Prediabetes: Diagnosis Thresholds, Lifestyle Prescription and When Metformin Is Reasonable

Prediabetes is defined by any one of three results: an A1c of 5.7 to 6.4 percent, a fasting plasma glucose of 100 to 125 mg/dL, or a 2-hour glucose of 140 to 199 mg/dL on a 75 g oral glucose tolerance test. Every patient with prediabetes needs a specific lifestyle prescription: weight loss of at least 5 to 7 percent of starting weight and at least 150 minutes a week of moderate-intensity activity, ideally through a structured diabetes prevention program. Metformin is reasonable for higher-risk adults, and the ADA Standards of Care in Diabetes 2026 name the features that define them: age 25 to 59, BMI of 35 or higher, fasting glucose of 110 mg/dL or higher, A1c of 6.0 percent or higher, or prior gestational diabetes.

The sections below cover who to screen, how to read and confirm the results, the components of the lifestyle prescription, the metformin decision, monitoring, special populations and the distractors a board item uses.

Who to screen

Two sources govern screening, and the exam expects both to be recognized.

ADA risk factors that make testing appropriate at any age in adults with overweight or obesity include a first-degree relative with diabetes, high-risk race or ethnicity, a history of cardiovascular disease, hypertension, low HDL cholesterol or high triglycerides, polycystic ovary syndrome, physical inactivity and other conditions associated with insulin resistance such as acanthosis nigricans. Women with prior gestational diabetes, people with HIV and those taking medications that raise glucose (for example glucocorticoids or some antipsychotics) are also tested.

Diagnostic thresholds

Confirming and reconciling results

• Diabetes requires two abnormal results, either from the same sample (for example A1c and fasting glucose both in the diabetes range) or from two separate samples, unless there is unequivocal hyperglycemia with classic symptoms.
• When two different tests disagree, the result above the diagnostic cut point is repeated. A patient with an A1c of 6.6 percent and a fasting glucose of 112 mg/dL does not yet have a confirmed diagnosis of diabetes; repeat the A1c.
• A prediabetes result does not need the same confirmation rule to start a lifestyle prescription, but repeat testing over time is part of the plan.
• A1c is unreliable with hemoglobin variants, anemia or altered red cell turnover, recent transfusion or blood loss, erythropoietin therapy, advanced CKD and pregnancy. Use glucose-based criteria in those patients.
• The three tests do not identify the same people. The OGTT finds more people with impaired glucose tolerance; the A1c is more convenient and needs no fasting.

Assessing the whole patient

Prediabetes is a cardiovascular risk marker as well as a diabetes risk marker. At the same visit, the provider:

• Measures weight, BMI, waist circumference and blood pressure.
• Checks a lipid panel and estimates cardiovascular risk.
• Screens for associated conditions: obstructive sleep apnea, steatotic liver disease, polycystic ovary syndrome.
• Reviews medications that raise glucose and whether alternatives exist.
• Asks about tobacco, alcohol, sleep and physical activity, and about food access and cost, which shape what is realistic.

The lifestyle prescription

The ADA recommends referral to an intensive behavioral lifestyle program modeled on the Diabetes Prevention Program, with these targets:

• Weight: reduce and maintain at least 5 to 7 percent of initial body weight.
• Activity: at least 150 minutes a week of moderate-intensity physical activity, such as brisk walking.
• Diet: a reduced-calorie eating pattern. The 2026 Standards cite Mediterranean-style and low-carbohydrate patterns as having the best evidence for preventing or delaying type 2 diabetes in high-risk people.
• Format: CDC-recognized diabetes prevention programs deliver this in person or online, and the Medicare Diabetes Prevention Program covers eligible beneficiaries.

In the original Diabetes Prevention Program trial, intensive lifestyle change reduced the incidence of diabetes by 58 percent and metformin by 31 percent compared with placebo over about 3 years. Lifestyle was effective across ages and was especially effective in older participants; metformin performed best in younger and heavier participants. Those findings explain the shape of the ADA metformin criteria.

"Eat better and exercise" is not a prescription. The exam expects specific, measurable elements: a percentage weight goal, a weekly activity target and a referral.

When metformin is reasonable

The 2026 ADA Standards recommend considering metformin to prevent type 2 diabetes in high-risk adults with prediabetes, especially those with any of the following:

Practical points:

• Metformin is added to lifestyle change, never instead of it.
• Use is off-label for prevention. The DPP used 850 mg twice daily; in practice, start low (for example 500 mg once or twice daily with meals, per the labeled titration) and increase as tolerated.
• Check eGFR first: the label prohibits starting metformin at eGFR 30 to 45 and contraindicates it below 30.
• The ADA recommends periodic vitamin B12 assessment with long-term use, especially with anemia or peripheral neuropathy.
• Metformin is a poor fit for a 74-year-old with an A1c of 5.8 percent, where lifestyle change is more effective and the absolute benefit of a drug is small.
• Obesity pharmacotherapy can be appropriate in its own right for patients who meet obesity treatment criteria (ADA Section 8); confirm prediabetes-specific wording in the current guideline.

Pioglitazone and other glucose-lowering drugs have prevention data, but their adverse effects (weight gain, edema, fractures and heart failure with pioglitazone) keep them out of routine prevention.

Monitoring and follow-up intervals

Special populations

• Prior gestational diabetes: lifelong testing for diabetes or prediabetes at least every 3 years (confirm the interval in the current Section 2), and these women are among the best candidates for metformin.
• Youth: risk-based testing in children and adolescents with overweight or obesity and risk factors follows the pediatric section of the Standards.
• Pregnancy planning: metformin and lifestyle goals are revisited before conception; A1c is unreliable for diagnosis in later pregnancy.
• Older adults: prediabetes progresses more slowly, and the focus is on activity, nutrition and cardiovascular risk rather than drugs.

Referral triggers

• Results in the diabetes range on repeat testing: manage as diabetes.
• Rapid weight loss, ketosis or a lean younger adult with rising glucose: evaluate for type 1 diabetes or another form of diabetes.
• Discordant or unreliable A1c in a patient with a hemoglobin variant: glucose-based testing and possibly hematology input.
• Severe obesity not responding to lifestyle measures: obesity medicine or metabolic surgery evaluation.

Patient education

• Prediabetes is common, often reversible, and the best time to act.
• A loss of 5 to 7 percent of body weight is enough to matter; for a 100 kg patient that is 5 to 7 kg.
• Build up to 150 minutes a week of brisk activity and limit long periods of sitting.
• Choose an eating pattern that can be sustained, with fewer refined carbohydrates and sugary drinks.
• Metformin, if prescribed, is taken with meals; GI effects usually settle over a few weeks.
• Return for testing at least yearly, and report thirst, frequent urination or unexplained weight loss sooner.

How the exam tests this

• Threshold confusion. A fasting glucose of 124 mg/dL is prediabetes, not diabetes; an A1c of 6.5 percent is diabetes, not prediabetes.
• One result labeled as diabetes. A single A1c of 6.7 percent without symptoms needs confirmation before the diagnosis is made.
• Vague lifestyle advice. The correct option names a weight percentage, an activity target or a structured program.
• Metformin for everyone, or for no one. The trap is giving metformin to an older adult at low risk or withholding it from a 40-year-old with BMI 38, A1c 6.2 percent and prior gestational diabetes.
• Wrong screening population. USPSTF limits its recommendation to adults 35 to 70 with overweight or obesity; the ADA starts universal screening at 35.
• Wrong interval. Retest prediabetes at least yearly, not every 3 years.

Practice question

A 41-year-old woman presents for an annual visit. She had gestational diabetes 6 years ago and has not been tested since. BMI is 37 and blood pressure is 128/80 mm Hg. A1c is 6.2 percent and fasting plasma glucose is 116 mg/dL. eGFR is 98 mL/min/1.73 m2. She has no symptoms. Which of the following is the most appropriate plan?

• A. Diagnose type 2 diabetes and start metformin with an SGLT2 inhibitor
• B. Give dietary advice and repeat testing in 3 years
• C. Refer to an intensive lifestyle program targeting 7 percent weight loss and discuss starting metformin
• D. Order a 75 g oral glucose tolerance test to confirm prediabetes before giving any advice

Answer: C. Both results are in the prediabetes range, and she has several features that the ADA associates with benefit from metformin (BMI 35 or higher, A1c 6.0 percent or higher, fasting glucose 110 or higher, prior gestational diabetes, age 25 to 59) in addition to an intensive lifestyle program. Option A misreads the thresholds: neither value reaches the diabetes range. Option B gives nonspecific advice and uses the interval for normal results; prediabetes is retested at least yearly. Option D delays care, because two concordant prediabetes results already define the condition and an OGTT would not change the plan.

Key points

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