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

Differential Diagnosis for Flank Pain

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 and identifying alternative diagnoses in approximately one-third of patients. 1, 2, 3

Primary Differential Diagnoses

Urinary Tract Causes (Most Common)

• Urolithiasis/Nephrolithiasis: The leading cause of acute flank pain, affecting 19% of men and 9% of women by age 70, presenting with colicky, wave-like severe pain independent of body position 2, 4
• Ureteral obstruction with hydronephrosis: Can occur from stones or other obstructive causes 4
• Urothelial masses: May be detected on excretory phase imaging 4
• Renal cell carcinoma: Can present with flank pain and hematuria, mimicking nephrolithiasis 5
• Page kidney: Subcapsular hematoma causing renal parenchymal compression and secondary hypertension 6

Gastrointestinal Causes

• Right colonic diverticulitis: Can present as right-sided flank pain 1
• Gastroenteritis and colitis: Common non-appendiceal diagnoses on CT 1
• Inflammatory bowel disease: Identified in patients with flank/lower quadrant pain 1

Gynecologic Causes (in women)

• Benign adnexal masses: Frequently identified on CT in patients with flank pain 1
• Pelvic congestion syndrome: Can cause lower flank/pelvic pain 1

Musculoskeletal Causes

• Paraspinal muscle strain: Pain occurring after prolonged static positioning suggests musculoskeletal origin 2
• Facet joint pathology: Can cause referred flank pain 2
• Lumbar spine pathology: May present as referred flank pain 2

Diagnostic Approach Algorithm

Step 1: Clinical Assessment

• Assess pain characteristics: Colicky, wave-like pain independent of position suggests renal colic; positional pain suggests musculoskeletal origin 2
• Check for hematuria: Even microscopic hematuria shifts probability toward stone disease 2
• Identify red flags: Fever, chills, visible blood in urine, inability to urinate, or decreased urine output require urgent evaluation 2

Step 2: Initial Imaging Selection

For most patients:

• Non-contrast CT abdomen/pelvis is the gold standard with 98-100% sensitivity and specificity for stones, identifies extraurinary causes in one-third of patients, requires no contrast agent, and takes only 5 minutes 1, 2, 3, 7

For specific populations:

• Pregnant patients: Ultrasound is first-line to avoid radiation exposure 1, 4
• Patients with radiation concerns or known renal disease: Ultrasound is appropriate 8
• Recurrent stone formers: Consider limited imaging or ultrasound to avoid cumulative radiation 4

When to use contrast-enhanced CT:

• Flank pain with negative urinalysis and no stone history—contrast better characterizes alternative diagnoses 8, 4
• Initial non-contrast CT is inconclusive 1
• Contrast helps differentiate ureteral stones from phleboliths via "soft tissue rim" sign 4

Step 3: Ultrasound Interpretation (When Used)

• Moderate to severe hydronephrosis in patients with moderate/high risk of ureteric calculi provides sufficient diagnostic information without immediate CT 8
• Absence of hydronephrosis makes larger ureteral stones (>5 mm) less likely 1, 2, 8
• Sensitivity for obstruction: Up to 100% sensitive and 90% specific for hydronephrosis, ureterectasis, and perinephric fluid 1

Key Clinical Distinctions

Renal Colic vs. Musculoskeletal Pain

• Renal colic: Colicky, severe, wave-like, independent of body position 2
• Musculoskeletal: Pain worsens with movement or after prolonged static positioning 2

Stone Disease vs. Alternative Diagnoses

• Presence of hematuria: Strongly suggests urinary tract pathology 2
• Fever with flank pain: Suggests pyelonephritis or infected obstructed system—requires urgent evaluation 2
• No hydronephrosis on ultrasound within first 2 hours: May be falsely negative as secondary signs of obstruction take time to develop 1

Common Pitfalls to Avoid

• Do not assume all flank pain is kidney-related: Positional nature of symptoms is key to distinguishing musculoskeletal from renal causes 2
• Do not rely solely on KUB radiography: Only 72% sensitive for large stones (>5 mm) in proximal ureter and 29% sensitive overall for stones of any size 1
• Do not use contrast-enhanced CT as initial study for suspected stones: Contrast can obscure stones within the renal collecting system 8
• Do not dismiss flank pain with hematuria as always being nephrolithiasis: Consider renal cell carcinoma, Page kidney, and other serious pathology 6, 5
• Do not perform ultrasound immediately after symptom onset expecting to see hydronephrosis: Secondary signs of obstruction may not develop within the first 2 hours 1

Management Implications Based on Diagnosis

When CT Identifies Stones

• Stone size and location are the two most important factors for management decisions 9
• Stones <5 mm typically pass spontaneously 1
• Larger stones or those causing complete obstruction may require endoscopic removal 7

When CT Shows No Obstruction

• CT can diagnose or exclude most other abnormalities causing flank pain 9
• In patients without CT diagnosis, only 14% require hospitalization and 4% undergo surgical intervention, compared to 41% hospitalization and 22% intervention rates when non-appendiceal CT diagnosis is made 1