Prediabetes is diagnosed when blood sugar is higher than normal but not yet high enough to qualify as type 2 diabetes. The most widely used criteria in the United States, set by the American Diabetes Association, define prediabetes as a fasting plasma glucose between 100 and 125 mg/dL, a two-hour glucose of 140 to 199 mg/dL after an oral glucose tolerance test, or an HbA1c between 5.7% and 6.4%. Those numbers, however, are not universal. Other organizations draw the lines differently, and which test you take can change whether you receive a prediabetes label at all.
The Three Tests and Their Cutoffs
Doctors have three standard ways to measure where your blood sugar stands, and each uses a different window into glucose metabolism.
- Fasting plasma glucose (FPG): A blood draw after at least eight hours without eating. Under ADA criteria, a result of 100 to 125 mg/dL counts as prediabetes (called “impaired fasting glucose”). Below 100 is normal; 126 or above on two separate tests indicates diabetes.1JAMA Pediatrics. Fasting Plasma Glucose Levels Within the Normoglycemic Range in Childhood as a Predictor of Prediabetes and Type 2 Diabetes in Adulthood: The Bogalusa Heart Study
- Oral glucose tolerance test (OGTT): You drink a sugary solution, and blood is drawn two hours later. A two-hour glucose of 140 to 199 mg/dL is classified as “impaired glucose tolerance.” This test captures how well your body clears a sugar load, which fasting glucose alone can miss.
- HbA1c (glycated hemoglobin): This reflects your average blood sugar over the previous two to three months. An HbA1c of 5.7% to 6.4% falls in the prediabetes range under ADA guidelines. It does not require fasting, making it convenient, but it can be affected by conditions that alter red blood cell lifespan.
You do not need all three tests to be abnormal. A single abnormal result on any one of these is enough for a prediabetes diagnosis. That said, the tests do not always agree with each other. Someone with a normal fasting glucose can still have impaired glucose tolerance on the OGTT, or vice versa. Research suggests these discordant patterns reflect different aspects of the same underlying problem: impaired fasting glucose tends to involve the liver’s handling of sugar, while impaired glucose tolerance is driven more by how muscles take up glucose after a meal.2Diabetes Care. Contributions of β-Cell Dysfunction and Insulin Resistance to the Pathogenesis of Impaired Glucose Tolerance and Impaired Fasting Glucose
Why the World Health Organization Uses Different Numbers
A common source of confusion is that the ADA and the World Health Organization do not agree on where the fasting glucose cutoff should sit. The WHO defines impaired fasting glucose as 110 to 125 mg/dL (6.1 to 6.9 mmol/L), a full 10 mg/dL higher than the ADA’s threshold of 100 mg/dL (5.6 mmol/L).3PLOS ONE. Optimal Cut-Off Points of Fasting Plasma Glucose for Two-Step Strategy in Estimating Prevalence and Screening Undiagnosed Diabetes and Pre-Diabetes in Harbin, China The WHO also does not formally endorse HbA1c as a standalone diagnostic criterion for prediabetes, though many countries use it in practice.
The practical consequence of this disagreement is enormous. In a large study of over 8,800 people, prediabetes prevalence was about 40% using ADA criteria but only 16% using WHO criteria, meaning ADA guidelines roughly doubled the number of people labeled prediabetic.4BMJ Open Diabetes Research & Care. Lifetime risk to progress from pre-diabetes to type 2 diabetes among women and men: comparison between American Diabetes Association and World Health Organization diagnostic criteria Other major organizations, including the European Association for the Study of Diabetes, the International Diabetes Federation, and Diabetes UK, each have their own variations on the criteria, creating a patchwork of definitions globally.5PubMed. Divergence in prediabetes guidelines – A global perspective
This is not just an academic quibble. Which definition your doctor uses determines whether you get a diagnosis, whether you are enrolled in a prevention program, and how urgently you are monitored. If you are tested abroad or switch healthcare systems, the same blood sugar reading could be labeled differently. The debate over where to draw the line reflects a genuine trade-off: a lower threshold catches more people early but also labels millions who may never develop diabetes, while a higher threshold is more specific but misses some who will progress.
What Happens Inside the Body at Prediabetic Levels
By the time blood sugar climbs into the prediabetic range, the body has already been compensating for a while. Insulin resistance, where cells respond less effectively to insulin, forces the pancreas to produce more of the hormone to keep glucose in check. Eventually the insulin-producing beta cells begin to tire. Research on people in the upper range of impaired glucose tolerance has found that they have typically lost about 70% to 80% of their beta-cell function and are near-maximally insulin resistant.6The Journal of Clinical Endocrinology & Metabolism. Preservation of β-Cell Function: The Key to Diabetes Prevention That means much of the metabolic damage is already underway before a person receives a diabetes diagnosis.
This is one reason prediabetes carries real health consequences even if it never formally progresses to diabetes. A person sitting at a fasting glucose of 115 mg/dL is not in some harmless holding zone; their body is already working harder than it should to manage sugar.
Cardiovascular and Other Health Risks
Prediabetes is sometimes dismissed as a warning rather than a condition in its own right, but the evidence tells a different story. A large meta-analysis found that people with prediabetes had about a 15% higher risk of cardiovascular disease and a 13% higher risk of dying from any cause compared with people who had normal blood sugar, over a median follow-up of nearly ten years.7BMJ. Association between prediabetes and risk of all cause mortality and cardiovascular disease: updated meta-analysis An earlier meta-analysis reported similar findings, with impaired fasting glucose and impaired glucose tolerance each associated with a roughly 20% increase in cardiovascular risk.8PubMed. Pre-diabetes and the risk for cardiovascular disease: a systematic review of the evidence
Beyond the heart, observational data link prediabetes to early forms of kidney disease, nerve damage (small fiber neuropathy), and even diabetic retinopathy, conditions most people associate only with full-blown diabetes.9PubMed Central. Prediabetes: A high-risk state for developing diabetes These risks are not hypothetical or distant; the same study of people in the upper range of impaired glucose tolerance noted that roughly one in ten already had signs of diabetic retinopathy.6The Journal of Clinical Endocrinology & Metabolism. Preservation of β-Cell Function: The Key to Diabetes Prevention All of this reinforces that the prediabetic range is not merely a bureaucratic classification; it reflects a metabolic state with measurable consequences for blood vessels, nerves, and organs.
How Quickly Does Prediabetes Progress to Diabetes?
Reviews estimate that somewhere between 5% and 10% of people with prediabetes convert to type 2 diabetes each year.10PubMed Central. Prediabetes diagnosis and treatment: A review A meta-analysis looking specifically at HbA1c-defined prediabetes found a pooled incidence rate of about 36 cases of new diabetes per 1,000 person-years, which was similar to the rate seen with ADA-defined impaired fasting glucose.11PubMed. Progression rates from HbA1c 6.0-6.4% and other prediabetes definitions to type 2 diabetes: a meta-analysis People who had both impaired fasting glucose and impaired glucose tolerance together faced a steeper rate, around 70 per 1,000 person-years, roughly double the risk of either alone.
That said, progression is not inevitable. Data from the Diabetes Prevention Program Outcomes Study showed that people who managed to return from prediabetes to normal glucose levels, even temporarily, had a significantly lower risk of developing diabetes later on. Regaining normal glucose regulation was associated with better beta-cell function and improved insulin sensitivity, regardless of which treatment group participants had been assigned to.12The Lancet. Effect of regression from prediabetes to normal glucose regulation on long-term reduction in diabetes risk: results from the Diabetes Prevention Program Outcomes Study This is one of the more encouraging findings in the field: the metabolic clock can be wound back, and every period spent at normal glucose levels appears to count.
Lifestyle Changes and Metformin
The strongest evidence for reversing prediabetes comes from the Diabetes Prevention Program, a landmark trial in the United States. Participants who were assigned to an intensive lifestyle program lost a modest amount of weight (about 7% of body weight) through diet and at least 150 minutes of moderate exercise per week. This reduced their rate of developing diabetes by 58% compared with placebo. Metformin, taken twice daily, reduced the rate by 31%.13PubMed Central. Reduction in the Incidence of Type 2 Diabetes with Lifestyle Intervention or Metformin In practical terms, about seven people needed to do the lifestyle program for three years to prevent one case of diabetes, versus fourteen for metformin.
A more recent meta-analysis pooling multiple randomized trials found that the risk of developing diabetes was not significantly different between metformin and lifestyle intervention overall, though the trend still slightly favored lifestyle changes.14PubMed Central. Comparison of the Efficacy of Metformin and Lifestyle Modification for the Primary Prevention of Type 2 Diabetes: A Meta-Analysis of Randomized Controlled Trials The benefits of lifestyle intervention also appear to extend beyond blood sugar. Long-term follow-up from the Diabetes Prevention Program found that participants in the lifestyle group had a 21% lower risk of accumulating multiple chronic conditions (multimorbidity) compared with placebo, a benefit that was not seen with metformin alone.15JAMA. Lifestyle and Metformin Interventions and Risk of Multimorbidity in Adults With Prediabetes
The upshot is that moderate, sustained changes to diet and activity are the first-line recommendation for anyone with prediabetes. Metformin may be considered as well, especially for people under 60, those with a BMI above 35, or women with a history of gestational diabetes, but it is typically offered alongside lifestyle changes rather than instead of them.
Who Should Be Screened
The U.S. Preventive Services Task Force recommends screening for prediabetes and diabetes in asymptomatic adults aged 35 to 70 who have overweight or obesity. This 2021 update lowered the starting age from 40, and the task force also suggested that clinicians consider screening earlier for racial and ethnic groups with disproportionately high diabetes risk at younger ages or lower BMI.16PubMed Central. Screening for Prediabetes and Diabetes: Clinical Performance and Implications for Health Equity The ADA’s screening criteria are somewhat broader and have been shown to catch a higher proportion of undiagnosed cases. In one analysis of over 50,000 participants, ADA criteria identified more than 99% of those with undiagnosed prediabetes or diabetes, compared with about 57% to 67% captured by USPSTF guidelines.17Diabetes. Assessing Prediabetes and Diabetes Screening Eligibility and Detection in Adults by ADA and USPSTF Guidelines in the NIH’s All of Us Research Program
If you have a family history of type 2 diabetes, had gestational diabetes, or belong to a racial or ethnic group at higher risk (including Black, Hispanic, Native American, Asian American, and Pacific Islander populations), it is worth asking your doctor about screening even if you do not fall neatly into the USPSTF criteria. Prediabetes is asymptomatic in the vast majority of cases, so waiting for symptoms is effectively waiting for diabetes.
Prediabetes in Children and Adolescents
Prediabetes is not confined to middle-aged adults. Rising rates of childhood obesity have pushed the incidence in adolescents to somewhere between 4% and 23%, depending on the population studied, and the trend is climbing.18PubMed Central. Prediabetes in children and adolescents: A ticking bomb! What makes pediatric prediabetes particularly concerning is that youth-onset type 2 diabetes tends to follow a more aggressive course than the adult version, with faster beta-cell decline and earlier organ damage.19PubMed Central. Prediabetes in children and adolescents: An updated review
Screening guidelines for children generally focus on those who are overweight and have at least one additional risk factor, such as a family history of type 2 diabetes, signs of insulin resistance (like darkened skin patches on the neck or armpits), or membership in a high-risk ethnic group. The diagnostic thresholds are the same as for adults, though interpreting HbA1c in growing children introduces some additional uncertainty.
After Gestational Diabetes
Women who develop diabetes during pregnancy (gestational diabetes) are at sharply elevated risk for prediabetes afterward. In one study of over 1,000 women tested six to nine weeks after delivery, about 35% overall met criteria for prediabetes. The rate varied widely by the type of gestational diabetes: women whose glucose was elevated both fasting and after meals had the highest rate, at nearly 56%, while those with only post-meal elevations had the lowest at about 24%.20JAMA Network Open. Early Postpartum Glucose Tolerance Reclassification by Gestational Diabetes Subtype Another study found that insulin use during pregnancy and a family history of diabetes were particularly strong predictors of postpartum prediabetes or type 2 diabetes.21PubMed. Risk factors for postpartum prediabetes and type 2 diabetes in women with gestational diabetes: insights from the edugest study in Argentina
Despite guidelines recommending postpartum glucose testing, follow-through rates are notoriously low. If you had gestational diabetes, getting tested at the recommended postpartum visit and then at regular intervals afterward is one of the clearest opportunities to catch prediabetes early, when lifestyle changes are most effective.
Continuous Glucose Monitors and What They Add
Continuous glucose monitors (CGMs), once reserved for people with type 1 diabetes, have become increasingly accessible and are now generating research interest in people with prediabetes. These devices track glucose every few minutes via a small sensor under the skin, offering a much richer picture than a single fasting blood draw.
Early research suggests that CGM metrics can capture glucose patterns that standard tests miss. In one community-based study, people with prediabetes spent about 77% of their time with glucose in the 70 to 140 mg/dL range, compared with 46% for people with diabetes.22PubMed Central. Defining Continuous Glucose Monitor Time in Range in a Large, Community-Based Cohort Without Diabetes Interestingly, the relationship between HbA1c and real-time CGM data appears to weaken in the prediabetic range, meaning your HbA1c may not fully capture what your glucose is actually doing throughout the day.23PubMed Central. Lack of Association Between Hemoglobin A1c and Continuous Glucose Monitor Metrics Among Individuals with Prediabetes and Normoglycemia Researchers are still working out the best CGM metrics for identifying risk in this population, including how much glucose variability, those spikes and dips after meals, matters on its own.24PubMed Central. Continuous Glucose Monitoring for Prediabetes: What Are the Best Metrics?
CGMs are not currently recommended as a routine diagnostic tool for prediabetes, and insurers generally do not cover them for people without diabetes. But as the technology becomes cheaper and the evidence base grows, the gap between what a once-a-year fasting glucose tells you and what continuous monitoring reveals is becoming harder to ignore.
The Controversy Over Labeling
Not everyone in the medical community is enthusiastic about the prediabetes label. The current ADA definition captures a staggeringly large share of the population: by some estimates, a third or more of U.S. adults qualify. Critics argue that casting a net this wide labels many people who will never develop diabetes, potentially generating anxiety and unnecessary treatment while straining healthcare resources.25PubMed Central. The prediabetes conundrum: striking the balance between risk and resources The WHO’s decision to use a higher fasting glucose cutoff (110 vs. 100 mg/dL) reflects this more conservative philosophy, aiming to limit the label to people at genuinely elevated risk.
Defenders of the broader definition counter that even modestly elevated glucose levels carry real cardiovascular risk, as the meta-analyses described earlier demonstrate, and that the whole point is to intervene before irreversible damage accumulates. They also note that lifestyle interventions are low-cost and beneficial regardless of whether someone actually develops diabetes. The debate is unlikely to be resolved soon, because it ultimately involves a value judgment about how aggressively to screen and intervene in a condition with a wide spectrum of outcomes. The honest answer is that a fasting glucose of 101 mg/dL and one of 124 mg/dL are both “prediabetes” by ADA standards, but they represent very different levels of risk. Where you sit within the range matters at least as much as whether you technically cross the threshold.
Factors That Can Skew Your Results
Several non-glucose factors can push your test results in misleading directions, and awareness of them can save you from an inaccurate diagnosis or a false sense of security.
HbA1c is sensitive to anything that affects how long red blood cells live. Iron-deficiency anemia, for example, tends to artificially raise HbA1c because older red blood cells accumulate more glucose tagging. Conversely, conditions that increase red blood cell turnover, like sickle cell trait or recent blood loss, can lower HbA1c and mask true prediabetes. Racial and ethnic variation in HbA1c at the same glucose levels has also been documented; at identical average blood sugar, Black individuals tend to have slightly higher HbA1c than white individuals. This has prompted some researchers to question whether a single HbA1c threshold is appropriate for all populations.
Fasting glucose can be thrown off by acute stress, illness, certain medications (especially corticosteroids), poor sleep the night before, or the timing of your last meal. If your result lands right on a threshold, a repeat test on a different day is reasonable before making any decisions. The OGTT is the most informative single test but also the most inconvenient and the most sensitive to preparation: an unusually low-carbohydrate diet in the days leading up to the test can paradoxically raise the two-hour result by reducing the body’s practice at clearing glucose loads.
None of these caveats mean the tests are unreliable. They mean a single borderline result should be interpreted in context, ideally confirmed with a second test or a different method, rather than treated as a definitive verdict on your metabolic health.