Most urinary tract infections (UTIs) involve only the bladder and urethra (the lower urinary system). Pyelonephritis results when a UTI progresses to involve the upper urinary system (the kidneys and ureters).

Pyelonephritis is a potentially serious kidney infection that can spread to the blood, causing severe illness. Fortunately, pyelonephritis is almost always curable with antibiotics.

The kidneys filter the blood to produce urine. Two tubes called the ureters carry urine from the kidneys down to the bladder. Urine travels from the bladder out of the body through the urethra.

Most cases of pyelonephritis are complications of common bladder infections. Bacteria enter the body from the skin around the urethra. They then travel up the urethra to the bladder.

Sometimes, bacteria escape the bladder and urethra, traveling up the ureters to one or both kidneys. The resulting kidney infection is called pyelonephritis.

The urethra is much shorter in women than in men, which is one reason why women are more vulnerable to UTIs and pyelonephritis.
Symptoms of Pyelonephritis
At least half of women have experienced the discomfort with urination caused by a urinary tract infection: painful, urgent, or frequent urination.

Pyelonephritis may start with similar symptoms. However, once the infection has spread to the kidney, signs of more severe illness usually result:

Back pain or flank pain
Fever (usually present) and/or chills
Feeling sick (malaise)
Nausea and vomiting
Confusion (especially in the elderly)
Pyelonephritis may cause noticeable changes in the urine, such as:

Blood in the urine (hematuria)
Cloudy or foul-smelling urine
Pain when urinating
Increased frequency or urgency of urination

Causes of Pyelonephritis
Most often, the bacteria that cause pyelonephritis are the same as those that cause ordinary urinary tract infections. Bacteria found in stool (such as E. coli or klebsiella) are most common. Uncommonly, bacteria from the skin or the environment cause pyelonephritis.

Conditions that create reduced urine flow make pyelonephritis more likely. When urine flow slows or stops, bacteria can more easily travel up the ureters. Some causes of urine obstruction include:

Benign prostatic hypertrophy (BPH)
Abdominal or pelvic masses (as from cancer)
Stones in the bladder, ureters, or kidneys
Kidney stones contribute to pyelonephritis by providing a place for bacteria to grow while evading the body's defenses.
People with diabetes or conditions that impair the immune system are more likely to get pyelonephritis.

How Pyelonephritis Is Diagnosed
Doctors may rely on various tests to diagnose pyelonephritis:

History. Telling the story of your illness and specific symptoms helps a doctor make the diagnosis of pyelonephritis.

Physical examination . A doctor notes a person's general appearance, vital signs, and presses over the kidneys to check for tenderness.

Urinalysis. In pyelonephritis, microscopic analysis of the urine virtually always shows signs of infection. This can include an excess of white blood cells and bacteria.