Acute Pyelonephritis

What is acute pyelonephritis?

Acute pyelonephritis is a bacterial infection of the kidneys. While most episodes clear without causing lasting renal damage, it can be severe and is usually quite painful, although few patients will need to be hospitalised.[1]

This condition is most common among young adult women, but can affect anyone of any age.[2] Men are at relatively low risk of acute pyelonephritis, unless they are older than 65 years of age.[3]

Causes and risk factors

Most cases of acute pyelonephritis occur when bacteria that are present in the gastrointestinal tract come into contact with the urethra, move into the bladder, and travel upward from the bladder to the kidneys.[1] In rare cases, the condition can arise hematogenously, which means that the bacteria which cause it originate and travel in the blood.

Some of the bacteria most commonly implicated in such infections are:[2][3]

  • Escherichia coli (more commonly referred to as e. coli)
  • Pseudomonas aeruginosa
  • Staphylococcus saprophyticus
  • Group B streptococci
  • Enterococci
  • Other enterobacteriaceae
  • Klebsiella spp.
  • Corynbacterium urealyticum
  • Mycobacteria

In rare cases, yeasts, and fungi may also be implicated. However, e. coli is by far the most common bacteria causing acute pyelonephritis. Vesicoureteral reflux (VUR), the backward flow of urine from the bladder to the kidneys, may also lead to acute pyelonephritis, as the refluxing urine can possibly carry bacteria.[3]

Although pyelonephritis can occur among any group of people, young women are most at risk, particularly between the ages of 15 and 29. This may have to do with the role of frequent sexual intercourse in introducing bacteria to region of the exterior opening of the urethra and thence to the bladder. The use of spermicidal contraceptives can increase the risk of pyelonephritis (as well as cystitis and other urinary tract infections) because they destroy the natural protective barrier of bacteria that should be in the vaginal area, allowing enteric bacteria to colonise the area instead.[2] New sexual partners can also increase one’s risk of a urinary tract infection (UTI), due to the potential introduction of “new” bacteria and more frequent sexual intercourse. There is some medical evidence that women with diabetes may be at slightly higher risk for UTIs and consequently acute pyelonephritis.[4]

Urinary tract infections (UTIs) also increase an individual’s susceptibility to acute pyelonephritis. Many of the bacteria associated with pyelonephritis are the same as those which can cause cystitis. In addition, women whose mothers have a history of UTIs may be more vulnerable to acute pyelonephritis.

Other factors that increase an individual’s risk of developing acute pyelonephritis include:[2][3]

  • Diabetes mellitus
  • Stress urinary incontinence
  • A structurally or functionally abnormal genitourinary tract, such as might result from infected cysts, underdeveloped kidneys, spinal cord injuries, or VUR.
  • Catheterisation
  • Stents or drainage procedures in or around the kidneys
  • Pregnancy
  • Biliary cirrhosis (a chronic condition where bile ducts in the liver are damaged)
  • Prostate enlargement
  • Immunocompromised states, such as might occur during chemotherapy.


The most distinctive features of acute pyelonephritis are pain in the back, flank or belly, and fever (at or above 38℃/100.4℉), chills, nausea and vomiting. If flank, belly or back pain is not present, doctors should consider other diagnoses. Other symptoms of pyelonephritis include:[1][2]

  • Urine that has an unusual or unpleasant smell
  • Hematuria (blood in the urine)
  • Urinary frequency
  • Urinary urgency
  • Dysuria (painful or difficult urination)
  • Oliguria (lack of urine)
  • Hypotension (low blood pressure).

In babies and toddlers, a high fever may be the only symptom. In men and women of older than 65 years of age, the above-mentioned symptoms may be absent and additional symptoms may include:

  • Confusion
  • Jumbled speech
  • Hallucinations

Complicated pyelonephritis

Some conditions increase the risk of acute pyelonephritis becoming complicated. In these case, hospitalisation may be required. These conditions include:[1]

  • Anatomical abnormalities of the genitourinary system
  • Multi-drug-resistant bacteria underlying the infection
  • A weakened immune system, for example as a result of chemotherapy or HIV/AIDS
  • Obstructions in the urethra, bladder or ureters
  • Use of inappropriate antibiotics as the first course of treatment
  • Underlying conditions such as diabetes mellitus, existing renal dysfunction, liver or cardiac disease, or urological disorders

Diagnosing acute pyelonephritis

Apart from noting the symptoms mentioned above, doctors examining a patient with suspected acute pyelonephritis will obtain a urine sample from them, and perform a basic test for abnormal color and odor. Additionally, they may perform tests such as:[2][5]

  • Dipstick analysis, which may be positive for blood, nitrite or leukocyte esterase. The latter is an enzyme produced by white blood cells.
  • Urinalysis (analysis of the urine) to check for white blood cell casts and microscopic pyuria, pus in the urine.
  • A bacterial culture of the urine to determine what kind of bacteria has caused the pyelonephritis. This may be particularly useful if the first line of antibiotic treatments does not clear up the infection, suggesting that the bacteria is drug-resistant or that another pathogen is responsible.
  • Blood tests such as a complete blood count should be performed in more complicated cases, along with analysis of inflammation markers. Markers that can indicate declining kidney function, and measurement of electrolytes may be necessary, too..
  • Imaging tests such as an MRI, CT scan, or ultrasound.

Doctors will also take a history from the patient to determine whether any underlying disorders (such as a genitourinary abnormality or diabetes mellitus) are present, as well as gather information about their lifestyle (for example, whether or not the patient is sexually active, experiences stress [urinary incontinence](/condition/urinary-incontinence, etc.) to find out what may have caused the infection to set in.


Pyelonephritis is treated with antibiotics. Simple pain-relieving medications (paracetamol, ibuprofen) are helpful in reducing pain. Drinking plenty of fluids can help flush bacteria from the kidneys and bladder. In severe cases, it might be necessary to receive intravenous antibiotics (through a drip) or have a surgical procedure to drain pus from around the kidneys.


Receiving treatment for cystitis (bladder infections) may help prevent some cases of pyelonephritis. Avoiding contraceptives that use spermicide might also be helpful. In people who suffer recurrent pyelonephritis, long-term antibiotics may help prevent new episodes.

Other names for acute pyelonephritis

  • kidney infection

  1. BMJ Best Practice. “Acute pyelonephritis”. November 2017. Accessed 12 March 2018.

  2. American Family Physician. “Diagnosis and Treatment of Acute Pyelonephritis in Women”. 1 September 2011. Accessed 12 March 2018.

  3. “Pyelonephritis”. 1 April 2016. Accessed 12 March 2018.

  4. Journal of General Internal Medicine. “Sexual Intercourse and Risk of Symptomatic Urinary Tract Infection in Post-Menopausal Women”. May 2008. Accessed 19 March 2018.

  5. National Institute for Diabetes and Digestive and Kidney Diseases. “Kidney Infection (Pyelonephritis)”. Accessed 12 March 2018.