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Renal abscess, emphysematous pyelonephritis and XGP
Fever lasting more than 72 hours despite appropriate antibiotics in pyelonephritis requires CT to look for an abscess, obstruction or gas; a small abscess is treated with antibiotics and a large one by percutaneous drainage, and emphysematous pyelonephritis in a diabetic is treated with antibiotics, glucose control and drainage — and nephrectomy when treatment fails.
Why it matters in the exam
The exam shows a patient, usually diabetic, with flank pain and a high fever that does not settle, a high white count and rising creatinine. A renal abscess forms by ascending spread (E. coli, with obstruction, stones or reflux) or by the bloodstream (Staphylococcus aureus, mainly in people who inject drugs and with skin infections), in which case it is cortical. Guidance: an abscess smaller than 3 cm — antibiotics alone; 3 to 5 cm — antibiotics and consider drainage; over 5 cm — percutaneous drainage. A perinephric abscess almost always needs drainage.
Emphysematous pyelonephritis is a necrotising infection with gas in the parenchyma, almost only in diabetic patients, usually women, due to E. coli or Klebsiella, sometimes with obstruction. The CT classification of Huang and Tseng: class 1 — gas in the collecting system only; class 2 — gas in the parenchyma without extension; class 3A — extension into the perinephric space; class 3B — extension beyond the fascia; class 4 — bilateral disease or a solitary kidney. Treatment in classes 1 and 2, and in classes 3 and 4 with few risk factors: antibiotics, glucose control and percutaneous drainage or relief of obstruction. Nephrectomy is needed when treatment fails or in extensive disease with risk factors: thrombocytopenia, acute kidney injury, reduced consciousness and shock.
Xanthogranulomatous pyelonephritis (XGP) is chronic inflammation that destroys the kidney, usually with a staghorn stone, infection with Proteus or E. coli, weight loss and a non-functioning kidney. CT shows an enlarged kidney with dilated calyces around a stone — the "bear paw" appearance. Treatment is nephrectomy, often difficult because of the surrounding inflammation. Papillary necrosis, which occurs in diabetes, long-term analgesic use and sickle cell disease, may cause obstruction from a sloughed papilla.
Key points
- Fever beyond 72 hours — CT.
- Abscess under 3 cm — antibiotics; over 5 cm — drainage.
- EPN — diabetics, gas in the parenchyma.
- EPN — drainage first, nephrectomy on failure.
- XGP — staghorn stone, Proteus, nephrectomy.
Summary table
| Condition | CT finding | Treatment |
|---|---|---|
| Abscess smaller than 3 cm | Parenchymal collection | Antibiotics |
| Abscess over 5 cm or perinephric | Large collection | Percutaneous drainage and antibiotics |
| EPN class 1 to 2 | Gas in the collecting system or parenchyma | Antibiotics, glucose, drainage |
| EPN class 3 to 4 with risk factors | Gas outside the kidney or bilateral | Drainage; nephrectomy on failure |
| XGP | Large kidney, staghorn stone, bear paw | Nephrectomy |
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Sources
- EAU Guidelines on Urological Infections, 2025 edition — Complicated UTI, renal abscess and emphysematous pyelonephritis
- Huang JJ, Tseng CC. Emphysematous pyelonephritis: clinicoradiological classification, management, prognosis, and pathogenesis, 2000 — CT classification
- Campbell-Walsh-Wein Urology, 12th edition (2020) — Infections of the Urinary Tract
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.