An apparatus includes a source and a detector. The source is operable to transmit light. The detector is operable to receive at least a portion of the light transmitted by the source. The source and the detector can be positioned on a nail of a digit such that the light enters the nail through an entrance region. The light further exits the nail through an exit region. The entrance region is defined by a lateral portion of the nail. The light is directed to interact with at least one analyte.