Renal Corticomedullary Abscess Workup

• CBC count with differential: Elevated peripheral leukocytosis with a left shift is a common finding in patients with renal corticomedullary abscess. Anemia may be present in patients with xanthogranulomatous pyelonephritis (XGP).
• Basic metabolic panel

  • BUN and creatinine levels are often elevated, usually secondary to prerenal azotemia. Hypovolemic states are caused by vomiting with GI fluid loss or decreased renal perfusion, which is observed in patients with sepsis. Patients who develop acute renal failure, as demonstrated by rising BUN and creatinine levels, show altered renal resorptive capabilities.
  • Calculate the fractional excretion of sodium (FENa) and measure urinary electrolyte levels to aid in diagnosis. The following formula is used to calculate the FENa:
    • FENa (%) = (urinary sodium X plasma creatinine)/(plasma sodium X urine creatinine) X 100
  • The calculated FENa is less than 1% in prerenal azotemia. If the FENa is greater than 1%, acute tubular necrosis is more likely.
• Urinalysis: Pyuria and proteinuria are common features of renal corticomedullary abscess. However, bacteria and pyuria may be absent if the ureter and/or collecting system is completely obstructed.
• Urine culture: Although urine culture results are often positive, cultures may fail to grow a causative organism. Seventy-five percent of urine cultures are positive in emphysematous pyelonephritis and XGP. The most common pathogens recovered include E coli, P mirabilis, and Klebsiella species.
• Blood cultures: Blood cultures are positive in more than 50% of patients with renal corticomedullary abscess and are particularly useful in patients with urosepsis. Isolated organisms are usually the same gram-negative microbes isolated from the urine.

Many imaging modalities are available to evaluate renal corticomedullary abscess, each with certain advantages and disadvantages. Renal ultrasonography and CT scanning are the fundamental tests for diagnosing corticomedullary abscesses. CT scanning is the study of choice in evaluating intrarenal abscesses. Ultrasonographic findings are less specific than findings on CT scanning, and MRI usually offers no additional information yielded by CT scanning.

• Radiography: This modality is often unhelpful in identifying intrarenal abscess; however, it may show radiopaque stones in patients with emphysematous pyelonephritis who have calculus-induced obstruction or intraparenchymal gas.
• Intravenous pyelography

  • Intravenous pyelography (IVP) limits the characterization of acute renal parenchymal infections.
  • Although IVP is an inexpensive study that may offer functional assessment of the kidneys, the risks associated with patient exposure to intravenous contrast and radiation outweigh the benefits, as this study offers low sensitivity in the detection of renal abscesses.
• Ultrasonography^{[4, 5]}

  • Ultrasonography is a readily accessible, rapid, and relatively inexpensive initial screening tool that reveals renal lesions and anatomic abnormalities.
  • The disadvantages of ultrasonography include its operator dependence, limited imaging capability in patients with a large body habitus, decreased sensitivity in comparison to CT scanning, and inability to functionally assess the kidneys.
  • Ultrasonographic findings that suggest renal abscess include an ill-defined renal mass (either hyperechoic or hypoechoic) with low-amplitude internal echoes and disruption of the corticomedullary junction, possible posterior acoustic enhancement, and lack of vascularity on Doppler imaging (to distinguish a complex abscess from malignancy).
• CT scanning^{[6, 7, 8]}

  • This is the most useful modality in diagnosing intrarenal abscess and planning operative procedures for treatment.
  • Noncontrast CT scans may demonstrate renal enlargement, inflammatory changes, and variable attenuation but poorly demonstrate intrarenal abscesses. When renal abscess is suspected, obtain both contrast and noncontrast images for comparison. Contrasted CT scan that demonstrates a corticomedullary abscess in a 27-year-old patient with diabetes mellitus who has a history of multiple urinary tract infections. Note the heterogeneous hypodense lesion in the right kidney. Extracapsular extension is not present.
  • CT scanning is extremely useful to characterize renal infections as diffuse or focal, to detect the presence of gas, and to help the clinician evaluate for perinephric extension.
  • Disadvantages of CT scanning include high cost, radiation exposure, and the use of iodinated contrast.
  • Findings suggestive of renal abscess include a poorly defined, wedge-shaped, hypodense area that may involve liquefaction and focal renal involvement. The characteristic appearance consists of a low-attenuation (0-20 Hounsfield units), distinctly marginated, parenchymal lesion that fails to enhance after contrast administration. Other features may include perirenal fluid and inflammatory stranding with thickening of Gerota fascia.
• MRI

  • MRI is highly sensitive in demonstrating renal abnormalities that may contribute to renal corticomedullary abscess but does not offer information in addition to that obtained with CT scanning.
  • Advantages of MRI include no ionizing radiation or iodinated contrast exposure.
  • Disadvantages of MRI include high cost, low availability, longer imaging times, motion artifacts, and a lower sensitivity for renal calculi than CT scanning.