an optical sensor includes photoemitter and photodetector elements at multiple spacings for the purpose of measuring the bulk absorptivity of an area immediately surrounding and including a hemodialysis access site , and the absorptivity of the tissue itself . at least one photoemitter element and at least one photodetector element are provided , the total number of photoemitter and photodetector elements being at least three . the photoemitter and photodetector elements are collinear and altematingly arranged , thereby allowing the direct transcutaneous determination of vascular access blood flow .