What is the appropriate evaluation and treatment approach for a patient presenting with flank pain, considering their age, medical history, and potential underlying conditions such as kidney stones or urinary tract infections?

Evaluation and Management of Flank Pain

Immediate Assessment and Red Flags

For patients presenting with acute severe flank pain, provide medical evaluation and analgesia within 30 minutes, and if this cannot be achieved in your setting, immediately transfer to the emergency department. 1, 2

Critical red flags requiring immediate hospital admission include:

• Fever, chills, or signs of systemic infection (risk of progression to urosepsis) 3, 2
• Hemodynamic instability or shock 1, 2
• Inability to urinate or decreased urine output 1
• Failure of analgesia after 1 hour 1
• Age >60 years with severe pain (consider leaking abdominal aortic aneurysm) 2
• Women of reproductive age with delayed menses (exclude ectopic pregnancy) 1, 2

Diagnostic Workup

Initial Laboratory Testing

• Urinalysis with microscopy (assess for white blood cells, red blood cells, and nitrite) 3
• Urine culture and antimicrobial susceptibility testing if pyelonephritis suspected 3
• Note: Over 20% of confirmed urinary stones may have negative urinalysis, so normal results do not exclude stone disease 1

Imaging Strategy

Non-contrast CT of the abdomen and pelvis is the gold standard imaging study for flank pain, with 98-100% sensitivity and specificity for detecting urinary stones. 1, 4 This modality also identifies alternative extraurinary diagnoses in approximately one-third of patients. 1, 4

Specific imaging recommendations by patient population:

• Non-pregnant adults: Non-contrast CT abdomen/pelvis as first-line 1, 4
• Pregnant patients: Ultrasound first to avoid radiation exposure 1
• Patients with history of urolithiasis, renal dysfunction, or high urine pH: Ultrasound evaluation of upper urinary tract to rule out obstruction or stones 3

Avoid plain KUB radiography as it has only 72% sensitivity for large stones (>5 mm) and 29% sensitivity overall. 1

When to Escalate Imaging

• Contrast-enhanced CT or excretory urography if patient remains febrile after 72 hours of treatment or shows clinical deterioration 3
• MRI for pregnant women if complications suspected (avoids radiation) 3

Clinical Differentiation

Distinguishing Renal Colic from Musculoskeletal Pain

Classic renal colic presents as colicky, wave-like severe pain that is independent of body position, often radiating to the groin or genitals with abrupt onset. 1, 5 In contrast, pain occurring after prolonged static positioning that changes with movement is more likely musculoskeletal in origin, involving paraspinal muscles or facet joints. 1, 5

Uncomplicated Pyelonephritis Presentation

Typically presents with fever (>38°C), chills, flank pain, nausea, vomiting, or costovertebral angle tenderness, with or without cystitis symptoms. 3 Prompt differentiation between uncomplicated and potentially obstructive pyelonephritis is crucial, as obstructive pyelonephritis can rapidly progress to urosepsis. 3

Treatment Approach

Pain Management

Provide rapid analgesia within 30 minutes of assessment, with diclofenac intramuscular injection preferred. 1 Reassess pain control after 1 hour—failure to respond mandates immediate hospital admission. 1

Antimicrobial Therapy for Pyelonephritis

Outpatient oral therapy (uncomplicated pyelonephritis):

• Fluoroquinolones or cephalosporins are the only recommended agents for empiric oral treatment 3
• Avoid nitrofurantoin, oral fosfomycin, and pivmecillinam due to insufficient efficacy data 3
• Oral cephalosporins achieve significantly lower blood and urinary concentrations than IV route 3

Inpatient IV therapy (requiring hospitalization):

• Fluoroquinolone, aminoglycoside (±ampicillin), or extended-spectrum cephalosporin/penicillin 3
• Reserve carbapenems and novel broad-spectrum agents only for culture-confirmed multidrug-resistant organisms 3
• Tailor choice to local resistance patterns 3

Stone Management

• Stones <5 mm typically pass spontaneously 1
• Stones >5 mm or causing complete obstruction require urgent urology referral for consideration of endoscopic removal 1
• Instruct patients to maintain high fluid intake and strain urine to catch stones 1

Follow-Up Protocol

For outpatient management:

• Telephone follow-up 1 hour after initial assessment and analgesia 1
• Fast-track radiology within 7 days 1
• Urgent urology appointment within 7-14 days if calculus identified requiring intervention 1

Common Pitfalls to Avoid

• Do not assume all flank pain is kidney-related—consider gastrointestinal causes (right colonic diverticulitis, inflammatory bowel disease) and gynecologic causes in women (adnexal masses, pelvic congestion syndrome) 1, 5
• Do not delay imaging in young females—consider gynecologic emergencies requiring urgent intervention 1
• Do not assume absence of hematuria excludes stone disease—clinical presentation may have evolved 1
• Do not miss ectopic pregnancy in women with delayed menses presenting with flank pain 1, 2
• Do not rely on normal urinalysis to exclude significant pathology—over 20% of stone patients have negative urinalysis 1