Also known as: acute pyelonephritis, upper UTI, kidney infection
Overview
Bacterial infection of the renal parenchyma and collecting system, typically resulting from ascending lower urinary tract infection. Characterized by fever, flank pain, costovertebral angle (CVA) tenderness, and pyuria, often with bacteremia.
Epidemiology
Incidence ~250,000 cases/year in the US. More common in women (especially aged 15-29), but more severe in men, the elderly, pregnant women, and diabetics. Hospital admission required in ~20%.
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Question 1RenalMedium
A 58-year-old diabetic woman presents with 3 days of urgency, frequency, and flank pain. Temperature is 38.8°C, HR 104. Urinalysis shows large leukocyte esterase, nitrites, many WBCs, and WBC casts. Urine culture grows E. coli, and local fluoroquinolone resistance among E. coli is below 10%. She is hemodynamically stable and tolerating oral intake. What is the most appropriate treatment?
AOral TMP-SMX DS for 3 days
BOral ciprofloxacin for 7 days
COral moxifloxacin for 7 days
DOral amoxicillin for 7 days
EOral nitrofurantoin for 5 days
Reveal answer & full explanation
Correct answer: B — Oral ciprofloxacin for 7 days
AOral TMP-SMX DS for 3 days
BOral ciprofloxacin for 7 days✓
COral moxifloxacin for 7 days
DOral amoxicillin for 7 days
EOral nitrofurantoin for 5 days
Why Oral ciprofloxacin for 7 days is correct
Pyelonephritis is an upper urinary tract infection (UTI) characterized by flank pain, fever, and WBC casts (pathognomonic of renal origin)
Mild-to-moderate pyelonephritis in a hemodynamically stable outpatient tolerating oral intake is treated with oral ciprofloxacin for 7 days
Trimethoprim-sulfamethoxazole (TMP-SMX) for 7 days is an alternative if the organism is susceptible
Why the others are wrong
Oral nitrofurantoin for 5 days — nitrofurantoin does not achieve adequate renal tissue concentrations and is not appropriate for pyelonephritis; it is used only for uncomplicated lower UTIs
Oral TMP-SMX DS for 3 days — a 3-day course of double-strength (DS) TMP-SMX is the regimen for uncomplicated cystitis; pyelonephritis needs a longer course (7 days of TMP-SMX when the isolate is susceptible), so 3 days risks treatment failure and relapse
Oral amoxicillin for 7 days — aminopenicillins such as amoxicillin have high E. coli resistance rates and poor empiric reliability for pyelonephritis
Oral moxifloxacin for 7 days — a fluoroquinolone like ciprofloxacin, so the low local fluoroquinolone resistance makes it look equivalent, but moxifloxacin is eliminated largely by nonrenal routes and reaches low urinary concentrations, so it is not recommended for UTI or pyelonephritis; ciprofloxacin and levofloxacin are the fluoroquinolones used for urinary infections
Additional high-yield points
If local fluoroquinolone resistance exceeds 10%, give a one-time dose of a long-acting parenteral agent such as ceftriaxone 1 g IV before starting the oral course
Question 2RenalMedium
A 65-year-old man with diabetes has fever, flank pain, and pyuria. CT shows gas in the renal parenchyma. Which of the following is the most likely diagnosis?
AEmphysematous pyelonephritis
BObstructive pyonephrosis
CXanthogranulomatous pyelitis
DClear cell renal carcinoma
EAcute renal papillary necrosis
Reveal answer & full explanation
Correct answer: A — Emphysematous pyelonephritis
AEmphysematous pyelonephritis✓
BObstructive pyonephrosis
CXanthogranulomatous pyelitis
DClear cell renal carcinoma
EAcute renal papillary necrosis
Why Emphysematous pyelonephritis is correct
Gas within the renal parenchyma comes from gas-forming organisms (usually E. coli) fermenting glucose in poorly perfused diabetic tissue.
Diabetes is the dominant risk factor and is present here.
Fever, flank pain, pyuria, and parenchymal gas are the classic tetrad.
Why the others are wrong
Clear cell renal carcinoma — RCC gives a solid mass with hematuria, not acute pyuria with gas; this is an imaging-mass trap.
Acute renal papillary necrosis — Papillary necrosis sloughs papillae causing hematuria and flank pain but produces no gas.
Xanthogranulomatous pyelitis — Xanthogranulomatous disease is a chronic staghorn-associated lipid-laden mass, not an acute gas-forming infection.
Obstructive pyonephrosis — fever, flank pain, and pyuria in a diabetic also fit an infected, obstructed collecting system, but pyonephrosis appears as hydronephrosis with high-attenuation fluid or a fluid-debris level, not as gas within the renal parenchyma; an obstructed-infection trap.
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Uropathogens (E. coli causes ~75-90%) ascend from the bladder via the ureters to the renal pelvis and parenchyma. P-fimbriae allow attachment to uroepithelial cells of the upper tract. Inflammatory response with neutrophil infiltration causes parenchymal damage, microabscesses, and systemic inflammatory response with fever and bacteremia. Obstruction worsens prognosis by impairing antibiotic delivery and promoting abscess formation.
Clinical presentation
Symptoms
Fever (often >38.5°C), chills, rigors
Flank or back pain (unilateral or bilateral)
Nausea and vomiting
Lower UTI symptoms — dysuria, frequency, urgency — preceding or accompanying upper tract symptoms
Malaise, fatigue, anorexia
Elderly: may present atypically with confusion, falls, or simply nonspecific decline
Signs / physical exam
Fever, tachycardia; hypotension in severe cases (urosepsis)
Costovertebral angle (CVA) tenderness — classic finding on percussion
Urinalysis with microscopy — pyuria, bacteriuria, WBC casts (specific for upper tract), nitrites, leukocyte esterase
Urine culture and susceptibility — ALWAYS obtain (vs cystitis where culture is optional)
Blood cultures × 2 if septic, hospitalized, immunocompromised, pregnant, or diagnosis uncertain; not routinely needed in mild outpatient cases
CBC — leukocytosis with left shift
BMP — assess renal function, electrolytes
Lactate, procalcitonin if sepsis suspected
Beta-hCG in reproductive-age women
Imaging
Imaging not required in most uncomplicated pyelonephritis with prompt improvement
CT abdomen/pelvis with contrast if: failure to improve after 48-72 h of appropriate antibiotics, sepsis, suspected obstruction or abscess, recurrent pyelonephritis, atypical course, history of stones, men
Renal ultrasound — first-line in pregnancy and children, or if CT contrast contraindicated
Children <2 years (the AAP 2026 guideline covers 8 days–5 years) with a first febrile UTI (presumed pyelonephritis): renal and bladder ultrasound; VCUG is not routine and is reserved for an abnormal ultrasound or recurrent febrile UTI, to detect vesicoureteral reflux (risk of renal scarring, hypertension, CKD)
Diagnostic algorithm
Feature
Cystitis
Pyelonephritis
Anatomic site
Bladder
Renal parenchyma + pelvis
Fever / chills
Absent
Present
Flank pain / CVA tenderness
Absent
Present
Systemic illness
Absent
Often present
WBC casts
Absent
Present (specific)
Urine culture
Optional
Required
Blood cultures
Not indicated
If septic, hospitalized, or high risk
Imaging
Not required
If failure to improve, sepsis, obstruction suspected
Outpatient (mild, hemodynamically stable, tolerating oral, no obstruction):
Fluoroquinolone (pyelo) — ciprofloxacin 500 mg PO BID × 7 days or levofloxacin 750 mg PO daily × 5 days — first-line if local resistance <10%
TMP-SMX DS PO BID × 7 days if the isolate is susceptible (avoid empiric use if susceptibility unknown); IDSA 2025: 5-7 days of a fluoroquinolone or 7 days of another active agent is enough for patients who are improving
Consider initial single dose of long-acting IV antibiotic (ceftriaxone 1 g IV, ertapenem 1 g IV) followed by oral therapy
Avoid nitrofurantoin and fosfomycin (do NOT achieve adequate parenchymal levels for pyelonephritis)
Ceftriaxone 1-2 g IV daily, or piperacillin-tazobactam, or carbapenem (meropenem, imipenem, ertapenem) if ESBL risk
Add ampicillin if Enterococcus suspected (or empiric if catheter-related)
Aminoglycoside (gentamicin) addition in critically ill or if pseudomonal coverage needed
Transition to oral once culture susceptibilities available and patient improving; total course about 7 days (5-7 with a fluoroquinolone), including with bacteremia (IDSA 2025); longer or individualized for pregnancy (see below), severe sepsis, abscess, undrained obstruction or slow response
Second-line / adjunct
Drain obstruction if present — percutaneous nephrostomy or ureteral stent — urgent if obstructed infected system
Drain abscess if >3-5 cm — percutaneous drainage
Aggressive IV fluid resuscitation, vasopressors as needed for septic shock
Antipyretics and analgesics
Pregnancy: hospitalize for IV antibiotics; ceftriaxone, ampicillin-gentamicin, or cefepime; total 10-14 days
Reassess at 48-72 h — failure to improve warrants imaging to exclude obstruction or abscess
Complications
Urosepsis with multi-organ dysfunction
Renal/perinephric abscess
Emphysematous pyelonephritis — gas in renal parenchyma; almost exclusively in diabetics; surgical emergency
WBC casts in the urine are specific for upper tract infection (pyelonephritis), not cystitis.
Fever in a patient with a urinary stone = obstructed infected system → urgent decompression (stent or percutaneous nephrostomy) before definitive stone treatment.
Emphysematous pyelonephritis: gas in renal parenchyma on CT, primarily in diabetics; high mortality without prompt drainage and broad-spectrum antibiotics.
Pregnancy pyelonephritis ALWAYS warrants hospitalization and IV antibiotics — high risk of preterm labor and sepsis.
Always image (CT) if patient does not improve within 48-72 hours of appropriate antibiotics to exclude obstruction or abscess.
In men, treat 7-14 days and evaluate for prostatitis (digital rectal exam, prolonged course if prostatic involvement).
References
IDSA/ESCMID 2011 — International Clinical Practice Guidelines for the Treatment of Acute Uncomplicated Cystitis and Pyelonephritis in Women: A 2010 Update by IDSA and ESCMID (Gupta et al., Clin Infect Dis 2011)
EAU 2024 — European Association of Urology Guidelines on Urological Infections 2024
IDSA 2025 — Clinical Practice Guidelines by IDSA: 2025 Guidelines on Management and Treatment of Complicated Urinary Tract Infections (Trautner et al., Clin Infect Dis 2026;82 Suppl 3)
AAP 2026 — Clinical Practice Guideline for the Diagnosis and Treatment of Urinary Tract Infection in Children From 8 Days to 5 Years of Age (AAP, Pediatrics, Oct 2026)
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