Differential Diagnosis of Left-Sided Flank Pain with Negative Urinalysis

Initial Diagnostic Considerations

The American College of Radiology recommends obtaining a non-contrast CT abdomen and pelvis as the primary diagnostic study, which has 98-100% sensitivity and specificity for stones, and identifies alternative diagnoses in approximately one-third of patients 1, 2
Approximately one-third of patients with flank pain have extraurinary pathology, emphasizing the need to systematically exclude non-renal causes before attributing symptoms to the urinary tract 1, 2

Urolithiasis can cause flank pain in up to 15% of patients with negative urinalysis and no history of stones, with small stones potentially not causing hematuria 4, 2
Left colonic diverticulitis can present as left-sided flank pain mimicking renal pathology 1
Gastroenteritis and colitis, including inflammatory bowel disease, can manifest with flank pain 1
Splenic pathology, such as splenic infarction or subcapsular hematoma, can cause left flank pain with referred pain patterns 3
Pancreatic tail pathology, including pancreatitis or masses, may present with left flank pain 3
Lumbar spine pathology, such as facet joint arthropathy or disc disease, can cause referred flank pain that is position-dependent 1, 3
Paraspinal muscle strain typically worsens with movement and palpation 1
Benign adnexal masses can cause left-sided flank pain in women 1
Pelvic congestion syndrome may present with chronic flank pain in women 1

Fever, chills, or signs of systemic infection may indicate pyelonephritis, even with negative urinalysis, and warrant urine culture 5, 6
Inability to urinate or decreased urine output suggests complete obstruction requiring urgent intervention 1, 3
Hemodynamic instability may indicate vascular catastrophe, including aortic pathology or renal artery dissection 3

Non-contrast CT abdomen and pelvis is the gold standard initial study, detecting stones with 98-100% sensitivity and identifying extraurinary causes in one-third of patients 1, 2, 3
Contrast-enhanced CT should be considered if non-contrast CT is non-diagnostic and the patient has no history of urolithiasis with negative urinalysis 4, 2
Ultrasound with Doppler may be considered for radiation concerns or when evaluating for nutcracker syndrome, though sensitivity is lower compared to CT 3
MRI is reserved for equivocal cases or when radiation must be avoided 4