What is the most likely diagnosis—psoas (retroperitoneal) abscess, perinephric abscess, or septic arthritis of the right hip—in a 66-year-old patient with recent urinary tract infection, two days of malaise, low‑grade fever, right lower abdominal, back and flank pain, difficulty walking, a limp with right hip flexion, tenderness in the right lower abdomen, pain on passive extension of the right hip (positive psoas sign), mild tachycardia, leukocytosis, and a negative urine dipstick?

Psoas Abscess

The most likely diagnosis is a psoas (retroperitoneal) abscess. The clinical presentation—pain on passive hip extension (positive psoas sign), relief with hip flexion, recent UTI, fever, leukocytosis, and difficulty walking with a limp—is pathognomonic for iliopsoas abscess rather than perinephric abscess or septic hip arthritis.

Clinical Reasoning

Why Psoas Abscess is Most Likely

The positive psoas sign is the key diagnostic feature here. Pain with passive hip extension that improves with flexion directly indicates psoas muscle irritation or infection 1, 2. This patient's recent hospitalization for UTI 10 days ago provides the likely source—secondary psoas abscess from genitourinary tract infection, though this etiology is less commonly described than gastrointestinal sources 1.

The clinical triad supporting psoas abscess includes:

• Hip flexion posture and limp: Patients hold the hip in flexion to minimize psoas stretch
• Pain on hip extension but NOT rotation: Distinguishes from hip joint pathology
• Right lower abdominal/flank pain: Reflects retroperitoneal location

Why NOT Perinephric Abscess

Perinephric abscess typically presents with:

• Persistent flank/costovertebral angle tenderness as the dominant feature
• Failure to respond to antibiotics for pyelonephritis 3
• Less prominent hip symptoms
• The negative urine dipstick makes active pyelonephritis or perinephric extension less likely, though blood and urine cultures can be negative in 30-40% of perinephric abscesses 3

Why NOT Septic Hip Arthritis

Septic arthritis would show:

• Pain with ALL hip movements, especially rotation 4
• This patient has pain only with extension, not rotation
• More acute presentation with inability to bear weight
• Joint effusion on examination

Diagnostic Approach

Immediate imaging with CT abdomen/pelvis with IV contrast is indicated 5. While guidelines state CT is not needed for uncomplicated pyelonephritis 6, this patient has:

• Failed recent UTI treatment (hospitalized 10 days ago)
• Persistent fever and systemic symptoms
• Concerning physical findings suggesting complicated infection

CT will demonstrate:

• Psoas muscle enlargement with rim-enhancing fluid collection
• Possible gas within the abscess
• Extension patterns (thigh, gluteal region, or mediastinum in severe cases) 7
• Primary source if gastrointestinal (appendicitis, diverticulitis)

Management Priorities

1. Blood cultures and repeat urine culture before antibiotics, even though urine dipstick is negative—cultures may still yield organisms 1, 8

2. Empiric broad-spectrum antibiotics targeting gram-negative Enterobacteriaceae (most common from GU source) and gram-positive cocci 9:

   • Fourth-generation cephalosporin or carbapenem if ESBL risk
   • Add anaerobic coverage if GI source suspected

3. Source control via drainage:

   • CT-guided percutaneous drainage is first-line for accessible collections 10
   • Open surgical drainage if extensive fascial necrosis or multi-compartment spread 7
   • Success rates for percutaneous drainage: 70-90% 10

Critical Pitfalls

• Delayed diagnosis is common due to nonspecific symptoms 3, 2. The insidious presentation (malaise, low-grade fever) can mimic many conditions
• Don't be falsely reassured by negative urine studies—blood and urine cultures are often negative in retroperitoneal abscesses 3
• Diabetes increases risk for complications and atypical presentations 6
• Follow-up for 1 year: Relapse occurs in 15-36% of cases 1
• Always search for the primary source—psoas abscess is often diagnosed before the underlying disease is identified 1

The combination of recent UTI, positive psoas sign, and systemic symptoms in a 66-year-old makes psoas abscess the diagnosis until proven otherwise.