Poncet's disease with high titers of rheumatoid factor and anti-citrullinated peptide antibodies mimicking rheumatoid arthritis.

Reactive arthritis accompanying tuberculosis (TB), also known as Poncet's disease, is a rare condition. In the present report, we describe the case of a patient with Poncet's disease, who presented with high titers of rheumatoid factor (RF) and anti-citrullinated peptide antibodies (ACPA), which mimicked rheumatoid arthritis (RA). A 69-year-old man with a childhood history of chronic left gonitis suffered from right knee arthritis for 3 years. Chronic monoarthritis in his right knee and positive results obtained on interferon-gamma release assay were suggestive of tuberculous arthritis. However, there was no evidence of TB infection. Moreover, the high titers of RF and ACPA suggested a diagnosis of RA. Surprisingly, the culture of a small sample from his bony ankylosed left knee that had no focal signs of infection, exhibited a positive result for TB infection. Thus, based on these findings, the patient was diagnosed with Poncet's disease. His symptoms improved after initiation of anti-TB therapy, which supported the accuracy of the diagnosis. In addition, we analyzed the characteristics of Poncet's disease by conducting a literature review, and identified that the presence of extra-articular manifestation and negative results for RF and ACPA tests were the features that facilitated distinguishing between typical Poncet's disease and RA; however, since tuberculous patients occasionally exhibit positive results for ACPA tests, the differential diagnosis is essential in ACPA-positive arthritic patients.