Diagnostic Approach to Lower Abdominal and Back Pain with Negative Urinalysis
Order CT abdomen and pelvis with IV contrast as the next diagnostic step to identify the underlying cause of lower abdominal and back pain when urinalysis is negative. 1, 2
Immediate Clinical Assessment
Before imaging, perform a focused evaluation to exclude emergent conditions:
- Check vital signs (pulse, blood pressure, temperature) to exclude shock or systemic infection 1
- Assess for urinary retention by measuring post-void residual volume, as retention has 90% sensitivity for cauda equina syndrome and can present with lower abdominal pain 1, 3
- Examine the abdomen for peritoneal signs, masses, or pulsatile abdominal aortic aneurysm, particularly if patient is over 60 years old 1
- Perform straight-leg raise testing and brief neurological exam (motor strength, reflexes, sensation) to assess for radiculopathy if back pain radiates to legs 1
- In women with delayed menses, exclude ectopic pregnancy with β-hCG testing even if urinalysis is negative 1
Recommended Imaging Strategy
CT abdomen and pelvis with IV contrast is the preferred initial imaging modality because:
- It provides comprehensive evaluation of both abdominal and retroperitoneal structures that can cause combined lower abdominal and back pain 1
- In patients with flank/back pain and negative urinalysis without history of kidney stones, contrast-enhanced CT identifies clinically significant non-urological diagnoses in 15% of cases 1, 2
- It has superior sensitivity (90.8%) and specificity (94.2%) for detecting appendicitis, which can present atypically with back pain 1, 4
- IV contrast is essential when urinalysis is negative, as it better characterizes alternative diagnoses beyond urolithiasis 2, 5
Alternative Imaging Considerations
- Non-contrast CT ("stone protocol") should only be used if there is high clinical suspicion for kidney stones despite negative urinalysis, but this has limited utility for other causes 1
- Ultrasound can be considered first-line in pregnant patients or those with radiation concerns, but has lower sensitivity (87.1%) and specificity (89.2%) compared to CT 5, 4
- MRI abdomen and pelvis is appropriate for patients with contraindications to iodinated contrast or when CT findings are equivocal 2
Differential Diagnosis to Consider
Based on the combination of lower abdominal and back pain with negative urinalysis, the imaging should evaluate for:
- Appendicitis (can present with atypical pain patterns and negative urinalysis) 1, 4, 6
- Gynecological pathology in women (ovarian torsion, tubo-ovarian abscess, endometriosis) 1
- Early urinary obstruction with minimal hydronephrosis that hasn't yet caused hematuria 2
- Musculoskeletal causes including vertebral compression fracture, particularly in older patients or those with osteoporosis risk 1
- Gastrointestinal pathology (diverticulitis, inflammatory bowel disease) 1
- Vascular causes (aortic aneurysm in patients >60 years) 1
Common Pitfalls to Avoid
- Do not assume negative urinalysis excludes urological pathology: Early obstruction or anatomical abnormalities may not produce hematuria initially 2
- Do not order non-contrast CT reflexively: Without history of kidney stones, contrast-enhanced CT provides more diagnostic information 2, 5
- Do not delay imaging in patients with severe pain: If pain fails to respond to analgesia within 1 hour, immediate hospital admission and imaging are warranted 1
- Do not overlook red flags: History of cancer, unexplained weight loss, age >50 years, fever, or progressive neurological deficits require urgent evaluation 1
Follow-up Based on CT Results
- If CT shows minimal hydronephrosis: Follow-up ultrasound in 1-6 months to ensure resolution 2
- If CT is negative but symptoms persist: Consider interstitial cystitis/bladder pain syndrome, which requires cystoscopy if Hunner lesions are suspected 1
- If musculoskeletal cause identified: Manage according to low back pain guidelines with conservative therapy unless red flags present 1