When charts indicate mild/moderate and severe hypoglycemia, is this based on the symptoms or on blood sugar readings?

It would be impossible to give exact numbers to distinguish between levels of hypoglycemia so the chart needs to be based on symptoms. A lot depends on how quickly the blood glucose is falling, the body’s own ability to bring the sugar back up, and each individual’s threshold. Sometimes, if blood glucose values have been elevated for a time, and then rapidly fall, a person with diabetes can experience symptoms of a hypo because it takes the brain an hour or two to catch up with the level of glucose in the blood. There is an old saying, “When in doubt, treat” any symptoms that appear to be a low blood sugar, even if you don’t know the actual value.

SS