What is the differential diagnosis and management for a patient presenting with flank pain?

Flank Pain Differential Diagnosis and Management

Primary Diagnostic Approach

Non-contrast CT of the abdomen and pelvis is the imaging study of choice for evaluating flank pain, with 98-100% sensitivity and specificity for detecting urinary stones while identifying alternative non-urinary diagnoses in approximately one-third of patients. 1, 2, 3

Key Differential Diagnoses

Urinary Tract Causes

• Urolithiasis remains the most common cause, affecting 19% of men and 9% of women by age 70, presenting with colicky, wave-like severe pain independent of body position that radiates to the groin or genitals 1, 3
• Acute pyelonephritis can present with flank pain even with negative initial urinalysis, requiring urine culture if fever or systemic signs are present 1, 2
• Ureteral obstruction with hydronephrosis represents a serious complication requiring urgent intervention 3
• Urothelial masses may be detected on excretory phase imaging 3
• Page kidney (subcapsular hematoma causing renal compression and secondary hypertension) can mimic nephrolithiasis with flank pain and hematuria 4

Gastrointestinal Causes

• Colonic diverticulitis (right-sided or left-sided) can present as flank pain mimicking renal pathology 1, 2
• Gastroenteritis and colitis, including inflammatory bowel disease, may manifest with flank pain 1, 2
• Pancreatic tail pathology (pancreatitis or masses) can cause left-sided flank pain 2

Gynecologic Causes (Women)

• Ectopic pregnancy must be considered in women with delayed menses presenting with flank pain 1
• Benign adnexal masses and ovarian cyst complications (including torsion) can cause flank pain 1, 2
• Pelvic congestion syndrome may present with chronic flank pain 1, 2

Musculoskeletal Causes

• Lumbar spine pathology (facet joint arthropathy or disc disease) causes position-dependent referred flank pain that worsens after prolonged static positioning 1, 2
• Paraspinal muscle strain typically worsens with movement and palpation 2

Vascular and Splenic Causes

• Splenic pathology (infarction or subcapsular hematoma) can cause left flank pain with referred pain patterns 2
• Aortic pathology or renal artery dissection should be considered with hemodynamic instability 2

Imaging Algorithm

First-Line Imaging

• Non-contrast CT abdomen/pelvis is the gold standard for all patients except pregnant women, detecting stones with 98-100% sensitivity and identifying extraurinary causes in one-third of cases 1, 2, 3
• Ultrasound is the first-line modality for pregnant patients to avoid radiation exposure, with up to 100% sensitivity for hydronephrosis and 90% specificity 1, 3

Second-Line Imaging Considerations

• Contrast-enhanced CT should be obtained if non-contrast CT is non-diagnostic and the patient has negative urinalysis with no stone history, as it better characterizes alternative diagnoses and differentiates stones from phleboliths 2, 3
• Ultrasound with Doppler may be considered for radiation concerns or recurrent stone formers to avoid cumulative radiation, though sensitivity is lower than CT 2, 3
• MRI is reserved for equivocal cases or when radiation must be absolutely avoided 2

Critical Red Flags Requiring Urgent Evaluation

• Fever, chills, or signs of systemic infection may indicate pyelonephritis or infected obstructed system requiring urgent urine culture and intervention 1, 2
• Inability to urinate or decreased urine output suggests complete obstruction requiring urgent intervention 1, 2
• Hemodynamic instability or shock may indicate vascular catastrophe requiring immediate evaluation 1, 2
• Failure of analgesia after 1 hour mandates immediate hospital admission 1

Management Based on Findings

Stone Disease Management

• Stones <5 mm typically pass spontaneously with conservative management including high fluid intake and urine straining 1, 3
• Stones >5 mm or complete obstruction require urgent urology referral within 7-14 days for consideration of endoscopic intervention 1
• Rapid analgesia with diclofenac intramuscular injection (preferred) should be provided within 30 minutes, with reassessment after 1 hour 1

Follow-Up Protocol

• Telephone follow-up 1 hour after initial assessment and analgesia administration 1
• Fast-track radiology within 7 days if managed outpatient 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—the positional nature of symptoms distinguishes musculoskeletal from renal causes 1
• Do not assume absence of hematuria excludes stone disease—over 20% of patients with confirmed urinary stones may have negative urinalysis 1, 2
• Do not delay imaging in young females—consider gynecologic causes including ectopic pregnancy that may require urgent intervention 1
• Do not miss ectopic pregnancy in women with delayed menses presenting with flank pain 1
• Do not rely on KUB radiography alone—it has only 72% sensitivity for large stones and 29% sensitivity overall 1