Pitfalls in Diagnosing Seropositive Rheumatoid Arthritis: Haematologic Malignancy Presenting as an Autoimmune Disease

Carcinomatous Polyarthritis (CP) is defined as the development of arthritis in association with a malignancy but distinct from that associated with metastasis or direct invasion. It can occur before, with, or after onset of malignancy or with treatment. CP usually affects large joints and is negative for Anti Citrullinated Protein Antibodies (ACPA). CP is rarely if ever considered in the differential diagnosis of a case of symmetric polyarthritis with ACPA positivity. Presence of autoimmune disease related antibodies further obscures the diagnosis. We present a case of a young lady with symmetrical inflammatory polyarthritis with positive ACPA who was found to have haematological malignancy. This is one of the very few cases of CP of small joints with ACPA positivity reported worldwide and highlights the need to be vigilant for red flags while evaluating any case presenting with rheumatologic symptoms.