What diagnostic tests should be ordered for a patient with bilateral flank pain?

Bilateral Flank Pain: Diagnostic Testing

Order a non-contrast CT of the abdomen and pelvis as the initial imaging study for bilateral flank pain, as it provides 98-100% sensitivity and specificity for detecting urinary stones while identifying alternative diagnoses in approximately one-third of patients. 1, 2

Initial Laboratory Workup

Before or concurrent with imaging, obtain:

• Urinalysis with microscopy to assess for hematuria, pyuria, bacteriuria, crystals, and casts 1
• Urine culture if infection is suspected (fever, dysuria, or pyuria present) 1
• Serum creatinine to evaluate renal function 1
• Complete blood count to assess for leukocytosis suggesting infection 1
• C-reactive protein (CRP) if systemic inflammation is suspected 1
• Beta-hCG in all women of reproductive age before imaging to exclude ectopic pregnancy 1

Primary Imaging: Non-Contrast CT Abdomen/Pelvis

This is the gold standard for evaluating bilateral flank pain because it:

• Detects stones of any size, location, or composition with 98-100% sensitivity and specificity 1, 2
• Identifies alternative diagnoses (abscess, pyelonephritis, diverticulitis, bowel obstruction, vascular pathology) in ~33% of cases 1, 2
• Characterizes stone burden, density, and degree of obstruction 1
• Low-dose CT protocols achieve 92-99% sensitivity while reducing radiation exposure and should be requested when available 1

Alternative Imaging in Specific Populations

Ultrasound of kidneys and bladder is the first-line modality in:

• Pregnant patients to avoid ionizing radiation 1, 2
• Patients with severe contrast allergy or renal insufficiency 1

However, recognize these limitations:

• Ultrasound has only 24-57% sensitivity for direct stone visualization, particularly for stones <5 mm 1, 2
• Sensitivity for hydronephrosis is excellent (up to 100%), but secondary signs of obstruction may not appear within the first 2 hours of symptom onset 1, 2
• Absence of hydronephrosis makes larger stones (>5 mm) unlikely 1, 2
• Combining ultrasound with plain abdominal radiography (KUB) increases sensitivity to 79-90% for clinically significant stones 1

When to Add Contrast-Enhanced CT

Reserve CT with IV contrast for:

• Suspected complicated infection (pyelonephritis, perinephric abscess) with fever and systemic symptoms 1
• Non-diagnostic non-contrast CT where stone versus phlebolith differentiation is unclear 1
• Lack of clinical improvement after initial therapy 1

Note that contrast-enhanced CT provides additional diagnostic information in only 5-18% of cases and changes management in just 2-3% of patients 1

Critical Red Flags Requiring Urgent Evaluation

Obtain imaging immediately and consider hospital admission if:

• Fever, chills, or signs of sepsis (suggests obstructive pyelonephritis or urosepsis) 1, 2
• Hemodynamic instability or shock 1, 2
• Inability to urinate or anuria (suggests bilateral obstruction) 2
• Solitary kidney with obstruction 1

Common Pitfalls to Avoid

• Do not assume absence of hematuria excludes stone disease—more than 20% of patients with confirmed stones have negative urinalysis for blood 2, 3
• Do not delay imaging in women of reproductive age—always consider gynecologic causes (ectopic pregnancy, ovarian torsion, pelvic inflammatory disease) that may require urgent intervention 1, 2
• Do not rely on plain radiography (KUB) alone—it has only 29% sensitivity for stones of any size and 72% sensitivity even for large proximal stones 2
• Do not order MRI routinely—it has only 69% sensitivity for identifying the site of stone impaction compared to 100% for CT 1

Bilateral Disease Considerations

When bilateral flank pain is present, consider:

• Bilateral urolithiasis (occurs in patients with recurrent stones, metabolic disorders, or strong family history) 1
• Bilateral pyelonephritis (more common in diabetics, immunocompromised patients, or those with anatomic abnormalities) 1
• Non-urologic causes: bilateral renal vein thrombosis, retroperitoneal pathology, or referred pain from lumbar spine disease 2

Follow-Up Based on CT Results

If stones <5 mm are identified:

• Conservative management with hydration and analgesia 2
• Strain urine to catch stones 2
• Outpatient follow-up with repeat imaging in 7 days if symptoms persist 2

If stones >5 mm or complete obstruction:

• Urgent urology referral within 7-14 days for consideration of endoscopic intervention 2, 3

If no stones but alternative diagnosis:

• Manage according to specific pathology identified (e.g., antibiotics for pyelonephritis, surgical consultation for diverticulitis) 1