Two Different Kinds of Prediabetes

Prediabetes is not one condition. There are two versions, they are identified by different tests, and you can have either or both.

Impaired fasting glucose means your baseline is raised — a fasting glucose of 100 to 125 mg/dL. This reflects your liver releasing more glucose than it should overnight, and reduced sensitivity to insulin at rest.

Impaired glucose tolerance means your response to a load is impaired — a two-hour value of 140 to 199 mg/dL on a glucose tolerance test. Your baseline may be fine; what you cannot do well is clear a large amount of glucose after eating.

The important practical consequence is a testing one. A fasting glucose test cannot detect impaired glucose tolerance. If your fasting result is normal and nobody has ever done a glucose tolerance test, one version of prediabetes has simply never been looked for.

This matters in India, where fasting glucose is by far the most commonly performed test and glucose tolerance testing is comparatively rare outside pregnancy.

Having both carries more risk than having either alone.

So if you have several risk factors and a fasting result at the upper end of normal, asking specifically for a glucose tolerance test is a reasonable thing to do rather than assuming the question has been settled.