What is the appropriate workup for a patient presenting with flank pain, considering potential underlying conditions such as kidney stones, urinary tract infections, or kidney disease?

Flank Pain Workup

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

Initial Clinical Assessment

Key Historical Features to Elicit

• Pain characteristics: Classic renal colic presents as colicky, wave-like severe pain that is independent of body position, radiating into the groin or genitals with abrupt onset 1
• Positional pain: Pain occurring after prolonged static positioning suggests musculoskeletal origin involving paraspinal muscles, facet joints, or referred pain from lumbar spine pathology 1
• Associated symptoms: Presence of hematuria (even microscopic) shifts probability toward stone disease 1
• Gynecologic history: In women with delayed menses, ectopic pregnancy must be considered urgently 1

Red Flags Requiring Urgent Evaluation

• Fever, chills, or signs of systemic infection 1
• Hemodynamic instability or shock 1
• Inability to urinate or decreased urine output 1
• Visible blood in urine with systemic symptoms 1

Imaging Strategy

First-Line Imaging: Non-Contrast CT Abdomen/Pelvis

This is the gold standard for flank pain evaluation because it detects ureteral stones with 98-100% sensitivity and specificity regardless of size, location, or chemical composition 2, 3. The American College of Radiology recommends this as the most accurate initial imaging modality 2.

Key advantages:

• Identifies extraurinary causes of flank pain in one-third of patients 1, 2
• No contrast agent needed, avoiding nephrotoxicity and allergic reactions 3
• Time-efficient, performed within 5 minutes 3
• Can detect alternative diagnoses such as renal infarction, subcapsular hematoma, or thoracic disc herniation 4, 5, 6

Alternative First-Line: Ultrasound (Specific Populations)

Ultrasound should be the initial imaging modality for 1, 2:

• Pregnant patients (to avoid radiation exposure)
• Patients with radiation concerns
• Patients with known renal disease
• When hydronephrosis has already been identified

Ultrasound performance characteristics:

• Up to 100% sensitive and 90% specific for hydronephrosis, ureterectasis, and perinephric fluid 1
• Absence of hydronephrosis makes larger ureteral stones (>5 mm) less likely 1, 2
• When moderate to severe hydronephrosis is detected with moderate or high risk of ureteric calculi, it can provide sufficient diagnostic information without immediate CT 2

Important caveat: Ultrasound cannot distinguish early renal infarction from non-abscessed acute pyelonephritis, requiring CT or MRI for definitive diagnosis 5

When to Use Contrast-Enhanced CT

• When non-contrast CT is non-diagnostic and alternative diagnoses are suspected 2, 7
• In patients with flank pain and negative urinalysis without history of urolithiasis, as it better characterizes alternative diagnoses 2

Pitfall to avoid: Contrast-enhanced CT can obscure stones within the renal collecting system 2

Avoid KUB Radiography

• Only 72% sensitive for large stones (>5 mm) in the proximal ureter 1
• Only 29% sensitive overall for stones of any size 1

Additional Laboratory Workup

Urinalysis

• Hematuria (even microscopic) shifts probability toward stone disease 1
• Critical caveat: Over 20% of patients with confirmed urinary stones may have negative urinalysis 1
• Normal urinalysis does not exclude significant urologic pathology 1

When Proteinuria is Present

• Requires additional evaluation beyond standard stone workup, as this combination may indicate glomerular disease, renal vein thrombosis, or other renal parenchymal pathology 7
• Check serum albumin to determine if nephrotic syndrome is present 7
• Consider renal biopsy when nephrotic-range proteinuria is present without clear etiology on imaging 7

Immediate Management

Pain Control

• Provide rapid analgesia with NSAIDs (diclofenac intramuscular injection preferred) within 30 minutes 1, 7
• Reassess within one hour 1, 7
• Failure of analgesia after 1 hour mandates immediate hospital admission 1

Supportive Care

• Instruct patient to maintain high fluid intake 1
• Strain urine to catch stones 1

Disposition and Follow-Up

Outpatient Management (Stones <5 mm)

• These typically pass spontaneously 1
• Telephone follow-up 1 hour after initial assessment and analgesia administration 1
• Fast-track radiology within 7 days if managed outpatient 1

Urgent Urology Referral (Within 7-14 Days)

• Stones >5 mm or complete obstruction 1, 7
• Calculus identified requiring intervention 1

Immediate Hospital Admission Required

• Fever, hemodynamic instability, or signs of sepsis 1
• Obstructive pyelonephritis (urologic emergency that can result in sepsis and death) 8
• Failure of analgesia after 1 hour 1

Common Pitfalls to Avoid

• Do not assume all flank pain is kidney-related: The positional nature of symptoms is a key distinguishing feature between renal and musculoskeletal causes 1
• Do not assume absence of hematuria excludes stone disease: The clinical picture may have evolved, and over 20% of confirmed stones have negative urinalysis 1
• Do not delay imaging in young females: Consider gynecologic causes (ectopic pregnancy, adnexal masses, pelvic congestion syndrome) that may require urgent intervention 1
• Do not miss alternative diagnoses: Page kidney (subcapsular hematoma causing secondary hypertension), renal infarction, and thoracic disc herniation can all present with flank pain and hematuria 4, 5, 6
• Beware of delayed secondary signs: Ultrasound findings may be falsely negative early in obstruction before hydronephrosis develops 2

Prognosis Based on Imaging Results

• In patients without CT diagnosis: only 14% require hospitalization and 4% undergo surgical intervention 1
• When non-appendiceal CT diagnosis is made: 41% hospitalization and 22% intervention rates 1