A 77-year-old man has had progressive dementia and gait ataxia for the past 9 years. He succumbs to bronchopneumonia. Autopsy shows that the thoracic aorta has a dilated root and arch, giving the intimal surface a "tree-bark" appearance. The microscopic examination of the aorta shows an obliterative endarteritis of the vasa vasorum. Which of the following laboratory findings is most likely to be recorded in this patient's medical history?