A 66-year-old female treated for a urinary tract infection with bacteremia 10 days ago now presents with right lower abdominal, flank, and back pain, fever, difficulty walking, a limp with the right hip held in flexion, and pain on passive and active extension of the right hip. What is the most likely diagnosis?

Psoas Abscess

This patient most likely has a psoas abscess, a serious complication of her recent UTI with bacteremia that requires urgent imaging and intervention to prevent permanent disability and mortality.

Clinical Reasoning

The constellation of findings is pathognomonic for psoas abscess:

• Classic triad present: Fever, back/flank pain, and limp
• Pathognomonic positioning: Hip held in flexion with pain on extension (the psoas sign)
• Recent bacteremia: 10-day history of UTI with bacteremia provides the infectious source
• Difficulty walking: Indicates significant iliopsoas involvement

The psoas muscle originates from the lumbar vertebrae and inserts on the femoral lesser trochanter. When infected, patients reflexively hold the hip in flexion to minimize stretch on the inflamed muscle. Any attempt at hip extension (passive or active) stretches the psoas and causes severe pain.

Pathophysiology and Source

Psoas abscesses develop through three mechanisms:

1. Hematogenous spread (most likely in this case given recent bacteremia)
2. Direct extension from adjacent infection (vertebral osteomyelitis, pyelonephritis, diverticulitis)
3. Penetrating trauma

Given her recent UTI with bacteremia, hematogenous seeding from Gram-negative organisms (likely E. coli or Enterobacterales) is the most probable mechanism. The 10-day interval fits the timeline for abscess formation following bacteremia 1.

Immediate Diagnostic Approach

CT abdomen and pelvis with IV contrast is the diagnostic study of choice 2. This imaging modality:

• Has high positive predictive value (81.82%) for identifying septic foci 2
• Can identify psoas abscess, associated complications (vertebral osteomyelitis, epidural extension), and alternative diagnoses
• Guides percutaneous drainage procedures

While MRI provides superior soft tissue detail and can better evaluate for concurrent vertebral osteomyelitis/discitis 1, CT is more readily available in acute settings and sufficient for initial diagnosis and intervention planning.

Critical Pitfalls to Avoid

1. Do not delay imaging: The hip flexion posture with pain on extension is highly specific—empiric antibiotics alone are insufficient
2. Assess for vertebral involvement: Up to 60% of psoas abscesses have associated vertebral osteomyelitis or discitis 1
3. Consider epidural extension: Can cause spinal stenosis and neurological compromise requiring urgent neurosurgical evaluation 1
4. Blood cultures are essential: Even if recent cultures were positive, repeat cultures guide targeted antibiotic therapy

Management Implications

This diagnosis demands:

• Source control: Most psoas abscesses require CT-guided percutaneous drainage or surgical debridement
• Prolonged antibiotics: Typically 4-6 weeks of IV therapy, often with metronidazole for anaerobic coverage plus Gram-negative coverage 1
• Transition to oral therapy: Can occur once clinically improving and effective oral option available 3, but only after adequate source control

The mortality risk is significant if untreated, and delayed diagnosis can result in permanent hip dysfunction, septic shock, or epidural abscess with paralysis.