How should I evaluate and manage a patient with a bloody knee effusion, including aspiration, infection work‑up, and considerations for intra‑articular steroid injection?

Management of Bloody Knee Effusion

A bloody knee effusion should be aspirated immediately to rule out septic arthritis and establish the diagnosis, as joint aspiration is the definitive diagnostic method and early aspiration leads to significant reduction in treatment duration and better outcomes. 1, 2

Initial Evaluation and Aspiration Technique

Indications for Immediate Aspiration

• Bloody effusions require urgent aspiration to differentiate between septic arthritis (a medical emergency), hemarthrosis from trauma, crystal arthropathy, or other causes. 1, 2
• The presence of blood does not exclude infection—septic arthritis can present with hemarthrosis, particularly in patients with underlying coagulopathy or trauma. 3
• Aspiration should be performed before initiating antibiotics if infection is suspected, as preaspiration antibiotic treatment causes false-negative cultures. 4, 2

Technical Approach

• Use ultrasound or fluoroscopic guidance to improve aspiration accuracy, especially when dealing with complex effusions or when targeting specific compartments. 1
• The superolateral approach (1 cm above and 1 cm lateral to the superior lateral patella at 45 degrees) is most commonly used for knee aspiration. 5
• Employ strict sterile technique with sterile equipment and aseptic approach to minimize infection risk. 6

Synovial Fluid Analysis Protocol

Essential Laboratory Studies

• Order the following tests on all bloody effusions: 4, 2
  • Cell count with differential (critical for distinguishing inflammatory from non-inflammatory causes)
  • Gram stain and aerobic/anaerobic cultures (though Gram stain has poor sensitivity of only 45% but 100% specificity) 4, 7
  • Crystal analysis with polarizing microscopy (definitive for gout/pseudogout, which can present with bloody effusion)
  • Alpha-defensin testing if prosthetic joint (97% sensitivity, 96% specificity) 2, 8

Interpretation of Bloody Aspirate

• Distinguish true hemarthrosis from traumatic tap by observing fluid as it enters the syringe—uniform blood throughout indicates hemarthrosis, while initial blood that clears suggests traumatic tap. 3
• Hemarthrosis etiologies vary by age: in elderly patients, chondrocalcinosis with osteoarthritis is most common; in young adults, consider coagulopathy or synovial tumors. 3
• Purulent-appearing fluid may actually represent crystal-induced arthritis rather than infection—crystal arthritis is the most common cause of purulent-appearing synovial fluid. 3

Infection Work-Up

Serum Markers

• Obtain CRP and ESR as screening tools—when both are negative, infection is highly unlikely (misses only one infected knee). 4, 8
• CRP >10 mg/L warrants repeat aspiration or biopsy if initial aspiration is negative. 4
• CRP demonstrates 73-91% sensitivity and 81-86% specificity for prosthetic joint infection using cutoff of 13.5 mg/L. 8
• Peripheral WBC count is often normal in joint infections despite significant intra-articular inflammation—do not rely on this alone. 2

Critical Timing Considerations

• Patients must be off antibiotics for at least 2 weeks before aspiration when clinically feasible (up to 1 month may be necessary for cultures to become positive). 4, 2
• Monitor carefully for sepsis during antibiotic-free period. 4
• If first aspiration is negative but clinical suspicion remains high, perform weekly repeat aspirations—16% of patients require more than three aspirations for positive culture. 4, 2

Important Pitfall

• "Dry tap" does not exclude infection—absence of aspirated fluid does not indicate absence of infection. 4, 2

Intra-Articular Corticosteroid Injection Considerations

Absolute Contraindications

• Never inject corticosteroids in the presence of:
  • Acute local infection 9
  • Suspected septic arthritis (even if cultures are pending) 2
  • Bacteremia 5
  • Overlying soft tissue infection 5

Relative Contraindications in Bloody Effusions

• Hemarthrosis from trauma should be approached with caution—aspiration may provide temporary relief but effusion commonly reaccumulates within the first week. 10
• Corticosteroids suppress immune function and can mask signs of infection, exacerbate existing infections, and increase risk of disseminated infection. 9
• In traumatic brain injury patients, high-dose systemic corticosteroids are contraindicated due to increased mortality. 9

When Injection May Be Considered

• Only after infection is definitively ruled out through negative cultures and appropriate clinical follow-up. 2
• Corticosteroid injection can provide significant relief in acute exacerbations of osteoarthritis with effusion once infection is excluded. 5
• Typical doses for knee injection range from 20-80 mg depending on preparation used. 11
• Following injection, the joint should be moved gently to aid mixing of synovial fluid and suspension. 11

Special Populations

Prosthetic Joint Patients

• Aspiration demonstrates 100% sensitivity, specificity, and accuracy for diagnosing infection in patients with pain, instability, or loosening after total knee arthroplasty. 4, 2
• Combine aspiration with ESR/CRP screening and intraoperative frozen section analysis when indicated. 4
• Intraoperative synovial fluid re-cultures are necessary even if preoperative aspiration is positive. 4

Patients on Anticoagulation

• Bloody effusions are more common but infection must still be excluded through appropriate aspiration and analysis. 3
• Consider coagulopathy correction if safe to do so, but do not delay diagnostic aspiration. 3

Clinical Outcomes

• Early aspiration in suspected septic arthritis leads to significant reduction in treatment duration and better outcomes. 4, 1
• Aspiration provides only temporary symptomatic improvement (lasting approximately one week) due to early reaccumulation, but is essential for diagnosis. 10
• In non-traumatic effusions of unknown origin, aspiration aids in earlier establishment of diagnosis. 10