Differential diagnosis of knee effusion
Knee effusion causes range from trauma and degenerative disease to inflammatory, crystal-induced, and infectious etiologies. [1,3] Arthrocentesis with synovial fluid analysis is recommended for unexplained knee effusion to distinguish these etiologies. [1,2]
Immediate, high-risk causes
Septic arthritis is a high-risk diagnosis to evaluate urgently. [2,3] Crystal-induced arthritis (acute gout and calcium pyrophosphate crystal disease) is a major inflammatory mimic of septic arthritis. [1,2]
Infectious etiologies
Bacterial septic arthritis is characterized by inflammatory synovial fluid with marked leukocytosis and neutrophil predominance. [2,3] Low synovial WBC counts can occur early in infectious arthritis or after partial treatment. [4,3] Other infectious causes of acute monoarthritis that can involve the knee include viral and mycobacterial etiologies (including Lyme arthritis and tuberculous arthritis) in appropriate epidemiologic contexts. [5]
Crystal-induced arthropathies
Acute gout causes knee effusion via monosodium urate crystal deposition. [1,6] Calcium pyrophosphate crystal disease (pseudogout) can also present with knee effusion and acute inflammation. [1,6]
Inflammatory and autoimmune arthritides
Inflammatory arthritis can cause knee effusion, including rheumatoid arthritis and other systemic inflammatory arthritides. [1,3] Reactive arthritis and spondyloarthritis-associated arthritis can present as acute mono- or oligoarthritis involving the knee. [5]
Degenerative and mechanical causes
Osteoarthritis can cause knee effusion, typically in a degenerative/mechanical context. [1,5] Meniscal injury, ligament injury, and fractures are traumatic causes of knee effusion. [1]
Hemorrhagic causes
Hemarthrosis is a cause of knee effusion and is often associated with trauma or bleeding disorders. [7]
Serous effusions from systemic conditions and malignancy
Systemic rheumatologic, hematologic, vasculitic, and oncologic processes can present with knee effusion. [1]
Key clinical testing that separates diagnoses
Synovial fluid evaluation should include at minimum cell count with differential and microbiologic studies when infection is a concern. [3,2] Crystal analysis is recommended when crystalline arthritis is suspected. [3,6] A synovial WBC count greater than 50,000 cells/µL with at least 90% neutrophils is suggestive of bacterial septic arthritis. [2]
Important interpretation cautions
Crystals in synovial fluid do not exclude septic arthritis. [4] Normal or modest synovial WBC counts do not exclude septic arthritis. [4,3]
Common synovial fluid features to consider
Inflammatory synovial fluid is associated with crystal-induced arthritis and infection. [3] Neutrophil predominance is common in septic arthritis and can overlap with crystalline arthritis early. [2,4]