Overview
Renal Abscess
1. Clinical Overview
Summary
A Renal Abscess is a collection of pus within or around the kidney, representing a severe form of upper urinary tract infection. Abscesses can be classified as Intrarenal (Renal Corticomedullary Abscess) or Perinephric/Perirenal (Extending beyond the renal capsule into the perinephric fat). The vast majority arise from ascending urinary tract infections (Gram-negative organisms, Especially E. coli), often in the setting of urinary obstruction (e.g., Stones), Diabetes Mellitus, or Immunocompromise. Less commonly, Renal abscesses result from haematogenous spread (Staphylococcus aureus from skin/Soft tissue infections). Patients present with fever, flank pain, Dysuria, and signs of systemic illness. Diagnosis is by CT with Contrast (Gold standard). Treatment requires a combination of Broad-spectrum Intravenous Antibiotics and Drainage (Percutaneous or Surgical) for larger abscesses (>5cm) or those not responding to antibiotics alone. [1,2,3]
Clinical Pearls
"Pyelonephritis that Doesn't Get Better": Consider abscess if fever and flank pain persist after 48-72 hours of appropriate antibiotics.
"Diabetics are High Risk": Diabetes is a major risk factor. Papillary necrosis and poor host immunity contribute.
"Ascending > Haematogenous": Most renal abscesses are from ascending UTI (Gram-negatives). Haematogenous spread (Staph aureus) is less common.
"CT is King": Contrast-enhanced CT is the gold standard. USS may miss early or small abscesses.
2. Epidemiology
Demographics
| Factor | Notes |
|---|---|
| Incidence | Uncommon. More common with UTI risk factors. |
| Age | Any age. Risk increases with age and comorbidities. |
| Sex | Female slightly > Male (Ascending UTI more common in women). Haematogenous abscesses may be equal. |
Risk Factors
| Risk Factor | Notes |
|---|---|
| Diabetes Mellitus | Major risk factor. Impaired immunity. Papillary necrosis. |
| Urinary Obstruction | Stones, Strictures, BPH. Stasis promotes infection. |
| Immunocompromise | HIV, Transplant, Chemotherapy. |
| Recurrent UTIs | |
| Vesicoureteral Reflux | Especially in children. |
| IV Drug Use | Haematogenous spread (Staph aureus). |
| Indwelling Catheter | |
| Previous Renal Surgery / Instrumentation |
3. Classification
By Location
| Type | Description |
|---|---|
| Renal Corticomedullary Abscess | Within the renal parenchyma. |
| Renal Cortical Abscess (Renal Carbuncle) | Localised to cortex. Often haematogenous (Staph aureus). |
| Perinephric (Perirenal) Abscess | Extends beyond the renal capsule into perinephric fat (Within Gerota's fascia). More serious. |
| Paranephric Abscess | Beyond Gerota's fascia (Rare, Very advanced). |
By Route of Infection
| Route | Organisms | Notes |
|---|---|---|
| Ascending (Most Common) | Gram-negatives: E. coli, Klebsiella, Proteus, Pseudomonas. | From lower UTI/Pyelonephritis. Often with obstruction. |
| Haematogenous | Staphylococcus aureus. | From distant focus (Skin, Endocarditis, IVDU). Typically cortical abscess. |
4. Clinical Presentation
Symptoms
| Symptom | Notes |
|---|---|
| Fever | May be high-grade. Rigors. |
| Flank Pain | Unilateral. Constant. May radiate to groin. |
| Dysuria, Frequency | UTI symptoms. |
| Nausea / Vomiting | |
| Malaise | Non-specific. |
| Weight Loss | If chronic. |
| Referred Pain | Hip, Thigh (Psoas irritation – Suggests perinephric extension). Pain on hip flexion. |
Examination Findings
| Finding | Notes |
|---|---|
| Fever | Often high-grade. |
| Flank Tenderness | Marked. Renal angle. |
| Palpable Mass | Sometimes. Bulging flank. |
| Psoas Sign | Pain on hip extension (Psoas irritation – Perinephric abscess). |
| Scoliosis | Curvature towards affected side (Muscle spasm). |
| Sepsis Features | Tachycardia, Hypotension, Altered mental status. |
Differentiating Features (Perinephric Abscess)
Symptoms often insidious (Developing over days to weeks).
Common presentation.
More systemic illness.
Common presentation.
May have psoas irritation (Hip pain, Limp).
Common presentation.
Higher mortality.
Common presentation.
5. Investigations
Laboratory
| Test | Findings |
|---|---|
| FBC | Leucocytosis. Left shift. |
| CRP | Elevated. |
| U&Es | May show renal impairment. |
| Blood Cultures | Positive in ~30-50%. E. coli, Staph aureus. |
| Urine Microscopy and Culture | Pyuria. Bacteriuria. May be sterile if abscess is walled off or haematogenous. |
Imaging
| Modality | Findings |
|---|---|
| CT Abdomen/Pelvis with Contrast | Gold Standard. Rim-enhancing hypodense lesion. Gas within abscess (Gas-forming organisms). Perinephric fat stranding. Extension beyond kidney. |
| Ultrasound | May show hypoechoic collection. Less sensitive than CT. May miss early abscess. |
| Plain X-Ray (KUB) | May show loss of psoas shadow. Stones (If obstruction cause). |
CT Findings
| Feature | Description |
|---|---|
| Renal Abscess | Hypodense lesion with rim enhancement (Ring sign). |
| Perinephric Abscess | Fluid collection in perinephric space. Thickened Gerota's fascia. |
| Gas | Gas bubbles within abscess = Emphysematous pyelonephritis (Severe, Emergency). |
6. Management
Management Algorithm
SUSPECTED RENAL ABSCESS
(Fever, Flank pain, Not improving on antibiotics)
↓
INVESTIGATIONS
- Bloods: FBC, CRP, U&Es, Blood cultures
- Urine: MC&S
- **CT Abdomen/Pelvis with IV Contrast**
↓
CONFIRM ABSCESS
- Size
- Location (Intrarenal vs Perinephric)
- Ureteric obstruction (Stone)
↓
INITIAL MANAGEMENT
- IV Fluids
- Analgesia
- Broad-Spectrum IV Antibiotics
(Cover Gram-negatives + Consider Staph aureus)
e.g., Piperacillin-Tazobactam, Ceftriaxone + Metronidazole,
or Carbapenem if resistant organisms suspected
↓
ASSESS FOR DRAINAGE
┌────────────────┴────────────────┐
SMALL ABSCESS (less than 3-5cm) LARGE ABSCESS (>5cm)
Uncomplicated OR Failure to respond (48-72h)
No obstruction OR Multiloculated
↓ OR Ureteric obstruction
**IV ANTIBIOTICS ↓
ALONE** **DRAINAGE**
- Monitor clinically - Percutaneous (Image-guided) – First-line
- Repeat CT if - Surgical (Open/Laparoscopic) – If perc fails
not improving or complex
- Nephrectomy – Rarely, Non-functional kidney
↓
RELIEVE OBSTRUCTION (If Present)
- Ureteric stent or Nephrostomy
- Definitively treat stone
↓
DURATION OF ANTIBIOTICS
- Typically 2-4 weeks
- Guided by clinical response and imaging resolution
- Transition to Oral when improving
Antibiotics
| Setting | Regimen |
|---|---|
| Empirical (Ascending) | Broad-spectrum Gram-negative cover: Piperacillin-Tazobactam, Ceftriaxone, Carbapenem. |
| Suspected Staph aureus (Haematogenous) | Add Flucloxacillin or Vancomycin (If MRSA risk). |
| Adjust Based on Cultures | Tailor to sensitivities. |
Drainage Indications
| Indication | Notes |
|---|---|
| Abscess >5cm | Generally requires drainage. |
| Failure to Improve | After 48-72 hours of antibiotics. |
| Multiloculated Abscess | May need surgical drainage. |
| Perinephric Abscess | Higher threshold for drainage (More complex). |
| Obstructed System | Relief of obstruction (Stent/Nephrostomy) essential. |
Drainage Options
| Method | Notes |
|---|---|
| Percutaneous Drainage (CT/US-Guided) | First-line for larger abscesses. Catheter left in place. |
| Surgical Drainage | Open or Laparoscopic. If percutaneous fails. Complex collections. |
| Nephrectomy | Rarely required. Non-functional kidney. Life-threatening sepsis. Emphysematous pyelonephritis. |
7. Complications
| Complication | Notes |
|---|---|
| Sepsis / Septic Shock | Life-threatening. |
| Perinephric Extension | From renal to perinephric space. |
| Psoas Abscess | Extension into psoas muscle. |
| Fistula | To colon (Rare). |
| Chronic Kidney Disease | If extensive damage. |
| Recurrence | If underlying cause (Obstruction) not addressed. |
| Emphysematous Pyelonephritis | Gas-forming infection. Emergency. High mortality. |
8. Prognosis and Outcomes
| Factor | Notes |
|---|---|
| With Prompt Treatment | Good outcomes. Most patients recover fully. |
| Delayed Diagnosis | Increased morbidity and mortality. |
| Immunocompromised | Higher risk of complications. |
| Emphysematous Pyelonephritis | Mortality ~20-40% even with treatment. Emergency nephrectomy may be needed. |
9. Evidence and Guidelines
Key Guidelines
| Guideline | Organisation | Key Recommendations |
|---|---|---|
| Complicated UTI | EAU Guidelines | CT for diagnosis. IV antibiotics. Drainage for large abscesses. Relieve obstruction. |
10. Patient and Layperson Explanation
What is a Renal Abscess?
A renal abscess is a collection of pus inside or around the kidney, Usually caused by a severe kidney infection.
Who is at risk?
- People with diabetes.
- Those with kidney stones or blocked urine flow.
- People with weakened immune systems.
What are the symptoms?
- High fever and chills.
- Pain in the side or back (Flank).
- Feeling unwell.
- Sometimes burning or pain when passing urine.
How is it diagnosed?
A CT scan is the best test to find a kidney abscess.
What is the treatment?
- Antibiotics – Given through a drip (IV) initially.
- Drainage – If the abscess is large (More than 3-5 cm), It may need to be drained with a needle or tube (Guided by a scan), Or sometimes surgery.
- Treating the cause – If a kidney stone is blocking urine flow, This needs to be sorted out too.
Is it serious?
Yes, A kidney abscess can be serious if not treated promptly. Most people make a full recovery with the right treatment.
11. References
Primary Sources
- Coelho RF, et al. Renal and perinephric abscesses: analysis of 65 consecutive cases. World J Surg. 2007;31(2):431-436. PMID: 17219277.
- Lee BE, et al. Renal abscess. J Urol. 2019;201(4S):e659.
- EAU Guidelines on Urological Infections. 2023.
12. Examination Focus
Common Exam Questions
- Risk Factor: "What is the most common risk factor for renal abscess?"
- Answer: Diabetes Mellitus.
- Causative Organism (Ascending): "What is the most common organism in ascending renal abscess?"
- Answer: Escherichia coli (E. coli).
- Gold Standard Investigation: "What is the gold standard imaging for renal abscess?"
- Answer: CT Abdomen/Pelvis with IV Contrast.
- Drainage Indication: "When is drainage indicated?"
- Answer: Abscess >5cm, Failure to respond to antibiotics after 48-72 hours, Multiloculated abscess.
Viva Points
- Perinephric Abscess: More insidious. Psoas irritation (Hip flexion pain). Higher mortality.
- Haematogenous = Staph aureus: Cortical abscess. Source may be distant (IVDU, Skin).
- Emphysematous Pyelonephritis: Gas on CT. Emergency. Consider nephrectomy.
- Relieve Obstruction: Essential if stone or stricture present.
Medical Disclaimer: MedVellum content is for educational purposes and clinical reference. Clinical decisions should account for individual patient circumstances. Always consult appropriate specialists.