This patient should begin treatment with ciprofloxacin. She has signs and symptoms most consistent with acute pyelonephritis. Cystitis is infection of the bladder and lower urinary tract and commonly presents with dysuria, frequency, and urgency. However, the presence of fever or other systemic symptoms (chills, nausea, vomiting) and back or flank pain are more consistent with pyelonephritis, or kidney infection. Lower urinary tract symptoms may antedate fever and upper urinary tract symptoms by approximately 2 days. Most cases of pyelonephritis may be managed in the ambulatory setting; indications for hospitalization include hemodynamic instability, inability to tolerate oral medications, host factors such as pregnancy or presence of kidney stones or other obstructions, presence of comorbidities, and an unstable social situation that may compromise adherence or follow-up. A urine culture and susceptibility testing should always be performed in patients with suspected pyelonephritis to confirm the diagnosis and guide therapy. However, empiric treatment should be provided pending culture results, with initial therapy modified appropriately when the infecting organism is identified and susceptibilities are known. A fluoroquinolone is the preferred agent for empiric therapy when resistance in the community does not exceed 10%. If fluoroquinolone resistance exceeds 10%, an initial, one-time intravenous dose of a long-acting agent such as ceftriaxone or a 24-hour dose of an aminoglycoside should be administered. A 7-day course of a fluoroquinolone is as effective as a 14-day course in women. If high-dose levofloxacin (750 mg) is administered, the duration is 5 days. If once-daily oral ciprofloxacin (1000 mg extended release) is administered, it should be given for 7 days.

Ampicillin with gentamicin is an appropriate regimen for treating acute pyelonephritis in patients requiring hospitalization. Other choices for hospitalized patients include therapy with a fluoroquinolone, an extended-spectrum cephalosporin, an extended-spectrum penicillin, or a carbapenem such as ertapenem. However, despite this patient's systemic findings, she is clinically stable and can undergo outpatient therapy. Therefore, intravenous, broad-spectrum antibiotic therapy with either ampicillin with gentamicin or ertapenem is not indicated.

Nitrofurantoin or fosfomycin are indicated for treatment of uncomplicated cystitis and are not effective for pyelonephritis because they do not achieve adequate tissue levels in the kidney parenchyma. Therefore, they are not appropriate for patients with pyelonephritis.