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Acute pyelonephritis is a bacterial infection causing inflammation of the kidneys and is one of the most common diseases of the kidney. Pyelonephritis occurs as a complication of an ascending urinary tract infection (UTI) which spreads from the bladder to the kidneys and their collecting systems. Symptoms usually include fever, flank pain, nausea, vomiting, burning on urination, increased frequency, and urgency. The 2 most common symptoms are usually fever and flank pain. Acute pyelonephritis can be divided into uncomplicated and complicated. Complicated pyelonephritis includes pregnant patients, patients with uncontrolled diabetes, kidney transplants, urinary anatomical abnormalities, acute or chronic kidney failure, as well as immunocompromised patients and those with hospital-acquired bacterial infections. It is important to make a distinction between complicated and uncomplicated pyelonephritis, as patient management and disposition depend on it.The main cause of acute pyelonephritis is gram-negative bacteria, the most common being Escherichia coli. Other gram-negative bacteria which cause acute pyelonephritis include Proteus, Klebsiella, and Enterobacter. In most patients, the infecting organism will come from their fecal flora. Bacteria can reach the kidneys in 2 ways: hematogenous spread and through ascending infection from the lower urinary tract. Hematogenous spread is less common and usually occurs in patients with ureteral obstructions or immunocompromised and debilitated patients. Most patients will get acute pyelonephritis through ascending infection. Ascending infection happens through several steps. Bacteria will first attach to urethral mucosal epithelial cells and will then travel to the bladder via the urethra either through either instrumentation or urinary tract infections which occur more frequently in females. UTIs are more common in females than in males due to shorter urethras, hormonal changes, and close distance to the anus. Urinary tract obstruction caused by something such as a kidney stone can also lead to acute pyelonephritis. An outflow obstruction of urine can lead to incomplete emptying and urinary stasis which causes bacteria to multiply without being flushed out. A less common cause of acute pyelonephritis is vesicoureteral reflux, which is a congenital condition where urine flows backward from the bladder into the kidneys.


Symptoms usually appear within two days of infection. Common symptoms include:

a fever greater than 102°F (38.9°C)

pain in the abdomen, back, side, or groin

painful or burning urination

cloudy urine

pus or blood in the urine

urgent or frequent urination

fishy-smelling urine

Other symptoms can include:

shaking or chills



general aching or ill feeling


moist skin

mental confusion

Symptoms may be different in children and older adults than they are in other people. For example, mental confusion is common in older adults and is often their only symptom.


The main cause of acute pyelonephritis is gram-negative bacteria, the most common being Escherichia coli. Other gram-negative bacteria which cause acute pyelonephritis include Proteus, Klebsiella, and Enterobacter. In most patients, the infecting organism will come from their fecal flora.

Risk factors

Acute pyelonephritis

Any problem that interrupts the normal flow of urine causes a greater risk of acute pyelonephritis. For example, a urinary tract that’s an unusual size or shape is more likely to lead to acute pyelonephritis.

Also, women’s urethras are much shorter than men’s, so it’s easier for bacteria to enter their bodies. That makes women more prone to kidney infections and puts them at a higher risk of acute pyelonephritis.

Other people who are at increased risk include:

anyone with chronic kidney stones or other kidney or bladder conditions

older adults

people with suppressed immune systems, such as people with diabetes, HIV/AIDS, or cancer

people with vesicoureteral reflux (a condition where small amounts of urine back up from the bladder into the ureters and kidneys)

people with an enlarged prostate

Other factors that can make you vulnerable to infection include:

catheter use

cystoscopic examination

urinary tract surgery

certain medications

nerve or spinal cord damage

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A possible complication of acute pyelonephritis is chronic kidney disease. If the infection continues, the kidneys may be permanently damaged. Although rare, it’s also possible for the infection to enter the bloodstream. This can result in a potentially deadly infection called sepsis.

Other complications include:

recurring kidney infections

the infection spreading to areas around the kidneys

acute kidney failure

kidney abscess


Kidney infections can be prevented by keeping bacteria out of the urinary tract and bladder. Infections in the kidney often start as a lower tract infection in the bladder. By preventing these infections, you may be able to prevent kidney infections.

There are several ways to avoid infection and keep your kidneys healthy. These tips can include:

Hydration: It is important to drink adequate fluid (not only water) every day but there is no specific recommendation that applies to every patient.. The amount can change if you have certain medical conditions or live in a particularly hot climate. Talk to your healthcare provider about the recommended amount of water you need each day.

Urinate completely: When you need to urinate, empty your bladder completely. Holding in your urine can be harmful and can promote the growth of bacteria. Urinating every several hours can help to flush any bacteria out of your body, protecting it from an infection.

Urinate after having sex: Urinating after sex helps to remove any bacteria that may have gotten into the body. Men and women should both do this to prevent infections.

Practice good hygiene: Keeping yourself clean can help protect your body from infections. After a bowel movement, wipe from front to back to push bacteria away from the urethra (an opening in the body). This is especially important for women.