometimes lead to high blood pressure.
Sepsis: A life-threatening bloodstream infection that can occur if the infection spreads from the kidneys to the blood.
Kidney abscess: A collection of pus in the kidney.
Pregnancy complications: Pyelonephritis during pregnancy can increase the risk of premature labor and low birth weight.

Prevention:

Drink plenty of fluids: Helps flush bacteria out of the urinary tract.
Urinate frequently: Don't hold urine for long periods.
Wipe from front to back after using the toilet: Helps prevent bacteria from the rectum from entering the urethra.
Urinate after intercourse: Helps flush out any bacteria that may have entered the urethra.
Avoid irritating feminine products: Such as douches, feminine hygiene sprays, and scented pads, as these can disrupt the natural balance of bacteria in the vagina and increase the risk of UTIs.
Prompt treatment of UTIs: Treating bladder infections promptly can help prevent them from spreading to the kidneys.
Prophylactic antibiotics: In some cases, people with recurrent UTIs may be prescribed low-dose antibiotics to prevent future infections.

When to See a Doctor:

Seek immediate medical attention if you experience:
High fever
Severe flank pain
Nausea and vomiting
Blood in the urine
Confusion or altered mental state

In summary, pyelonephritis is a serious kidney infection that requires prompt medical attention. If you suspect you have a kidney infection, see a doctor as soon as possible for diagnosis and treatment to prevent potentially serious complications.