Researchers at the University of Michigan studied glucose in rabbits. They found that glucose levels are much lower in tears than in blood, but the difference is consistent. They thus aim to develop a sensitive enough system to detect sugar levels in tears.
In order to best control their glucose levels and prevent complications like kidney failure or limb amputation, some diabetics should be testing their blood multiple times a day. But the pain of jabbing a finger with a needle over and over keeps some patients from the frequent testing they need. With a pain-free test, that deterrent would vanish, making blood sugar tests a lot sweeter.
An excellent example of new ways of doing the same thing that make people suffering less! The other key point is to take advantage of fixed ratio or patterns between tears and blood sugar levels.