This tab covers infection of the kidney and its surrounds — from acute pyelonephritis through abscesses, the gas-forming and granulomatous variants, and parasitic disease. A recurring theme: obstruction or granulomatous infection turns a treatable infection into a surgical emergency.
Bacterial Nephritis: Overview
The classic triad of acute fever, chills, and flank pain usually signals renal infection, but the correlation is imperfect in both directions — significant renal infection can present insidiously or be entirely asymptomatic.
- Laboratory findings correlate poorly with renal infection. Bacteriuria and pyuria, the hallmarks of UTI, are not predictive of renal involvement, and a patient with significant renal infection may have sterile urine if the draining ureter is obstructed or the infection lies outside the collecting system. Pathologic and radiologic criteria can also mislead.
- Effect on renal function is variable. Acute or chronic pyelonephritis may alter function transiently or permanently, but non-obstructive pyelonephritis is no longer considered a major cause of renal failure. When combined with obstruction or granulomatous infection, however, it can rapidly cause inflammatory complications, renal failure, or death.
Acute Pyelonephritis
Pathogens
E. coli causes ≈80% of cases (a virulent subgroup). Suspect more resistant species — Proteus, Klebsiella, Pseudomonas, Serratia, Enterobacter, Citrobacter — in patients with recurrent UTIs, hospitalisation, indwelling catheters, or recent instrumentation. Apart from E. faecalis, S. epidermidis, and S. aureus, gram-positive bacteria rarely cause pyelonephritis. The differential includes acute appendicitis, diverticulitis, and pancreatitis (similar pain intensity, different location).
Diagnosis and Evaluation
The clinical spectrum runs from gram-negative sepsis to cystitis with mild flank pain.
- Symptoms — upper-tract signs are abrupt-onset chills, fever, and unilateral or bilateral flank/costovertebral-angle (CVA) pain or tenderness, often with LUTS (dysuria, frequency, urgency). Exam: CVA tenderness to deep palpation.
- Labs — CBC may show leukocytosis with neutrophil predominance. Urinalysis usually shows numerous WBCs (often in clumps) and bacterial rods or chains of cocci; granular or leukocyte casts suggest acute pyelonephritis. Urine cultures are usually positive, though ≈20% have <10⁵ cfu/mL and therefore a negative urine Gram stain. Blood cultures should not be obtained routinely in uncomplicated pyelonephritis in women (positive in ≈25%, mostly replicating the urine culture without changing management) — reserve them for men, anyone with systemic toxicity or needing hospitalisation, and risk factors such as pregnancy.
- Imaging — defer initial imaging in outpatient uncomplicated cases; renal ultrasound can exclude stones/obstruction when there is concern or poor access to follow-up. In known or suspected complicated disease, CT best defines the urinary tract and the severity/extent of infection. Use US/CT for initial complicated evaluation or to re-evaluate patients who fail to respond after 72 hours.
Management
Any substantial obstruction must be relieved expediently by the safest, simplest means — an obstructed kidney cannot concentrate or excrete antibiotics.
- Oral: amoxicillin/clavulanate 625 mg PO TID ×7 days (amoxicillin ± clavulanate if a gram-positive organism is suspected); ciprofloxacin 500 mg PO BID ×7 days; or levofloxacin 500 mg PO daily ×7 days. Many clinicians give a single parenteral dose (ceftriaxone, gentamicin, or a fluoroquinolone) before starting oral therapy.
- IV (for hospitalisation-level illness — high fever, high WBC, vomiting, dehydration, sepsis — complicated disease, or outpatient failure): ceftriaxone 1–2 g IV q24h; ciprofloxacin 400 mg IV TID; or gentamicin 5–7 mg/kg q24h. For gram-positive cocci, ampicillin/sulbactam ± an aminoglycoside.
- Follow-up: repeat urine cultures after 5–7 days of therapy and 10–14 days after stopping it. 10–30% relapse after a 14-day course; relapse is usually cured by a second 14-day course, occasionally requiring 6 weeks.
Focal and Multifocal Bacterial Pyelonephritis (Lobar Nephronia)
An uncommon, severe infection in which a heavy leukocyte infiltrate is confined to one renal lobe (focal) or several (multifocal) — a midpoint between pyelonephritis and renal abscess. Presentation resembles acute pyelonephritis but is usually more severe; ≈50% of patients are diabetic and sepsis is common. Diagnosis is radiologic (US or CT). Management is hydration plus IV antimicrobials for ≥7 days, then 7 days of oral therapy; failure to respond should prompt studies for obstructive uropathy, renal or perirenal abscess, renal carcinoma, or acute renal vein thrombosis.
Renal and Perinephric Abscess
A renal abscess is purulent material confined to the renal parenchyma (mostly gram-negative organisms); the primary pathway is ascending infection with tubular obstruction from prior infection or calculi. A perinephric abscess results from rupture of a cortical abscess into the perinephric space or haematogenous seeding; diabetes is present in ≈1/3.
- Presentation — renal abscess: fever, chills, abdominal/flank pain, occasionally weight loss and malaise, with marked leukocytosis. Perinephric abscess has a more insidious onset (symptoms >5 days in most) — suspect it with a UTI plus a flank mass or persistent fever after 4 days of antibiotics.
- Distinguishing perinephric abscess from acute pyelonephritis: uncomplicated pyelonephritis is symptomatic for <5 days before hospitalisation and defervesces within 4 days of appropriate antibiotics, whereas perinephric abscess is symptomatic for >5 days and stays febrile for ≥5 days (median 7).
- Diagnosis — CT is the procedure of choice; CT- or US-guided aspiration may distinguish abscess from a hypervascular tumour.
Drainage thresholds differ between the two:
| Renal abscess | Perinephric abscess | |
|---|---|---|
| Antibiotics alone | <3–5 cm in a clinically stable patient | <3 cm in an immunocompetent patient |
| Drainage | ≥5 cm (percutaneous) | >3 cm — early drainage |
Follow with serial US/CT until resolution, and once a perinephric abscess is drained, address the underlying cause.
Chronic Pyelonephritis
In patients without underlying renal or urinary-tract disease, chronic pyelonephritis from UTI is rare and an even rarer cause of chronic renal failure; with underlying functional or structural abnormalities, chronic infection can cause significant impairment. There are no symptoms until renal insufficiency develops, after which it resembles any chronic renal failure, and urinary findings correlate poorly. Imaging shows asymmetry and irregularity of the renal outline, blunting and dilation of one or more calyces, and cortical scars at the corresponding sites. Management is directed at treating infection, preventing recurrences, and monitoring and preserving renal function.
Infected Hydronephrosis and Pyonephrosis
Infected hydronephrosis is bacterial infection within a hydronephrotic kidney. Pyonephrosis is infected hydronephrosis with suppurative parenchymal destruction and total or near-total loss of renal function. The patient is usually very ill — high fever, chills, flank pain, and tenderness — and a prior history of calculi, infection, or surgery is common. (This is a drainage emergency.)
Emphysematous Pyelonephritis
An acute necrotising parenchymal and peri-renal infection caused by gas-forming uropathogens. E. coli is the most common cause (also of emphysematous cystitis), and it usually occurs in diabetics. Almost all patients show the classic triad of fever, vomiting, and flank pain.
- Imaging establishes the diagnosis by gas in the parenchyma or collecting system; a crescentic gas collection over the upper pole is distinctive, and gas extends into the perinephric space and retroperitoneum as it progresses. Do not confuse it with emphysematous pyelitis (gas confined to the collecting system) — a less serious, often non-diabetic, gas-forming bacterial UTI that usually responds to antimicrobials.
- Management is urgent. Most patients are septic, so fluid resuscitation and broad-spectrum antimicrobials are essential. Relieve an obstructed kidney with a stent or nephrostomy; a functioning kidney may be managed medically. Nephrectomy is indicated when the affected kidney is non-functioning and not obstructed (medical therapy alone is usually lethal) or when there is failure to improve after a few days of therapy.
Granulomatous Nephritis
Xanthogranulomatous Pyelonephritis (XGP)
A rare chronic infection, more common in women and in diabetics. The pathogenesis is a triad of nephrolithiasis (usually staghorn) → chronic obstruction → infection, producing diffuse destruction of an enlarged, non-functioning kidney; it begins in the pelvis and calyces and extends to destroy the parenchyma and adjacent tissue, and is usually unilateral. Proteus is the most common organism; E. coli is also common.
- Diagnosis — urinalysis shows pus and protein; urine culture is mixed in ≈10% and shows no growth in ≈1/3 (recent/current antibiotics), so the organism may emerge only on intra-operative tissue culture. CT is the most useful modality: unilateral renal enlargement, a large pelvic calculus without pelvic dilatation, and little or no function (low nephrographic enhancement), with the classic triad in 50–80%. CT often cannot distinguish XGP from renal cell carcinoma — it mimics virtually every other inflammatory renal disease and RCC. Pathology: accumulation of lipid-laden foamy macrophages (characteristic — frequently examined).
- Management — antibiotics may stabilise the patient preoperatively and occasionally (long-term) eradicate infection and restore function, but total nephrectomy (radical or simple) is usually performed, since the kidney is often diagnosed preoperatively as a tumour suspicious for cancer or is a source of recurrent infection. A retrospective cohort of 86 nephrectomies for XGP found the midline approach had longer operative time and slower return of oral intake than the flank retroperitoneal approach, with no difference in complications; laparoscopy has relatively high conversion rates and suits experienced surgeons, while open is better for the inexperienced. Localised XGP identified preoperatively or at exploration is amenable to partial nephrectomy.
Malacoplakia
Greek for "soft plaque," it probably arises from abnormal macrophage function in response to bacterial infection — most often E. coli. Consider it when one or more renal masses are seen, especially in women with recurrent E. coli UTIs, altered immune responses, or cystoscopic plaques/filling defects. Biopsy shows von Hansemann cells and Michaelis-Gutmann bodies (pathognomonic). Management is directed at controlling the UTIs, which stabilises the disease; long-term antibiotics (sulfonamides, rifampin, doxycycline, trimethoprim) are effective.
Renal Echinococcosis
A parasitic infection caused by the larval stage of the tapeworm Echinococcus granulosus. It behaves like a slowly growing tumour — most patients are asymptomatic or have a flank mass, dull pain, or haematuria — and because the cyst is focal it rarely affects renal function. Rarely it ruptures into the collecting system, causing severe colic and passage of debris resembling grape skins (hydatiduria). Excretory urography shows a thick-walled cystic mass, occasionally calcified. Diagnostic aspiration should not be performed because rupture and spillage of the highly antigenic contents risk fatal anaphylaxis. Surgery is the mainstay of treatment.