What is the appropriate work‑up and treatment for a deep fluid collection (possible seroma or infection) following total hip arthroplasty?

Deep Fluid Collection After Total Hip Arthroplasty

Assume infection until proven otherwise for any deep fluid collection after total hip arthroplasty and proceed immediately with image-guided aspiration for definitive diagnosis. 1

Initial Diagnostic Approach

Clinical Assessment

• Check inflammatory markers immediately: ESR, CRP, and white blood cell count with differential, as elevation strongly suggests infection 1
• Recognize that collections identified ≤6 weeks postoperatively are particularly challenging to interpret because normal postoperative changes overlap with infection findings 1
• Understand that fluid collections in muscles or perimuscular fat carry a 100% positive predictive value for infection 2, 1

Imaging Strategy

MRI with and without IV contrast is the primary diagnostic modality, providing 96% sensitivity and 94% specificity for periprosthetic infection 1, 3. Key imaging features include:

• Peripheral rim enhancement indicates abscess requiring drainage 1, 3
• Soft tissue edema demonstrates 86.7% sensitivity and >73.3% specificity for infection 2, 1
• Intramuscular edema yields 86-91% diagnostic accuracy for infection 2, 1
• Enlarged regional lymph nodes (compared to contralateral hip) identify infection with up to 93.1% accuracy 2, 1
• Lamellated hyperintense synovitis is highly specific for infection 2

Critical MRI protocol requirement: Always obtain pre-contrast sequences—contrast-only studies miss true enhancement patterns and cannot reliably differentiate abscess from seroma 1, 3

If MRI is contraindicated, use contrast-enhanced CT as an acceptable alternative 1. CT demonstrates:

• Joint distension (83% sensitivity, 96% specificity for infection) 2
• Fluid in muscle/perimuscular fat (100% PPV for infection) 2, 1
• IV contrast helps define abscess versus phlegmon 2

Important limitation: CT has only ~6% sensitivity for epidural extension, so MRI is mandatory when spinal involvement is suspected 1, 3

Definitive Diagnosis: Image-Guided Aspiration

Perform CT- or fluoroscopic-guided hip aspiration within 48-72 hours of collection detection—this is the gold standard with 68.6% sensitivity and 96.4% specificity for infection 2, 1

Send aspirated fluid for:

• Cell count with differential 1
• Gram stain 1
• Aerobic and anaerobic cultures 1
• Alpha-defensin and leukocyte esterase when available 2, 1

Critical caveat: Negative aspiration cultures do not exclude infection—up to 31.4% of infected joints have false-negative results 1

Treatment Algorithm

For Confirmed Infection (Abscess)

Collections ≥3 cm require percutaneous catheter drainage 1, 3:

• Use large-bore catheters (10-14 Fr) 1
• Seldinger (wire-guided) technique is preferred for complex or deep collections 1, 3
• Initiate broad-spectrum IV antibiotics immediately, then de-escalate based on culture results 1

Catheter removal criteria: Output <300 mL/24 hours AND follow-up imaging confirming resolution 1

If no clinical improvement within 48-72 hours, proceed to surgical debridement 1

For Sterile Collections (Seroma)

• Symptomatic seromas ≥3 cm: Perform single aspiration with large-bore needle 1, 4
• Asymptomatic seromas <3 cm: Conservative observation with serial clinical assessment 1, 3

Research evidence supports that CT-guided aspiration of symptomatic postoperative seromas is safe, with 50% achieving resolution or substantial improvement without requiring surgery 4

Critical Pitfalls to Avoid

• Never rely on imaging alone to differentiate abscess from seroma—clinical correlation and fluid analysis are mandatory 1, 3
• Do not delay drainage of symptomatic collections causing mass effect or neurologic symptoms, as early intervention prevents permanent injury 3
• Avoid performing MRI with contrast only—pre-contrast sequences are essential for accurate interpretation 1, 3
• Do not assume negative cultures exclude infection—consider repeat aspiration or biopsy when clinical suspicion remains high despite negative initial cultures 2, 1
• Recognize that FDG-PET/CT has inconsistent performance for periprosthetic hip infection (specificity as low as 38% in some studies), with false-positive rates up to 77% compared to culture 2