Adalimumab-Induced Pneumocystis jirovecii Pneumonitis (PCP) Misdiagnosed As Methotrexate-Induced Pneumonitis

<i>Pneumocystis jirovecii</i> pneumonia (PCP), a life-threatening opportunistic infection, occasionally mimics drug-induced pneumonitis, posing a diagnostic challenge in immunocompromised patients (such as those with rheumatoid arthritis, RA, on immunosuppressive therapy). We present a 46-year-old woman with seropositive RA treated with long-term methotrexate and recently initiated on adalimumab, who presented with progressive dyspnea, dry cough, and low-grade fever. Imaging revealed bilateral interstitial and ground-glass opacities, initially attributed to methotrexate-induced pneumonitis (MTX-IP). Methotrexate was discontinued, and corticosteroids were started, but clinical deterioration ensued. Bronchoalveolar lavage with polymerase chain reaction (PCR) confirmed PCP, leading to a revised diagnosis. The patient responded well to trimethoprim-sulfamethoxazole and corticosteroids, with full recovery and resolution of pulmonary findings. This case highlights the importance of distinguishing PCP from drug-induced lung toxicity, particularly in patients receiving tumor necrosis factor-α inhibitors like adalimumab, which significantly increase susceptibility to opportunistic infections. MTX-IP usually occurs within the first year, whereas PCP may develop insidiously. Elevated lactate dehydrogenase levels and poor response to corticosteroids should prompt further investigation. Early use of diagnostic tools such as bronchoalveolar lavage with PCR is critical for timely diagnosis and treatment. Clinicians must maintain a high index of suspicion for PCP in RA patients with new pulmonary symptoms while on biologic therapy.