What is the appropriate workup and initial management for an adult presenting with acute right flank pain without a prior history of kidney stones?

Acute Right Flank Pain: Workup and Management

Non-contrast CT of the abdomen and pelvis is the imaging study of choice for evaluating acute right flank pain, with 98-100% sensitivity and specificity for detecting urinary stones and 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 and radiates to the groin or genitals with abrupt onset 2, 3
• Positional relationship: Pain worsening with external flank pressure or after prolonged static positioning suggests pyelonephritis, perinephric abscess, or musculoskeletal origin rather than uncomplicated stone disease 2, 3
• Associated symptoms: Fever, chills, inability to urinate, or decreased urine output are red flags requiring urgent evaluation 2, 4
• Hematuria presence: Even microscopic hematuria shifts probability toward stone disease, though over 20% of confirmed stones may have negative urinalysis 1, 2

Physical Examination Priorities

• Vital signs: Hemodynamic instability or shock requires immediate hospital admission 2
• Flank tenderness: Significant worsening with external pressure suggests infection (pyelonephritis or perinephric abscess) rather than stone disease 3, 4

Diagnostic Imaging Algorithm

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

This is the gold standard and should be obtained in most patients without contraindications. 1, 2

Advantages:

• Detects stones of any size, location, or composition with 97-100% sensitivity and 96-100% specificity 1, 5
• Identifies extraurinary causes in approximately one-third of patients presenting with acute flank pain 2, 6, 5
• Completed within 5 minutes without need for contrast 6
• Low-dose CT protocols are non-inferior to standard-dose for detecting ureteral stones with 92-99% sensitivity 4

Alternative diagnoses commonly detected:

• Right colonic diverticulitis, appendicitis, inflammatory bowel disease 2
• Gynecologic pathology (adnexal masses, ovarian cyst torsion) 2, 7, 5
• Pyelonephritis or perinephric abscess 4, 7

When to Use Ultrasound Instead

Ultrasound is the first-line imaging modality for pregnant patients to avoid radiation exposure. 2

Ultrasound performance characteristics:

• Up to 100% sensitive and 90% specific for hydronephrosis, ureterectasis, and perinephric fluid 1, 2
• Only 24-57% sensitive for direct stone detection (reduced for stones <5 mm) 1
• Absence of hydronephrosis makes larger ureteral stones (>5 mm) less likely 1, 2
• Sensitivity for ureteral calculi up to 61% with 100% specificity, improved if obstruction present 1

Important caveat: Within the first 2 hours of presentation, secondary signs of obstruction may not have developed, reducing ultrasound sensitivity 1

Ultrasound combined with radiography achieves 79-90% sensitivity for clinically significant stones, though all missed cases in one study had spontaneous passage 1, 3

When to Add Contrast-Enhanced CT

Consider contrast-enhanced CT if: 4

• Strong clinical suspicion for complicated infection (fever with flank pain)
• Initial non-contrast CT is inconclusive
• No improvement after initial management

Note: Contrast-enhanced CT provides additional diagnostic information in 5-18% of cases but changes management in only 2-3% 1

Immediate Management

Pain Control

• Administer rapid analgesia within 30 minutes, preferably with NSAIDs (diclofenac intramuscular injection preferred) 2
• Reassess pain within 1 hour: Failure of analgesia mandates immediate hospital admission 2

Urinalysis and Urine Culture

• Obtain urinalysis to assess for hematuria and signs of infection 8
• Send urine culture if infection suspected (pyuria, fever) 8

Risk Stratification and Disposition

Immediate Hospital Admission Required If:

• Hemodynamic instability or signs of sepsis 2, 4
• Inability to urinate or decreased urine output 2, 4
• Failure of analgesia after 1 hour 2
• Fever with obstructing stone (suggests infected obstructed system) 4

Outpatient Management Appropriate If:

• Pain controlled with oral analgesia 2
• No signs of infection or obstruction 8
• Patient able to maintain oral hydration 2

Outpatient management protocol: 2, 8

• Instruct patient to maintain high fluid intake and strain urine to catch stones
• Telephone follow-up 1 hour after initial assessment
• Fast-track radiology within 7 days if not already obtained
• Follow-up imaging within 14 days to monitor stone position and assess for hydronephrosis

Medical Expulsive Therapy

• Consider alpha blocker for stones likely to pass spontaneously 8
• Stones <5 mm typically pass spontaneously 2

Urgent Urology Referral Within 7-14 Days If:

• Stone >5 mm identified 2, 8
• Complete obstruction present 2
• Stone >10 mm (requires active removal) 8

Common Pitfalls to Avoid

1. Do not assume absence of hematuria excludes stone disease – over 20% of confirmed stones have negative urinalysis 2

2. Do not delay imaging in young females – consider gynecologic causes (ectopic pregnancy, ovarian torsion) that may require urgent intervention 2, 7

3. Do not miss ectopic pregnancy in women with delayed menses presenting with flank pain 2

4. Do not assume all flank pain is kidney-related – positional nature and response to external pressure are key distinguishing features 2, 3

5. Do not rely on conventional radiography (KUB) – only 72% sensitive for large stones (>5 mm) and 29% sensitive overall 2

6. Do not perform ultrasound within first 2 hours expecting to see hydronephrosis – secondary signs of obstruction take time to develop 1