What is the next best step for a patient with left flank area pain that worsens with prolonged standing or seating, elevated C-Reactive Protein (CRP) and erythrocyte sedimentation rate (Sed rate), normal urinalysis (UA) and computed tomography (CT) of the abdomen, and normal complete blood count (CBC)?

Obtain MRI of the Spine with Contrast Urgently to Rule Out Spinal Infection

Given the combination of flank pain worsening with prolonged standing/sitting and elevated inflammatory markers (CRP and ESR), you must obtain an urgent MRI of the spine with and without IV contrast to exclude vertebral osteomyelitis or epidural abscess, as these life-threatening conditions can present with positional back pain and elevated inflammatory markers. 1

Critical Red Flags Present in This Case

Your patient has two concerning red flags that mandate urgent imaging:

• Elevated inflammatory markers (CRP and ESR) combined with back/flank pain constitutes a red flag requiring urgent imaging, regardless of normal CT abdomen and urinalysis 2
• Positional pain (worsening with prolonged standing or sitting) can indicate musculoskeletal pathology but does not exclude spinal infection, which has a mortality rate of 0-11% when diagnosis is delayed 1

Why MRI is Essential

• MRI of the spine with and without IV contrast is the diagnostic test of choice for spinal infection, with sensitivity of 96%, specificity of 94%, and accuracy of 92% 1
• CT is inadequate for detecting epidural abscess (sensitivity only 6%) and was already performed on the abdomen, not the spine 1
• The normal CT abdomen effectively ruled out intra-abdominal pathology (diverticulitis, appendicitis, renal stones) but did not evaluate the spine adequately 2

Immediate Actions Before MRI

Obtain blood cultures (two sets from separate sites) before starting any antibiotics if infection is suspected, as this is critical for identifying the causative organism 1

Assess for additional red flags immediately:

• Fever, night sweats, or unintentional weight loss (constitutional symptoms suggesting infection) 1
• Progressive weakness, numbness, or bowel/bladder dysfunction (suggesting epidural abscess or cord compression requiring emergency intervention) 1
• Risk factors for infection: IV drug use, recent spinal procedures, diabetes, immunosuppression, or indwelling catheters 1

Understanding the Inflammatory Markers

• CRP is more sensitive than ESR for acute inflammatory processes and correlates better with active disease 2, 3, 4
• ESR and CRP show poor agreement (kappa coefficient 0.38) in hospital practice, with discordance in 33% of cases 3
• Elevated ESR with normal CRP occurs in 28% of cases and can indicate resolving inflammation, confounding factors (renal dysfunction, anemia), or false-positive ESR 3, 5
• However, both markers elevated together increases concern for active inflammatory process requiring investigation 1, 5

Alternative Diagnoses to Consider After Negative MRI

If MRI shows no spinal infection, consider:

Musculoskeletal causes:

• Thoracic disc herniation can present with flank pain mimicking urinary disorders, and far lateral disc herniations may not be visible on routine imaging 6
• Facet joint arthropathy or sacroiliitis (especially if inflammatory markers suggest spondyloarthritis) 2

Inflammatory conditions:

• Polymyalgia rheumatica (PMR) if patient has bilateral shoulder/hip girdle pain with morning stiffness >45 minutes 1
• Inflammatory arthritis - check rheumatoid factor, anti-CCP antibodies, and ANA if joint symptoms present 1

Vascular causes:

• Nutcracker syndrome can present with atypical left flank pain worsening with standing and increased lumbar lordosis, though typically associated with hematuria or orthostatic symptoms 7

Management Algorithm Based on MRI Results

If MRI shows spinal infection:

• Start empiric IV antibiotics (vancomycin plus third-generation cephalosporin like ceftriaxone) immediately after blood cultures obtained 1
• Obtain tissue diagnosis via CT-guided biopsy when feasible to guide antibiotic selection 1
• Monitor inflammatory markers at 4 weeks - they should decline, though persistent elevation alone doesn't indicate treatment failure if clinical improvement occurs 1

If MRI is negative:

• Repeat ESR and CRP in 2-4 weeks to determine if elevation is persistent or transitory 1
• Consider MRI with specific sequences for spondyloarthritis if inflammatory back pain pattern present (STIR sequences sufficient to detect inflammation) 2
• Refer to rheumatology if inflammatory arthritis suspected based on clinical presentation 1

Common Pitfall to Avoid

Do not assume the normal CT abdomen and urinalysis exclude serious pathology. The CT abdomen evaluated intra-abdominal organs but not the spine adequately for infection or inflammatory conditions 2, 1. The combination of positional pain with elevated inflammatory markers requires dedicated spinal imaging regardless of negative abdominal workup 2, 1.