Not quite PIB-positive, not quite PIB-negative: Slight PIB elevations in elderly normal control subjects are biologically relevant

►Two cutoff approaches were employed to define PIB-positivity in elderly controls. ►A subset of elderly controls have PIB levels similar to Alzheimer's disease. ►A subset of elderly controls have ambiguously elevated PIB values. ►Ambiguously elevated PIB values may reflect early signs of amyloid deposition.