What is impaired glucose tolerance?

Impaired glucose tolerance (IGT) is a category of prediabetes defined by the American Diabetes Association as a 2-hour value of 140 to 199 mg/dL on an oral glucose tolerance test. Below 140 mg/dL at 2 hours is considered normal, and 200 mg/dL or higher falls in the diabetes range.

How is IGT different from impaired fasting glucose?

IGT is based on how the body handles a measured glucose drink, assessed by the 2-hour value of an oral glucose tolerance test. Impaired fasting glucose (IFG) is based on a fasting blood sample taken after an overnight fast. The two tests measure different things and can disagree; a person may have one, the other, or both.

Does IGT relate to blood sugar spikes after meals?

IGT reflects how efficiently the body clears glucose after a challenge, which is conceptually related to post-meal glucose rises. However, IGT is specifically defined by a standardized oral glucose tolerance test, not by everyday meals. A clinician can explain what a result means for you.

Prediabetes · testing

Impaired Glucose Tolerance (IGT)

6 min read · Updated July 2026

Impaired glucose tolerance, or IGT, is a form of prediabetes revealed by a specific test: the oral glucose tolerance test. Rather than measuring your baseline fasting glucose, it looks at how well your body clears a measured dose of glucose over two hours. That "challenge" view captures something fasting tests can miss — and it helps explain why IGT and impaired fasting glucose are related but not the same.

What IGT means

IGT is defined by the 2-hour value of an oral glucose tolerance test (OGTT). In this test, you drink a standardized glucose solution after fasting, and your blood glucose is measured two hours later to see how efficiently your body has handled the load. According to the American Diabetes Association (ADA), the 2-hour categories are:

Below 140 mg/dL — normal glucose tolerance.
140–199 mg/dL — impaired glucose tolerance (IGT), a form of prediabetes.
200 mg/dL or higher — in the diabetes range.

IGT is one of three standard ways prediabetes is identified, alongside the A1c test and fasting glucose. For the broader context, see the prediabetes guide and our overview of what prediabetes is.

Two-hour OGTT bands per the ADA. IGT is the 140–199 mg/dL range measured two hours after a glucose drink.

The post-glucose-challenge context

What makes IGT distinctive is that it is a dynamic measurement. A fasting test captures a single resting point; the OGTT deliberately stresses the system with a glucose load and then measures the response. A person whose fasting glucose looks fine can still clear a glucose load slowly, which the 2-hour value reveals. This is why the OGTT is sometimes used when clinicians want a fuller view of glucose handling than a fasting sample alone provides.

How IGT differs from IFG

IGT is closely related to, but distinct from, impaired fasting glucose (IFG). IFG is based on a fasting blood sample — 100–125 mg/dL per the ADA — reflecting the glucose your body maintains overnight. IGT is based on the post-load 2-hour value. Because they measure different aspects of metabolism, the two can disagree: some people have IFG with normal glucose tolerance, some have IGT with normal fasting glucose, and some have both. Our companion article on impaired fasting glucose covers the fasting side in detail, and both fall under the broader umbrella of dysglycemia.

Relation to post-meal spikes

Conceptually, IGT is related to the idea of blood sugar rising more than it should after eating, since both involve how quickly the body clears incoming glucose. But it is important to be precise: IGT is defined specifically by the standardized OGTT, not by everyday meals. Ordinary post-meal glucose rises vary with food, portion, and timing, and are not the same as a formal IGT result. Some people use continuous glucose monitoring, under professional guidance, to observe their own post-meal patterns, but that is a separate tool from the OGTT used to define IGT.

What IGT means for risk, and next steps

Like other forms of prediabetes, IGT signals elevated metabolic risk and a higher likelihood of developing type 2 diabetes over time, and it is frequently linked to insulin resistance. The NIDDK and CDC emphasize that prediabetes is often modifiable. If an OGTT result falls in the IGT range, the appropriate next step is a conversation with a clinician who can interpret it alongside your other results and history, arrange any confirmatory testing as outlined in how prediabetes is diagnosed, and discuss lifestyle steps — such as eating patterns and physical activity — that public-health programs describe as reducing the risk of progression. Endobits is clinical decision-support software used under clinician oversight; it helps put results in context but does not diagnose or treat.

Trying to understand a glucose tolerance result?

See how glucose data can be viewed against the standard ranges — a starting point for a conversation with your clinician.

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Sources

American Diabetes Association — Understanding Diagnosis (oral glucose tolerance test criteria). Centers for Disease Control and Prevention — Prediabetes — Your Chance to Prevent Type 2 Diabetes. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) — Diabetes Tests & Diagnosis.

This article is educational and not medical advice. Diagnostic thresholds are attributed to the American Diabetes Association and may be updated over time. A single test result is not a diagnosis; talk to a qualified healthcare provider about your results. Endobits is clinical decision-support software used under clinician oversight, not a diagnostic device.