How should I diagnose and manage impaired fasting glucose (fasting glucose intolerance) in an adult without diabetes?

Diagnosis and Management of Impaired Fasting Glucose

Diagnose impaired fasting glucose (IFG) with a fasting plasma glucose of 100-125 mg/dL (5.6-6.9 mmol/L), confirm with repeat testing, and immediately initiate intensive lifestyle intervention focused on 5-7% weight loss and 150 minutes weekly of moderate physical activity. 1

Diagnostic Criteria

IFG is defined as fasting plasma glucose between 100-125 mg/dL (5.6-6.9 mmol/L) measured after at least 8 hours of fasting. 1

Confirmation Testing

• Repeat the same test on a different day to confirm the diagnosis, as a single abnormal result requires verification unless unequivocal hyperglycemia is present. 1, 2
• Consider adding A1C measurement (5.7-6.4%) simultaneously, which provides both diagnostic confirmation and baseline glycemic assessment without requiring fasting. 1
• An oral glucose tolerance test (OGTT) can serve as alternative confirmation, measuring 2-hour post-load glucose of 140-199 mg/dL to identify impaired glucose tolerance (IGT). 1, 3

Important Testing Considerations

• Ensure proper sample handling: plasma must be separated immediately or kept on ice to prevent falsely low glucose from glycolysis. 1
• Avoid testing during acute illness, stress, or after recent physical activity, as these falsely elevate glucose levels. 1
• The patient should consume at least 150g of carbohydrates daily for 3 days prior to OGTT to avoid falsely elevated results. 1

Risk Stratification

Screen for IFG in adults with BMI ≥25 kg/m² plus one additional risk factor, or in all adults ≥35 years regardless of weight. 1

Additional risk factors warranting screening include: 1, 4

• First-degree relative with diabetes
• High-risk ethnicity (African American, Latino, Native American, Asian American, Pacific Islander)
• History of gestational diabetes or delivering infant >9 lbs
• Hypertension (≥140/90 mmHg or on therapy)
• HDL <35 mg/dL or triglycerides >250 mg/dL
• Polycystic ovary syndrome
• Physical inactivity

Rescreen every 3 years if initial testing is normal. 1

Management: Lifestyle Intervention is Primary Treatment

The cornerstone of IFG management is intensive behavioral intervention, which is significantly more effective than medications at preventing progression to diabetes. 1, 4

Specific Lifestyle Targets

• Weight loss goal: 5-7% of body weight 1, 4
• Physical activity goal: minimum 150 minutes per week of moderate-intensity exercise 1, 4
• Dietary counseling: focus on healthful diet with consistent carbohydrate intake 1

Structure of Effective Interventions

Effective programs require multiple contacts over extended periods, not single counseling sessions. 1 The Diabetes Prevention Program demonstrated 58% reduction in diabetes progression with intensive lifestyle intervention. 5

Pharmacologic Therapy: Secondary Option

Metformin or acarbose can delay diabetes onset but are less effective than lifestyle changes and should be reserved for patients unable to achieve lifestyle goals. 4, 5

• Metformin has demonstrated effectiveness in preventing diabetes progression, particularly in younger patients with BMI ≥35 kg/m². 4
• Acarbose also shows benefit but with more gastrointestinal side effects. 4
• Cost-effectiveness of pharmacologic therapy for IFG remains unproven. 4

Critical Distinction: IFG vs IGT

IFG and IGT represent different pathophysiologic states with different cardiovascular risk profiles. 5

• IFG reflects elevated hepatic glucose output and defects in early insulin secretion, more common in men, prevalence plateaus in middle age. 5
• IGT reflects peripheral insulin resistance, slightly more common in women, prevalence rises into old age. 5
• Only 20-30% of people with IGT also have IFG, demonstrating limited concordance between the two conditions. 5
• IGT carries stronger association with cardiovascular disease and mortality than isolated IFG. 5
• Highest diabetes risk occurs when both IFG and IGT coexist. 5

Clinical Implication

Consider performing OGTT in addition to fasting glucose, as this identifies IGT which carries higher cardiovascular risk and has stronger evidence for intervention benefit. 1, 3, 5 Most diabetes prevention trials demonstrating efficacy enrolled patients with IGT, not isolated IFG. 1, 3

Monitoring and Follow-up

• Annual fasting glucose or A1C testing to monitor for progression to diabetes. 1
• Screen and manage other cardiovascular risk factors aggressively, as IFG clusters with hypertension, dyslipidemia, and obesity. 1, 5
• Approximately 60% of people who develop diabetes have either IFG or IGT 5 years prior, while 40% progress from normal glucose tolerance. 5

Common Pitfalls to Avoid

• Do not rely on single abnormal glucose measurement—always confirm with repeat testing. 1, 2
• Do not prescribe medications without first attempting intensive lifestyle intervention, as lifestyle changes are more effective. 1, 4
• Do not assume IFG and IGT are interchangeable—they have different pathophysiology and risk profiles. 5
• Do not use point-of-care A1C for diagnosis unless the device is NGSP-certified and used in CLIA-certified settings with proper quality control. 1