# A patient with positive anti‐IFN‐γ autoantibody and monoclonal gammaglobulinemia masquerading as multiple myeloma: Case report and literature review

## TL;DR

A rare case of adult-onset immunodeficiency caused by anti-IFN-γ autoantibodies is mistaken for multiple myeloma, highlighting diagnostic challenges and treatment outcomes.

## Contribution

This case report emphasizes the importance of screening for anti-IFN-γ autoantibodies in patients with monoclonal gammaglobulinemia and NTM infection.

## Key findings

- Anti-IFN-γ autoantibodies can mimic multiple myeloma and lead to NTM infections.
- Anti-plasma cell therapy showed favorable outcomes in treating this condition.
- Monoclonal gammaglobulinemia with NTM infection is often misdiagnosed as multiple myeloma.

## Abstract

Adult‐onset immunodeficiency (AOID) is an emerging acquired immunodeficiency, characterized by multiple opportunistic infections including non‐tuberculous mycobacterium (NTM) due to the presence of anti‐IFN‐γ autoantibody (AIGA). This case highlights the challenges of accurate diagnosis of monoclonal gammaglobulinemia with NTM infection and favorable outcomes of anti‐plasma cell therapy in AOID.

Adult‐onset immunodeficiency (AOID) is an emerging acquired immunodeficiency due to anti‐IFN‐γ autoantibody (AIGA) with low morbidity, frequent disseminated infections, a prolonged course, difficult diagnosis and treatment, and a poor prognosis. Here, we report a patient with positive AIGA and monoclonal gammaglobulinemia who was mimicking symptomatic multiple myeloma and resulting in a non‐tuberculous mycobacterial (NTM) infection. While he achieved an excellent therapeutic effect with anti‐plasma cell therapy, it also serves as a warning that monoclonal gammaglobulinemia with NTM infection is easily misdiagnosed as symptomatic multiple myeloma, and the screening for AIGA should not be ignored in patients with NTM infection.