A food with a glycemic index of 28 means that it boosts blood sugar only 28 percent as much as pure glucose (pure table sugar). (healthyeating.sfgate.com)
The index was developed by testing the glucose response to a standard amount of carbohydrate against a reference food, usually pure glucose, whose GI value is 100. (fitnessandpower.com)
To do this, researchers commonly use pure glucose as the benchmark, measuring its effect on blood glucose and using that to set the bar. (healthyeating.sfgate.com)