From the Guidelines
The workup for knee effusion should begin with a thorough history and physical examination, followed by joint aspiration (arthrocentesis) for definitive diagnosis, as recommended by the Infectious Diseases Society of America 1. The initial evaluation includes assessing for trauma, pain characteristics, systemic symptoms, and examining for warmth, redness, and range of motion limitations.
- Laboratory tests should include:
- Complete blood count
- Inflammatory markers (ESR, CRP)
- Consideration of rheumatoid factor and anti-CCP antibodies if inflammatory arthritis is suspected
- Joint aspiration (arthrocentesis) is essential for definitive diagnosis, with fluid analysis including:
- Cell count
- Crystal examination
- Gram stain
- Culture
- Glucose levels This helps differentiate between inflammatory, infectious, traumatic, and degenerative causes. Imaging studies should start with plain radiographs to assess for fractures, osteoarthritis, or loose bodies.
- Advanced imaging like MRI may be indicated if internal derangement is suspected, particularly for meniscal or ligamentous injuries.
- Ultrasound can be useful for guiding aspiration and assessing soft tissue pathology. The diagnostic approach should be tailored to the clinical presentation, with urgent evaluation for suspected infection, which would require immediate arthrocentesis, antibiotics, and possible surgical intervention. For non-infectious causes, treatment depends on the underlying etiology, ranging from rest and anti-inflammatories for minor injuries to disease-modifying agents for inflammatory arthritis, as suggested by the EULAR recommendations 1.
From the Research
Work-up for Knee Effusion
- The work-up for knee effusion typically involves a combination of history taking, physical examination, and diagnostic imaging studies 2, 3.
- A thorough medical history is essential to identify the underlying cause of the knee effusion, including traumatic or atraumatic etiologies 2.
- Physical examination of the knee, including specific maneuvers such as the ballottement test, can help diagnose knee joint effusion 4.
- Diagnostic imaging studies, such as ultrasonography or MRI, can confirm the presence of effusion and assess for internal derangement of the knee 4, 3.
- Arthrocentesis, which involves aspirating synovial fluid from the knee joint, can be diagnostic or therapeutic and can help identify the cause of the knee effusion 5.
Diagnostic Value of History Taking and Physical Examination
- The combination of self-noticed swelling and the ballottement test has been shown to have diagnostic value for knee joint effusion 4.
- The likelihood ratio positive (LR+) for self-noticed swelling and the ballottement test is 1.5 and 1.6, respectively, and combining these two tests improves the diagnostic value to an LR+ of 3.6 4.
- The accuracy of clinical examination in detecting knee effusion can be affected by effusion depth and clinician experience, but not by patient obesity 6.
Role of Arthrocentesis
- Arthrocentesis can be used to diagnose and treat knee effusion, and can help identify the underlying cause of the effusion 5.
- Arthrocentesis can also aid in diagnosing crystal-induced arthritis, such as gout or pseudogout, or non-inflammatory arthritis, such as osteoarthritis 5.
- Urgent arthrocentesis is indicated if septic arthritis is suspected, and can help guide treatment and improve patient outcomes 5.