Q. A 28-year-old woman comes to clinic complaining of symptoms of shortness of breath and fatigue upon exertion. On review of systems, she also admits to a painful right knee and left wrist, as well, as a rash on her cheeks and a smoldering fever, which she assumed was secondary to an allergic reaction to dust mites. On clinical examination, there is, indeed, a rash on her face. Notably, it spares the nasolabial folds. You also detect several ulcers in her mouth, which she has struggled with but was embarrassed to tell you about. Laboratory studies show a hemoglobin concentration of 9 g/dl and a platelet count of 140 000 platelets per microliter. In addition, she has a positive Coombs test. Which additional laboratory finding would be consistent with this case?