This patient has had a persistent, high fever after 72 hours of appropriate antibiotic therapy; therefore, ultrasonography or contrast-enhanced CT should be performed to exclude an intrarenal or perinephric abscess. Both are acceptable imaging modalities, but CT is considered the gold standard because it offers better anatomic detail. MRI could also be performed to investigate for these complications. Abscess formation is an uncommon complication of urinary tract infection (UTI). The most common predisposing factors for perinephric abscess are diabetes mellitus and the presence of urinary tract calculi. Abscess formation within the kidney usually occurs from infective disruption of the kidney parenchyma secondary to obstruction, frequently by a stone. Perinephric abscesses may result from rupture of an abscess in the corticomedullary region of the kidney through the fascia surrounding the kidney and into the perinephric space. A smaller number of abscesses associated with UTI result from hematogenous spread of bacteria from the highly vascular kidney. Most intrarenal or perinephric abscesses are caused by gram-negative enteric bacilli, whereas gram-positive cocci are generally seen when the abscess occurs secondary to bacteremia. Infection may also be polymicrobial, and fungal organisms such as Candida may be causative in some abscesses. Abscess drainage is usually required except for very small collections or those for which the causative factor (such as a kidney stone) may be removed to allow drainage.

This patient is receiving appropriate therapy for pyelonephritis caused by Escherichia coli, and the isolate is known to be susceptible to ceftriaxone. Therefore, a change in antibiotic to gentamicin, which has significant toxicity, is not indicated, and changing the antibiotic therapy might delay diagnosis of a complication from her UTI.

Repeating the urine culture is not indicated. A culture was performed on admission and has already revealed the causative organism with susceptibility testing. It is highly unlikely that another pathogen would be identified or a resistant E. coli strain would emerge during appropriate and adequate therapy.

Because most patients respond to antibiotic therapy with defervescence within 72 hours, continued observation without any further diagnostic interventions would be inappropriate.