Diagnostic challenges in a case of reactive arthritis with concurrent calcium pyrophosphate disease

Peripheral arthritis can be triggered by a variety of causes, two of them being calcium pyrophosphate (CPP) disease and peripheral spondyloarthritis (pSpA). After the identification of pSpA, further paraclinical investigations guided by the clinical context of the patient can help differentiate between different subsets of this pathology such as reactive arthritis (ReA), psoriatic arthritis and spondyloarthritis (SpA) related to inflammatory bowel disease. As in many other rheumatologic conditions, diagnostic criteria for pSpA are currently lacking. Classification criteria were implemented for research purpose and were not designed to be used in the case of an individual patient. Nevertheless, used with caution, classification criteria can serve as a valuable tool in guiding the diagnostic approach. We describe the case of a 52 year-old man for which the final diagnosis of ankylosing spondylitis (AS) with overlapped ReA and concurrent CPP disease was made based on a complex decision-making process, along with a comprehensive differential diagnosis. The association between SpA and crystal- induced arthritis is extremely rare, with only a few cases being reported until now. In this particular case, ASAS classification criteria for pSpA were able to strengthen the diagnosis.