What are the appropriate evaluation and management steps for a patient, potentially a young or middle-aged adult with a history of urinary tract infections (UTIs) or kidney stones, presenting with left-sided flank pain, with or without accompanying symptoms such as fever, nausea, or vomiting?

Evaluation and Management of Left-Sided Flank Pain

For a patient presenting with left-sided flank pain, immediately assess for signs of infection (fever, leukocytosis) and obtain urinalysis with urine culture, as the presence of infection fundamentally changes management and requires urgent intervention to prevent progression to urosepsis. 1

Initial Clinical Assessment

Key Historical and Physical Examination Features

• Determine if infection is present: Fever >38°C, chills, costovertebral angle tenderness, nausea, or vomiting suggest pyelonephritis rather than uncomplicated nephrolithiasis 1
• Assess for systemic illness: Hypotension, altered mental status, or septic appearance requires immediate hospitalization and aggressive management 2
• Evaluate urinary symptoms: Dysuria, frequency, urgency, or hematuria help differentiate upper tract infection from lower UTI or stone disease 3
• Review risk factors: History of kidney stones, diabetes, immunosuppression, pregnancy, or anatomical abnormalities classify the patient as high-risk and alter management 1, 4

Critical Pitfall to Avoid

Do not delay imaging beyond 72 hours in patients with persistent fever despite appropriate antibiotics, as this may indicate complications such as obstruction, abscess, or emphysematous pyelonephritis requiring urgent intervention. 1, 4

Diagnostic Workup

Mandatory Initial Laboratory Studies

• Urinalysis: Evaluate for white blood cells, red blood cells, nitrites, and bacteria 1
• Urine culture with antimicrobial susceptibility testing: Required in all cases of suspected pyelonephritis before initiating antibiotics 1
• Complete blood count: Assess for leukocytosis indicating infection 2
• Renal function assessment: Creatinine and estimated glomerular filtration rate 2
• Blood cultures: If patient appears systemically ill or has high fever 1

Imaging Strategy

Initial imaging is NOT indicated for uncomplicated acute pyelonephritis in patients who can tolerate oral intake, have no signs of sepsis, and have normal renal function. 1, 4

However, imaging is mandatory in the following situations:

• Persistent fever after 72 hours of appropriate antibiotic therapy 1, 4
• Clinical deterioration at any time 1, 4
• History of urolithiasis, renal function alterations, or elevated urine pH 1
• Diabetic or immunocompromised patients (lower threshold for imaging due to higher risk of complications like emphysematous pyelonephritis or renal abscess) 1, 4, 5
• Suspected obstruction, abscess, or emphysematous pyelonephritis 2, 4

Imaging Modality Selection

• Ultrasound: First-line imaging to evaluate for hydronephrosis, obstruction, or stones 2, 1
• CT abdomen/pelvis without contrast: If ultrasound is inconclusive or when stone disease is suspected 2
• CT with contrast: When abscess, emphysematous pyelonephritis, or other complications are suspected 2, 1

Management Algorithm

For Patients WITH Infection (Pyelonephritis)

Outpatient Management Criteria

Patients can be treated as outpatients if ALL of the following are present: 4

• Uncomplicated disease (no anatomical abnormalities, not pregnant, not diabetic, not immunocompromised)
• Ability to tolerate oral intake and medications
• No signs of sepsis
• Reliable follow-up
• Normal or near-normal renal function

Outpatient antibiotic regimens: 1

• Ciprofloxacin 500-750 mg twice daily for 7 days, OR
• Levofloxacin 750 mg once daily for 5 days

Inpatient Management Criteria

Hospitalize immediately if any of the following are present: 4

• Signs of sepsis or hemodynamic instability
• Inability to tolerate oral intake
• Diabetes mellitus or immunosuppression
• Pregnancy (≥20 weeks)
• Suspected complications (abscess, emphysematous pyelonephritis, obstruction)

Inpatient parenteral antibiotic regimens: 1

• Ceftriaxone 1-2 g IV once daily (preferred first-line agent) 1
• Alternative options: Ciprofloxacin 400 mg IV twice daily, Levofloxacin 750 mg IV once daily, Cefotaxime 2 g IV three times daily, or Cefepime 1-2 g IV twice daily 1

Treatment duration: 7-14 days total, with transition to oral therapy once afebrile for 24-48 hours based on culture sensitivities 1

Management of Obstructive Pyelonephritis

If imaging reveals hydronephrosis with infection, urgent decompression is required to prevent urosepsis and renal damage: 2

• Retrograde ureteral stenting: Usually appropriate first-line intervention 2
• Percutaneous nephrostomy (PCN): If retrograde stenting fails or is not technically feasible 2
• For pregnant patients (≥20 weeks): Either retrograde stenting or PCN is appropriate, with fluoroscopy typically avoided 2

Critical warning: Prolonged manipulation during procedures in infected, obstructed systems can precipitate urosepsis; limit manipulation and monitor closely for clinical deterioration 2

For Patients WITHOUT Infection (Suspected Nephrolithiasis)

Conservative Management

• Medical expulsive therapy: Consider alpha-blockers or calcium channel blockers to facilitate stone passage 6
• Analgesia: NSAIDs are first-line; opiates for severe pain 6
• Avoid intensive hydration (not currently recommended) 6
• Imaging: Ultrasound or non-contrast CT to confirm stone presence and assess for obstruction 2

Indications for Urological Intervention

• Stone >10 mm (unlikely to pass spontaneously) 6
• Persistent pain despite adequate analgesia
• Progressive hydronephrosis
• Solitary kidney with obstruction
• Bilateral obstruction

Special Populations

Diabetic Patients

Diabetic patients with flank pain and fever require heightened vigilance: 1, 4, 5

• Higher risk of emphysematous pyelonephritis and renal abscess
• May lack typical flank tenderness in up to 50% of cases
• Lower threshold for admission and early imaging even if initially stable
• Require close monitoring for clinical deterioration

Pregnant Patients (≥20 weeks)

• Hydronephrosis is common due to uterine compression and does not always indicate pathology 2
• If infection is present with hydronephrosis, urgent decompression with retrograde stenting or PCN is required 2
• Avoid fluoroscopy; use ultrasound guidance for procedures 2
• Risk of preterm labor with urolithiasis; risk ceases once stone passes or is removed 2

Elderly or Frail Patients

Diagnosis requires specific criteria to avoid overtreatment of asymptomatic bacteriuria: 3

• Fever >37.8°C oral (or >37.5°C rectal, or 1.1°C increase from baseline) OR rigors/shaking chills OR clear-cut delirium
• PLUS recent-onset dysuria
• If systemic symptoms present, add one of: frequency, incontinence, urgency, or costovertebral angle pain/tenderness

Do NOT diagnose UTI based solely on: cloudy urine, urine odor, change in color, nocturia, decreased output, suprapubic pain alone, agitation, mental status change without delirium criteria, decreased intake, malaise, fatigue, weakness, or dizziness 3

Monitoring and Follow-Up

• 95% of patients with uncomplicated pyelonephritis become afebrile within 48 hours, and nearly 100% within 72 hours of appropriate antibiotic therapy 1
• If fever persists beyond 72 hours, imaging is mandatory to rule out complications 1, 4
• Repeat urine culture is not routinely needed if clinical improvement occurs 1
• For recurrent UTIs, consider prophylaxis strategies 1

Differential Diagnoses to Consider

While urinary tract pathology is most common, consider: 7, 8

• Page kidney (subcapsular hematoma causing secondary hypertension) in patients with hypertension and no trauma history 7
• Thoracic disc herniation (T10-11 level can present as flank pain mimicking urinary pathology) 8
• Musculoskeletal causes if urinalysis is negative and imaging unrevealing