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The glycemic index (GI) that is assigned to a particular food is an average of the glycemic responses of many individuals. It is possible for people to measure their own personal response to a food to determine how it affects their glucose level. By testing individually, people 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 (e.g., for diabetes).

The first step in determining how food affects the glucose level requires a glucose test immediately before and two hours after eating. This reading will give a general idea of the way the food raises glucose in the individual. 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.

Testing more frequently after eating a specific food can help. By testing at numerous times, a personal glucose curve can be produced. 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 the 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