Typical symptoms

• Fever or chills
• Pain in the side or back, below the ribs (flank pain)
• Nausea or vomiting
• Burning with urination, urgency, or frequent urination
• Cloudy, bloody, or foul-smelling urine
• Tenderness when the back over the kidney is examined

Cause and risk factors

• Pregnancy
• Kidney stones or urinary blockage
• Urinary catheter or recent urinary procedure
• Vesicoureteral reflux or structural urinary-tract problems
• Diabetes, impaired immunity, or a kidney transplant
• Recurrent UTIs

How it is diagnosed

• Urinalysis: may show white blood cells, bacteria, nitrites, or blood
• Urine culture and antibiotic-susceptibility testing, ideally collected before antibiotics
• Blood tests if illness is more severe
• Imaging is not routine for a straightforward case. A CT scan or ultrasound may be needed if there is sepsis, suspected obstruction or stone, renal impairment, atypical features, or no improvement within 48 to 72 hours. The AAFP evidence review outlines these indications.

Treatment

• People who are stable, can drink fluids, and have uncomplicated illness may often be treated at home with oral antibiotics and close follow-up.
• People with vomiting, dehydration, sepsis, urinary obstruction, pregnancy, unstable medical conditions, or inability to take oral medication often require hospital care, IV fluids, and IV antibiotics.
• If a stone or other obstruction is infected, it may require urgent drainage in addition to antibiotics.

Complications

• Bloodstream infection and sepsis
• Kidney or perinephric abscess
• Obstructed infected kidney
• Renal scarring, especially with recurrent infection or reflux
• In rare severe cases, emphysematous pyelonephritis, particularly in people with diabetes

Pregnancy

Seek urgent care now if

Clinical and test findings in acute pyelonephritis

Clinical findings

• Fever usually >38°C, chills or rigors
• Flank or loin pain, often unilateral
• Costovertebral-angle tenderness on percussion or palpation, often called renal-angle tenderness
• Nausea, vomiting, malaise, anorexia
• May also have lower-UTI symptoms: dysuria, urgency, frequency, suprapubic discomfort
• More severe illness: tachycardia, hypotension, confusion, or signs of sepsis

Urine tests

Blood tests

• CBC: leukocytosis, typically neutrophil predominance or left shift
• CRP and ESR: often elevated but nonspecific
• Serum creatinine and electrolytes: check for acute kidney injury, dehydration, or obstruction
• Blood cultures: not routine in simple cases, but appropriate with sepsis, immunosuppression, uncertain diagnosis, or severe illness

Imaging findings

• Suspected stone or urinary obstruction
• Sepsis or severe illness
• Poor response to appropriate antibiotics after 48 to 72 hours
• Recurrent infection, known structural urinary-tract abnormality, or reduced renal function

Important caveat

Culture findings that support the diagnosis

• A predominant single uropathogen, most often E. coli
• From a properly collected midstream clean-catch specimen, ≥10,000 CFU/mL (10⁴ CFU/mL) with compatible clinical features is a commonly used diagnostic threshold for acute pyelonephritis.
• ≥100,000 CFU/mL (10⁵ CFU/mL) is strongly supportive and is the traditional threshold. About 80% of women with pyelonephritis have counts above this level.
• Lower counts, roughly 10³ to <10⁴ CFU/mL, can still be meaningful in symptomatic patients, especially men, pregnant patients, patients already exposed to antibiotics, or when the specimen was collected by catheter. They should not be dismissed automatically.

Results that are less convincing

• Mixed growth of several organisms often indicates contamination, especially from a clean-catch sample. A repeat culture may be needed.
• No growth does not completely exclude pyelonephritis if antibiotics were taken before the sample or collection was inadequate.
• Bacteriuria without urinary symptoms or systemic signs may be asymptomatic bacteriuria, not pyelonephritis.