Pyelonephritis (Kidney Infection)

Overview

Pyelonephritis is a bacterial infection of one or both kidneys representing the most serious form of urinary tract infection. The condition occurs when bacteria ascend from the bladder through the ureters to infect the kidney tissue directly. Without prompt treatment, pyelonephritis can cause permanent kidney damage, sepsis, and potentially life-threatening complications.

Kidney infections affect approximately 250,000 Americans annually and are more common in women than men due to the shorter female urethra, which allows bacteria easier access to the urinary tract. Men with prostate problems and anyone with urinary tract abnormalities face increased risk.

Pyelonephritis presents in two main forms. Acute pyelonephritis develops suddenly with rapid onset of symptoms and typically responds well to antibiotic therapy. Chronic pyelonephritis involves recurring or persistent infections that cause progressive kidney damage, scarring, and reduced function over time. The condition is further classified as uncomplicated when occurring in otherwise healthy individuals with normal urinary tract anatomy, or complicated when structural abnormalities, kidney stones, diabetes, pregnancy, catheter use, or immunosuppression are present.

Symptoms

The hallmark symptoms of pyelonephritis distinguish it from simple bladder infections. High fever, often exceeding 38.5°C (101.3°F), accompanied by shaking chills signals that infection has reached the kidneys. Flank pain—a distinctive aching discomfort in the side and back below the ribs—is characteristic of kidney involvement. Costovertebral angle tenderness, pain elicited when the back over the kidney area is tapped, helps clinicians confirm the diagnosis. Nausea and vomiting commonly accompany severe infections.

Most patients also experience lower urinary tract symptoms including painful urination (dysuria), frequent urination, urgency, and a sensation of incomplete bladder emptying. Urine may appear cloudy or murky, have a strong or foul odor, or contain visible blood (hematuria) or pus.

Systemic symptoms include fatigue, weakness, loss of appetite, and general malaise. In elderly patients, confusion or altered mental status may be the only presenting sign, with fever absent or mild and classic symptoms less pronounced. Children may present with poor feeding, irritability, new-onset bedwetting, or abdominal pain rather than typical adult symptoms. Pregnant women may have subtle symptoms but require immediate treatment to prevent complications affecting both mother and fetus.

Causes

Escherichia coli (E. coli) bacteria cause 80-90% of kidney infections. Other causative organisms include Klebsiella species, Proteus mirabilis (more common in men and catheter users), Enterococcus species, and Staphylococcus saprophyticus (common in young women). Most infections occur through ascending spread—bacteria enter the urethra, travel to the bladder causing cystitis, then continue up the ureters to reach the kidneys. Rarely, bacteria spread to the kidneys through the bloodstream from another infection source, a pattern more common with Staphylococcus aureus.

Risk Factors

Several anatomical factors increase pyelonephritis risk. Female anatomy with its shorter urethra allows bacteria easier access to the urinary tract. Vesicoureteral reflux, where urine flows backward from the bladder toward the kidneys, carries bacteria upward with it. Urinary tract obstruction from kidney stones, enlarged prostate, or structural abnormalities creates stagnant urine where bacteria multiply. Urinary tract malformations present from birth also predispose to recurrent infections.

Medical conditions that impair immune function or bladder emptying significantly increase risk. Diabetes mellitus, immunosuppressive medications, and conditions causing weakened immunity allow infections to take hold more readily. Neurogenic bladder and other voiding disorders lead to incomplete bladder emptying and urinary stasis. Previous kidney infections increase the likelihood of recurrence, as does chronic kidney disease.

Behavioral and situational factors include urinary catheter use, recent urologic procedures, sexual activity in women, pregnancy, and spermicide use. Indwelling catheters provide a direct route for bacteria to enter the urinary tract, while pregnancy hormones and the enlarging uterus can impair urine drainage.

Diagnosis

Clinicians diagnose pyelonephritis through history, physical examination, and laboratory testing. The evaluation begins with assessment of symptoms, their duration, and vital signs including temperature. Physical examination includes testing for costovertebral angle tenderness and abdominal examination to assess for other causes of pain.

Urinalysis reveals white blood cells (pyuria), red blood cells, bacteria, and typically positive nitrites and leukocyte esterase. Urine culture identifies the specific bacteria causing infection and determines which antibiotics will be effective, guiding treatment decisions. Blood tests include complete blood count showing elevated white cells, kidney function tests (creatinine and BUN) to assess kidney involvement, and inflammatory markers. Blood cultures are obtained when sepsis is suspected.

Imaging studies are not routinely needed for uncomplicated pyelonephritis but become important when symptoms fail to improve with antibiotics, obstruction or abscess is suspected, infections recur, or the patient is male, diabetic, or immunocompromised. CT scan with contrast is the gold standard for complicated cases, detecting abscesses, stones, obstruction, and the extent of kidney involvement. Ultrasound serves as first-line imaging in pregnancy, detecting hydronephrosis and stones without radiation exposure. Voiding cystourethrogram evaluates for vesicoureteral reflux in children with recurrent infections.

Treatment

Treatment intensity depends on infection severity and patient factors. Uncomplicated pyelonephritis in otherwise healthy patients who can tolerate oral medications, are not vomiting, and have no underlying conditions can be managed at home with oral antibiotics for 7-14 days. Common oral options include fluoroquinolones (ciprofloxacin, levofloxacin), trimethoprim-sulfamethoxazole, or third-generation cephalosporins. Supportive care includes adequate fluid intake, pain relievers such as acetaminophen or NSAIDs, and rest.

Hospitalization is necessary for patients with high fever and severe symptoms, persistent vomiting, pregnancy, suspected sepsis, obstruction or abscess, significant underlying conditions, or failure to improve with outpatient treatment. Intravenous antibiotics are started immediately after cultures are obtained and adjusted based on culture results. Patients transition to oral antibiotics once symptoms improve. Additional interventions may include IV fluids for hydration, drainage of obstruction, or drainage of kidney abscess if present.

Treatment duration varies by severity: 7-14 days for uncomplicated infections, 14-21 days or longer for complicated cases. Patients with recurrent UTIs may require suppressive antibiotic therapy. Follow-up includes repeat urine culture after completing treatment, imaging if the infection was complicated or recurrent, and evaluation for underlying causes that might predispose to future infections.

When to Seek Emergency Care

Pyelonephritis can progress to life-threatening sepsis rapidly, making prompt recognition essential. Seek immediate medical attention for high fever above 39°C (102°F) with shaking chills, severe flank or back pain, persistent vomiting with inability to keep fluids down, or visible blood in urine.

Signs of sepsis require emergency evaluation: rapid heart rate, rapid breathing, confusion or disorientation, extreme weakness, and skin that appears cold, clammy, or mottled. Other urgent situations include symptoms that fail to improve after 48-72 hours of antibiotic treatment, decreased urine output suggesting kidney dysfunction, and any UTI symptoms during pregnancy.

Early treatment prevents serious complications including kidney abscess, permanent kidney scarring, chronic kidney disease, and sepsis. If you experience symptoms suggesting kidney infection, particularly fever combined with flank pain, contact your healthcare provider promptly rather than waiting to see if symptoms resolve on their own.