Reactive Arthritis is NOT Typically Associated with Eosinophilia
Reactive arthritis (ReA) is classically NOT characterized by eosinophilia, and the presence of significant eosinophilia should prompt investigation for alternative diagnoses or concurrent conditions. 1
Key Diagnostic Considerations
Reactive arthritis is a sterile inflammatory arthritis triggered by extra-articular infections (typically urogenital, gastrointestinal, or respiratory), manifesting as peripheral oligoarthritis, dactylitis, and sometimes axial involvement, often in HLA-B27 positive individuals 1. The typical laboratory profile does NOT include eosinophilia.
When Eosinophilia Appears with Arthritis: Alternative Diagnoses
If you encounter arthritis with eosinophilia, consider these distinct entities:
Primary considerations:
Eosinophilic Granulomatosis with Polyangiitis (EGPA): The most important diagnosis to exclude. Look for asthma, chronic rhinosinusitis with polyps, eosinophilia ≥1×10⁹/L, neuropathy (mononeuritis multiplex), lung infiltrates, and ANCA positivity 2. A cumulative score ≥6 on the 2022 ACR-EULAR criteria classifies EGPA with 85% sensitivity and 99% specificity 2.
Parasitic infections: The most common cause of eosinophilia worldwide, particularly in endemic regions. Can cause systemic disease and arthritis 3. The case report of ReA with eosinophilia involved Ankylostoma duodenale infestation 4.
Eosinophilic arthritis: A distinct entity described primarily in Asian populations, presenting as acute polyarthritis with mild-to-moderate eosinophilia (11-63%), affecting large joints without systemic involvement 5. This responds to NSAIDs, corticosteroids, or interestingly, anthelmintic drugs like diethylcarbamazine 5.
The Rare Exception: TB Hypersensitivity
One recent case report described HLA-B27-associated ReA with eosinophilia in the context of tuberculosis hypersensitivity 6. However, this represents an unusual overlap syndrome rather than typical ReA.
Clinical Pitfalls to Avoid
Do not dismiss eosinophilia as part of ReA. If present, it demands:
- Comprehensive parasitic workup (stool studies, serologies)
- Evaluation for EGPA (pulmonary function tests, chest imaging, ENT examination, nerve conduction studies, ANCA testing) 2
- Consideration of drug-induced eosinophilia
- Assessment for malignancy and idiopathic hypereosinophilic syndrome if other causes excluded 3
The correlation between eosinophil count and joint activity can help identify the target organ involvement 5.
Emerging Nuance: Regulatory Eosinophils
Recent research has identified a subset of "regulatory eosinophils" (rEos) in RA synovium that promote resolution of inflammation, particularly in remission states 7, 8. However, this represents a specialized immunologic phenomenon in established RA, not a feature of reactive arthritis, and these patients do not typically present with peripheral blood eosinophilia.
Bottom line: If you see reactive arthritis with eosinophilia, you're likely dealing with two separate processes or an alternative diagnosis entirely.