Clinical and Imaging Features of Chronic Occult Infectious Arthritis and Undifferentiated Oligoarthritis: A Comparative Analysis

<b>Background</b>: Undifferentiated arthritis is characterized by synovitis that does not meet the criteria for any specific rheumatic disease. However, a subset of chronic occult infectious arthritis, owing to atypical or overlapping clinical features, is often misclassified as undifferentiated oligoarthritis, potentially leading to diagnostic delays and suboptimal management. This study aimed to compare the clinical, laboratory, and imaging characteristics of these two types of oligoarthritis and to evaluate potential discriminatory markers. <b>Methods</b>: Patients older than 16 years with synovitis involving ≤2 joints at Beijing Jishuitan Hospital from September 2023 to December 2024 were included. Ultrasound-guided joint aspiration or synovial biopsy samples were analyzed by culture and next-generation sequencing, classifying patients as pathogen-positive or -negative. Clinical, laboratory, and imaging data (ultrasound, MRI, CT, X-ray) were compared, and multivariable logistic regression and ROC analyses were performed to identify predictors of infectious arthritis. <b>Results</b>: A total of 57 patients were included, with 20 (35.1%) categorized as pathogen-positive and 37 (64.9%) as pathogen-negative. The mean age was 41.7 ± 14.3 years, and 61.4% of the patients were female, with no significant demographic differences between groups. Monoarthritis was more common in pathogen-positive patients, accounting for 95% of cases (<i>p</i> = 0.02). Although the distribution of affected joints was similar between groups, ultrasound revealed a significantly higher bone erosion grade in pathogen-positive patients (<i>p</i> = 0.02), and CT/X-ray demonstrated articular surface destruction in 58.8% of infectious cases compared to 6.2% in pathogen-negative cases (<i>p</i> < 0.001). Serum albumin levels were significantly lower in the pathogen-negative group (20.7 ± 8.5 g/L vs. 41.1 ± 3.9 g/L, <i>p</i> < 0.001). ROC analysis determined that an albumin threshold >35.4 g/L predicted microbiological positivity with 100% sensitivity and 69.7% specificity. Multivariable logistic regression identified normal serum albumin levels, severe ultrasound-detected bone erosion, and imaging evidence of joint surface destruction as significant predictors of chronic occult infectious arthritis. <b>Conclusions</b>: Our findings suggest that, despite overlapping clinical and laboratory features, serum albumin levels, severe bone erosion on ultrasound and articular surface destruction on CT/X-ray may help differentiate chronic occult infectious arthritis from undifferentiated oligoarthritis. Further studies with larger cohorts are needed to confirm these preliminary results.