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Can you please tell me about Pyelonephritis

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Pyelonephritis

Pyelonephritis is an infection of the upper urinary tract - specifically the renal parenchyma and pelvicalyceal system. It is differentiated from cystitis primarily by clinical findings: flank pain or costovertebral angle (CVA) tenderness, fever, and a positive urine culture (≥10⁵ CFU/mL), often with systemic symptoms like nausea and vomiting. - Tintinalli's Emergency Medicine

Classification

Type	Description
Acute pyelonephritis	Active bacterial infection of the kidney with neutrophilic inflammation
Chronic pyelonephritis	Recurrent or persistent infection leading to scarring, typically at the upper or lower poles; associated with reflux (reflux nephropathy) or outflow obstruction
Uncomplicated	In a healthy, non-pregnant woman with no structural abnormality
Complicated	In the presence of structural/functional abnormalities, comorbidities, pregnancy, male sex, or immunosuppression

Routes of Infection

Ascending route (more common) - bacteria travel from the bladder up the ureters to the kidney
Hematogenous spread - less common; seen in IV drug users, bacteremia from other sources

Obstructive lesions of the urinary tract are important predisposing factors. - Robbins, Cotran & Kumar Pathologic Basis of Disease

Causative Organisms

The most common organisms are:

Escherichia coli (most common, ~80% of cases)
Klebsiella species
Proteus species
Other Enterobacter species

Virulence factors of E. coli:

P fimbriae - bind specific receptors on renal epithelial cells; critically important for ascending to the kidney and causing bloodstream invasion
Type 1 pili - mediate binding to the bladder urothelium to initiate infection
Toxins, iron-acquisition systems, biofilm formation, and bacterial capsules also contribute
Harrison's Principles of Internal Medicine, 22E

Risk Factors & Predisposing Conditions

Female sex (shorter urethra allows easier bacterial ascent)
Sexual activity; new sexual partner
Prior UTIs, especially before age 15 or maternal history of UTI
Obstructive uropathy (stones, strictures, BPH)
Vesicoureteral reflux
Indwelling catheters or urinary tract instrumentation
Pregnancy (progesterone-induced ureteral dilation; progesterone accelerates E. coli growth)
Diabetes mellitus
Immunosuppression (HIV, chemotherapy, transplant)
Male sex (complicated category): suspect structural abnormality in any man with culture-proven UTI

Genetic susceptibility also plays a role - mutations in innate immune genes (Toll-like receptors, IL-8 receptor) are linked to recurrent pyelonephritis. - Harrison's

Clinical Features

Classic presentation:

Fever and chills (often high-grade)
Flank pain or costovertebral angle tenderness
Nausea and vomiting
Lower urinary tract symptoms (dysuria, frequency, urgency) - may or may not be present
The patient may appear toxic with poor oral intake

Special populations where presentation is subtle or atypical:

Spinal cord injury patients (do not feel pain normally)
Immunocompromised patients
Elderly patients
A missed diagnosis of cystitis is unlikely to cause deterioration; a missed pyelonephritis can progress to untreated sepsis

Approximately 10-19% of sepsis cases have a genitourinary source. - Tintinalli's Emergency Medicine

Pathology (Gross and Microscopic)

Acute pyelonephritis: prominent neutrophilic inflammatory response in the interstitium and tubules; bacteria are the most common infectious agent; granulomatous inflammation suggests fungal or mycobacterial infection
Chronic pyelonephritis: irregular cortical scarring, more prominent at the upper or lower poles (where reflux is most common); leads to contracted, scarred kidneys - Robbins Pathology

Diagnosis

Urinalysis:

Pyuria (WBCs in urine)
Bacteriuria
WBC casts - pathognomonic for renal parenchymal infection
Leukocyte esterase and nitrite positive

Urine culture: Required (send BEFORE starting antibiotics); diagnostic criterion is ≥10⁵ CFU/mL

Blood tests: Leukocytosis with elevated neutrophils; blood cultures if systemic illness

Imaging:

Not routinely needed for uncomplicated cases
Indicated if: no response to treatment in 72 hours, suspected obstruction, immunocompromised, or atypical presentation
CT scan is the gold standard to detect renal/perirenal abscess, emphysematous pyelonephritis, or obstruction
Ultrasound is used in pregnancy (avoids radiation) - Schwartz's Surgery / Tintinalli's

Treatment

Outpatient (Mild-Moderate, Uncomplicated)

Patients can be treated as outpatients if they can tolerate oral intake, have adequate pain control, and have no systemic toxicity.

Antibiotic	Dose	Duration
Ciprofloxacin (1st line where resistance <10%)	500 mg twice daily	7 days
Levofloxacin	750 mg once daily	5 days
TMP-SMX (if susceptible)	160/800 mg twice daily	10-14 days

Important: Nitrofurantoin and fosfomycin do NOT achieve adequate renal tissue and blood levels - they are contraindicated for pyelonephritis (useful only for cystitis). - Rosen's Emergency Medicine / Schwartz's Surgery

Inpatient (Severe/Complicated - IV Antibiotics)

Hospitalize if: signs of sepsis, inability to tolerate oral intake, high-resistance organism suspected, or no outpatient improvement.

Initial IV options include: ceftriaxone, cefepime, piperacillin-tazobactam, aztreonam, or fluoroquinolone - transition to oral therapy once afebrile for 24-48 hours, complete 10-14 days total. Aminoglycoside (e.g., gentamicin) as initial IV therapy is associated with better outcomes in high-resistance areas. - Goldman-Cecil Medicine

Duration

7-14 days is the standard recommendation; 5-7 days is adequate with ciprofloxacin or levofloxacin

Persistent Fever >72 hours

If fever or symptoms persist beyond 72 hours of appropriate antibiotics, cross-sectional imaging (CT) is warranted to rule out:

Renal abscess
Perinephric abscess
Emphysematous pyelonephritis These complications usually require percutaneous drainage + IV antibiotics. - Schwartz's Surgery

Special Situations

Pregnancy

Hospital admission is standard practice for pregnant women with pyelonephritis
Initial broad-spectrum IV antibiotics, then culture-guided oral therapy
Prophylactic antibiotics are recommended until delivery to prevent recurrence and risk of preterm labor
Ultrasound of the kidneys is performed to assess anatomy; if obstruction is confirmed, percutaneous nephrostomy may be needed - Comprehensive Clinical Nephrology

Emphysematous Pyelonephritis

A urologic emergency - necrotizing infection with gas-forming organisms (usually in diabetics). Treated urgently with IV antibiotics and often nephrectomy.

Complications

Urosepsis / septic shock (most serious acute complication)
Renal/perinephric abscess
Emphysematous pyelonephritis
Chronic pyelonephritis with progressive renal scarring
Renal failure (especially with obstruction or repeated infection)
Preterm labor in pregnancy

Recent Evidence (2024-2026)

Recent systematic reviews are refining treatment protocols:

A 2025 living meta-analysis (PMID: 40228579) compared short vs. long antibiotic courses - supporting shorter regimens for uncomplicated cases
A 2025 network meta-analysis (PMID: 39817442) evaluated treatment options for complicated UTI including pyelonephritis, supporting individualized antibiotic selection
A 2026 systematic review (PMID: 40742430) found procalcitonin and urinary NGAL are useful biomarkers for predicting acute pyelonephritis and kidney scarring in children with febrile UTI

Key Clinical Pearl: Always send a urine culture before starting antibiotics. Fevers may persist up to 72 hours despite correct treatment - this is normal. Beyond 72 hours without improvement demands urgent imaging to rule out abscess formation.

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