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Kidney Infection



Talha H. Imam

, MD, University of Riverside School of Medicine

Last full review/revision Jul 2021| Content last modified Jul 2021
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Pyelonephritis is a bacterial infection of one or both kidneys.

  • Infection can spread up the urinary tract to the kidneys, or uncommonly the kidneys may become infected through bacteria in the bloodstream.

  • Chills, fever, back pain, nausea, and vomiting can occur.

  • Urine and sometimes blood and imaging tests are done if doctors suspect pyelonephritis.

  • Antibiotics are given to treat the infection.

Causes of Kidney Infection

Pyelonephritis is more common among women than men. Escherichia coli, a type of bacteria normally in the large intestine, causes about 90% of cases of pyelonephritis among people who are not hospitalized or living in a nursing home. Infections usually ascend from the genital area through the urethra to the bladder, up the ureters, into the kidneys. In a person with a healthy urinary tract, an infection is usually prevented from moving up the ureters into the kidneys by the flow of urine washing organisms out and by closure of the ureters at their entrance to the bladder. However, any physical blockage (obstruction) to the flow of urine, such as a structural abnormality, kidney stone, or an enlarged prostate gland, or the backflow (reflux) of urine from the bladder into the ureters increases the likelihood of pyelonephritis.

The risk of pyelonephritis is increased during pregnancy. During pregnancy, the enlarging uterus puts pressure on the ureters, which partially obstructs the normal downward flow of urine. Pregnancy also increases the risk of reflux of urine up the ureters by causing the ureters to dilate and reducing the muscle contractions that propel urine down the ureters into the bladder. Occasionally, a catheter that remains in the bladder can cause pyelonephritis by allowing bacteria to enter or remain in the bladder.

In about 5% of cases, infections are carried to the kidneys from another part of the body through the bloodstream. For instance, a staphylococcal skin infection can spread to the kidneys through the bloodstream.

The risk and severity of pyelonephritis are increased in people with diabetes or a weakened immune system (which reduces the body's ability to fight infection). Pyelonephritis is usually caused by bacteria, but rarely it is caused by tuberculosis (a rare bacterial cause of pyelonephritis), fungal infections, and viruses.

Some people develop long-standing infection (chronic pyelonephritis). Almost all of them have significant underlying abnormalities, such as a urinary tract obstruction, large kidney stones that persist, or, more commonly, reflux of urine from the bladder into the ureters (which occurs mostly in young children). Chronic pyelonephritis can cause bacteria to be released into the bloodstream, sometimes resulting in infections in the opposite kidney or elsewhere in the body. Rarely, chronic pyelonephritis can eventually severely damage the kidneys.

Symptoms of Kidney Infection

Symptoms of pyelonephritis often begin suddenly with chills, fever, pain in the lower part of the back on either side, nausea, and vomiting.

About one third of people with pyelonephritis also have symptoms of cystitis (bladder infection), including frequent, painful urination. One or both kidneys may be enlarged and painful, and doctors may find tenderness in the small of the back on the affected side. Sometimes the muscles of the abdomen are tightly contracted. Irritation from the infection or the passing of a kidney stone (if one is present) can cause spasms of the ureters. If the ureters go into spasms, people may experience episodes of intense pain (renal colic). In children, symptoms of a kidney infection often are slight and more difficult to recognize. In older people, pyelonephritis may not cause any symptoms that seem to indicate a problem in the urinary tract. Instead, older people may have a decrease in mental function (delirium or confusion), fever, or an infection of the bloodstream (sepsis).

In chronic pyelonephritis, the pain may be vague, and fever may come and go or not occur at all.

Diagnosis of Kidney Infection

  • Urinalysis

  • Urine culture

  • Sometimes imaging tests

The typical symptoms of pyelonephritis lead doctors to do two common laboratory tests to determine whether the kidneys are infected: (1) examination of a urine specimen under a microscope to count the number of red and white blood cells and bacteria and (2) a urine culture, in which bacteria from a urine sample are grown in a laboratory to identify the numbers and type of bacteria (see also Urinalysis and Urine Culture). Blood tests may be done to check for elevated white blood cell levels (suggesting infection), bacteria in the blood, or kidney damage.

Imaging tests are done in people who have intense back pain typical of renal colic, in those who do not respond to antibiotic treatment within 72 hours, in those whose symptoms return shortly after antibiotic treatment is finished, in those with long-standing or recurring pyelonephritis, in those whose blood test results indicate kidney damage, and in men (because they so rarely develop pyelonephritis). Ultrasonography or helical (spiral) computed tomography (CT) studies done in these situations may reveal kidney stones, structural abnormalities, or other causes of urinary obstruction.

Prognosis of Kidney Infection

Most people recover fully. Delayed recovery and the chance of complications are more likely if the person needs hospitalization, the infecting organism is resistant to commonly used antibiotics, or the person has a disorder that weakens the immune system (such as certain cancers, diabetes mellitus, or AIDS) or a kidney stone.

Prevention and Treatment of Kidney Infection

  • Antibiotics

  • Occasionally surgery (to correct abnormality of urinary tract)

Antibiotics are started as soon as the doctor suspects pyelonephritis and samples have been taken for laboratory tests. The choice of drug or its dosage may be modified based on the laboratory test results (including culture results), how sick the person is, and whether the infection started in the hospital, where bacteria tend to be more resistant to antibiotics. Other factors that can alter the choice or dosage of drug include whether the person's immune system is impaired and whether the person has a urinary tract abnormality (such as an obstruction).

Outpatient treatment with antibiotics given by mouth is usually successful if the person has:

  • No nausea or vomiting

  • No signs of dehydration

  • No other disorders that weaken the immune system, such as certain cancers, diabetes mellitus, or AIDS

  • No signs of very severe infection, such as low blood pressure or confusion

  • Pain that is controlled with drugs taken by mouth

Otherwise, the person is usually treated initially in the hospital. If hospitalization is needed and the person needs antibiotics, the antibiotics are given intravenously for 1 or 2 days, then they can usually be given by mouth.

Antibiotic treatment of pyelonephritis is given for 5 to 14 days so that infection will not recur. However, antibiotic therapy may continue for up to 6 weeks for men in whom the infection is due to prostatitis, which is more difficult to eradicate. A final urine sample is usually taken shortly after the antibiotic treatment is finished to make sure the infection has been eradicated.

Surgery is necessary only occasionally if tests show that something is chronically blocking the urinary tract, such as a structural abnormality or a particularly large stone. Removal of the infected kidney may be necessary for people with chronic pyelonephritis who are about to undergo kidney transplantation. Spread of infection to the transplanted kidney is particularly risky because the person takes immunosuppressant drugs, which prevent rejection of the transplanted kidney but also weaken the body's ability to fight infection.

People who have frequent episodes of pyelonephritis or whose infection returns after antibiotic treatment is finished may be advised to take a small dose of antibiotic on a long-term basis. The ideal duration of such therapy is unknown. If the infection returns, preventive therapy may be continued indefinitely. If a woman of child-bearing age is taking an antibiotic, she should avoid pregnancy or talk to her doctor about whether to use an antibiotic that is safe during pregnancy in case she becomes pregnant.

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Test your knowledge

Acute Tubular Necrosis
Acute tubular necrosis is a type of kidney injury caused by damage to the kidney tubule cells (kidney cells that reabsorb fluid and minerals from urine). Which of the following groups is most likely to develop acute tubular necrosis?
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