How pyelonephritis is tested on Step 1
Expect fever and flank pain after cystitis symptoms, a child with reflux, or a diabetic with a severe or gas-forming infection.
Acute vs chronic pyelonephritis
Chronic disease results from repeated or persistent infection with scarring.
| Finding | Pyelonephritis | Cystitis | Interstitial nephritis |
|---|---|---|---|
| Fever, flank pain | Yes | No | Fever possible; rash |
| WBC casts | Yes | No | Yes |
| Positive urine culture | Yes | Yes | No (sterile pyuria) |
| Eosinophilia | No | No | Often (drug-induced) |
WBC casts localize inflammation to the kidney; culture separates infection from drug-induced nephritis.
Complications
Diabetes, obstruction, and sickle cell disease predispose to severe forms.
| Variant | Key features |
|---|---|
| Papillary necrosis | Gross hematuria and flank pain; sloughed papillae may obstruct the ureter |
| Emphysematous pyelonephritis | Gas-forming infection (often E. coli, Klebsiella) in poorly controlled diabetes; high mortality |
| Xanthogranulomatous pyelonephritis | Chronic obstruction with staghorn stone (often Proteus); foamy lipid-laden macrophages; mimics tumor |
| Renal abscess | Persistent fever despite antibiotics; seen on CT |
Persistent fever after 48–72 hours of appropriate antibiotics should prompt imaging for obstruction or abscess.
Treatment
Choose drugs that reach kidney tissue.
- Outpatient: oral fluoroquinolone (ciprofloxacin, levofloxacin) or TMP-SMX if the organism is susceptible.
- Inpatient or severe: IV ceftriaxone, an aminoglycoside, or a carbapenem when resistant organisms are likely.
- Pregnancy: hospitalize and give an IV β-lactam such as ceftriaxone; avoid fluoroquinolones.
- Nitrofurantoin and fosfomycin do not achieve adequate kidney concentrations and are not used.
- Relieve obstruction promptly (ureteral stent or nephrostomy).
Worked example
Apply pathology to a pediatric case.
A 7-year-old with recurrent febrile UTIs has hypertension. Voiding cystourethrogram shows vesicoureteral reflux. Imaging shows small kidneys with coarse polar scars over blunted calyces. Diagnosis and histology?
- Recurrent upper-tract infections with reflux cause repeated injury.
- Coarse, asymmetric scars over blunted calyces at the poles indicate chronic pyelonephritis (reflux nephropathy).
- Histology shows interstitial fibrosis, chronic inflammation, and atrophic tubules filled with eosinophilic casts.
- The scarring and renin release contribute to hypertension and CKD.
Answer. Chronic pyelonephritis from vesicoureteral reflux, with thyroidization of tubules on histology.