The availability of effective immunosuppressive medication has greatly improved survival for patients with antineutrophil cytoplasmic antibody–associated vasculitis (AAV) but also conveys increased infection risk. Infections are the leading cause of death in the year following diagnosis1 and the most common reason for hospitalization in patients with AAV.2 To reduce these risks, multiple clinical practice guidelines, including those from KDIGO (Kidney Disease: Improving Global Outcomes),3 recommend prophylaxis against Pneumocystis jirovecii pneumonia (PJP) with trimethoprim/sulfamethoxazole (TMP/SMX) for patients with AAV receiving rituximab or cyclophosphamide.