A 19-year-old woman is bothered by a tremor at rest, which becomes progressively worse over the next 6 months. She exhibits paranoid ideation with auditory hallucinations and is diagnosed with acute psychosis. On physical examination, she has scleral icterus. A slit-lamp examination shows corneal Kayser-Fleischer rings. Laboratory findings include total serum protein, 5.9 g/dL; albumin, 3.1 g/dL; total bilirubin, 4.9 mg/dL; direct bilirubin, 3.1 mg/dL; AST, 128 U/L; ALT, 157 U/L; and alkaline phosphatase, 56 U/L. Which of the following additional serologic test findings is most likely to be reported in this patient?