The glycemic index (GI) that is assigned to foods is an average of the glycemic responses of many individuals. It is possible for people with diabetes to measure their own personal response to a food to determine how it affects the glucose (blood sugar) level. By testing individually, people with diabetes are able to get their own glycemic response for specific foods. The process requires consistent, timed blood testing that is usually more involved than what would be done for normal glucose monitoring.

The first step in determining how food affects the glucose level requires testing glucose immediately before and two hours after eating. This reading will give the person with diabetes a general idea of the way the food raises glucose. However, because different foods raise the glucose level at different speeds, this testing may not provide enough information. Eating an apple may peak the blood glucose at 60 minutes, whereas a piece of bread may peak at 90 minutes.

The way to overcome this problem is to test more often after eating a specific food. By testing at numerous times, the person with diabetes will produce a glucose curve. This curve will show the baseline, when the blood glucose peaks and when it returns back to the baseline.

To be most accurate, the testing of foods should be done at different times of the day. A person may respond differently to a food in the morning or after activity. If a person chooses to do glycemic testing, it should be done at the time when the food is usually eaten.

The difficulties in calculating personal glycemic responses for food include:

• Multiple finger sticks and testing  (always use same blood source - either capillary or vein)
• Time consuming
• Must follow strict schedule for accurate results
• Responses may vary for many reasons
• Questionable accuracy due to variables with testing

Glycemic testing can be a difficult process with less-than-accurate results due to the variables. If a person with diabetes wishes to calculate the GI for foods, this should be done after consulting medical professionals for specific recommendations and guidelines.