Kidneys obstructed by calculi are at a high risk because of infection in the calculi. Urine is an excellent culture medium for Gram-negative bacilli and an obstructed kidney can rapidly be damaged by a pyonephrosis. The danger of a Gram-negative septicemia should prompt urgent imaging and, if necessary, a percutaneous nephrostomy (Fig 3).
As with obstructive renal failure, ultrasound is the first-line imaging modality ( LevineJ 994). It may detect a perirenal abscess as a complication of the pyonephrosis, which can be treated with a percutaneous drain inserted at the time of the nephrostomy. In very obese patients, retroperitoneal collection may be difficult to exclude with certainty on ultrasound and a CT scan may be indicated. If access to an abscess is then thought to be awkward, a catheter drain can be inserted under CT guidance.
Infected perinephric hematoma after renal biopsy. Large collections such as this are easily diagnosed by ultrasound in a patient of normal body size. CT is more accurate with small collections or in the obese.
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