# Pseudogout Masquerading as Septic Arthritis in a Patient With Rheumatoid Arthritis

## TL;DR

A patient with rheumatoid arthritis showed symptoms of septic arthritis, but was diagnosed with pseudogout through synovial crystal analysis.

## Contribution

Highlights the importance of synovial crystal analysis to avoid unnecessary antibiotic use in immunosuppressed patients.

## Key findings

- Synovial aspiration revealed rhomboid, weakly positively birefringent crystals consistent with CPPD.
- Discontinuation of antibiotics led to rapid improvement with corticosteroids.
- Use of biomarkers and early arthrocentesis is essential for accurate diagnosis.

## Abstract

Septic arthritis is a medical emergency requiring prompt diagnosis and treatment to prevent joint destruction. However, crystal arthropathies such as calcium pyrophosphate deposition (CPPD) disease can closely mimic infection, particularly in immunosuppressed patients, leading to diagnostic uncertainty and potentially unnecessary antibiotic use. We describe a 67-year-old wheelchair-bound man with seropositive rheumatoid arthritis on methotrexate, hydroxychloroquine, and prednisone who presented with acute left-wrist pain, swelling, and limited range of motion following minor trauma. Laboratory studies showed leukocytosis, elevated CRP, ESR, and mildly increased procalcitonin. Radiographs revealed chondrocalcinosis, and MRI demonstrated pancarpal joint effusion and erosive arthropathy. Given immunosuppression and inflammatory markers, empiric vancomycin was started for presumed septic arthritis. Synovial aspiration yielded scant fluid; microscopy demonstrated numerous rhomboid, weakly positively birefringent crystals consistent with CPPD. Gram stain and culture were negative. Antibiotics were discontinued, and the patient improved rapidly on corticosteroids. This case underscores the importance of synovial crystal analysis in differentiating septic arthritis from pseudogout, particularly in patients with rheumatoid arthritis or immunosuppression. Judicious use of biomarkers (calprotectin, procalcitonin) and early arthrocentesis are essential to avoid overtreatment with antibiotics.