Do not use smartwatches or smart rings to measure blood glucose. They are not authorized by the FDA. Using them could result in inaccurate blood glucose measurement, which can lead to serious injury or death if used to make medical decisions. Only use FDA-authorized devices to measure blood glucose.

http://www.fda.gov/medical-devices/safety-communications/do-not-use-smartwatches-or-smart-rings-measure-blood-glucose-levels-fda-safety-communication