A middle-aged man with diabetes mellitus type 2 presented with fever, chills and malaise. Urinalysis revealed pyuria and was positive for nitrite. Renal sonogram demonstrated a heterogeneous mass-like structure in the mid right kidney extending over the lower pole.

CT scan and MRI with contrast were suggestive of renal abscess. A drainage catheter was placed and patient started on intravenous antibiotics. Fluid culture grew Klebsiella pneumoniae and the therapy was tailored accordingly. 1-week later, a repeat sonogram demonstrated near-complete resolution of the collection.

On a sonogram, renal abscess appears as a well-defined hypoechoic area often with thick walls and internal echoes. Can be associated with surrounding diffusely hypoechoic, enlarged kidney due to pyelonephritis or hydronephrosis interspersed with echogenicities if it ruptures into the collecting system = pyonephrosis. It may appear similar to an organizing hematoma or renal cell carcinoma as in the above case where history becomes crucial and further imaging should be obtained. Small abscesses can be totally missed on ultrasound. Following are two more examples.

For renal abscesses <5 cm in diameter, antibiotic therapy alone without drainage is appropriate initial management. These lesions usually respond well to prolonged antibiotic treatment, and moreover, radiographic localization for drainage can be difficult. If clinical symptoms and radiographic findings persist after several days of antimicrobial therapy, drainage should be considered. Renal abscesses >5 cm should be managed with percutaneous drainage in addition to antimicrobial therapy.

Here is another example shared by Dr. Jones obtained from a patient who was being treated for pyelonephritis presenting with worsening left flank pain. Note the relatively well-defined heterogeneous structure in the lower pole of the kidney (medial aspect) = likely abscess. Differential includes renal cell carcinoma but in the given clinical context, abscess seems more likely. Moreover, there is no internal flow on color Doppler. When in doubt, get a CT scan with contrast.