How to Diagnose Prediabetes
Prediabetes is diagnosed using any one of three glycemic tests: fasting plasma glucose (FPG) of 100-125 mg/dL, 2-hour plasma glucose during a 75-gram oral glucose tolerance test (OGTT) of 140-199 mg/dL, or hemoglobin A1C of 5.7-6.4%. 1
Diagnostic Criteria
The diagnosis requires meeting at least one of these three criteria 1:
- Impaired Fasting Glucose (IFG): FPG 100-125 mg/dL (5.6-6.9 mmol/L)
- Impaired Glucose Tolerance (IGT): 2-hour plasma glucose 140-199 mg/dL (7.8-11.0 mmol/L) during 75-gram OGTT
- Elevated A1C: 5.7-6.4% (39-47 mmol/mol)
Critical Testing Considerations
Do not rely on fasting glucose alone—you will miss the majority of prediabetic patients. Research demonstrates that 2-hour postload glucose identifies far more patients with prediabetes (23%) than fasting glucose (7%) or A1C (5%) 2. Approximately one-third of patients with normal fasting glucose have impaired glucose tolerance or elevated A1C 2.
Risk Stratification Based on Number of Criteria Met
The number of abnormal tests matters significantly for risk stratification 3:
- Elevated risk: One prediabetes criterion met
- High risk: Two prediabetes criteria met
- Very high risk: All three prediabetes criteria met
Patients with A1C >6.0% (>42 mmol/mol) or those meeting both IFG and IGT criteria warrant aggressive intervention and vigilant follow-up 1.
Who Should Be Screened
Adults (Starting at Age 35)
Screen all adults beginning at age 35 years 1. For those with risk factors, begin screening earlier and include:
- BMI ≥25 kg/m² (≥23 kg/m² in Asian Americans) with additional risk factors:
- First-degree relative with diabetes
- High-risk race/ethnicity (African American, Latino, Native American, Asian American, Pacific Islander)
- History of cardiovascular disease
- Hypertension (≥130/80 mmHg or on therapy)
- HDL <35 mg/dL and/or triglycerides >250 mg/dL
- Polycystic ovary syndrome
- Physical inactivity
- Conditions associated with insulin resistance (severe obesity, acanthosis nigricans)
Special Populations Requiring Different Screening Intervals
- People with existing prediabetes: Test yearly 1
- Women with prior gestational diabetes: Lifelong testing at least every 3 years 1
- Normal results: Repeat at minimum 3-year intervals 1
- Patients on second-generation antipsychotics: Screen at baseline, repeat 12-16 weeks after initiation, then annually 1
- People with HIV: Screen with FPG before starting antiretroviral therapy, when switching therapy, 3-6 months after starting/switching, then annually if normal 1
Children and Adolescents
Screen youth with overweight (≥85th percentile) or obesity (≥95th percentile) after puberty onset or age 10 years, whichever is earlier, if they have additional risk factors 1:
- Maternal diabetes or gestational diabetes during gestation
- Family history in first- or second-degree relative
- High-risk race/ethnicity
- Signs of insulin resistance (acanthosis nigricans, hypertension, dyslipidemia, PCOS)
Clinical Implications
Prediabetes is not a benign condition—it carries increased risk for cardiovascular events (8.75 excess events per 10,000 person-years) and mortality (7.36 excess deaths per 10,000 person-years) over 6.6 years 4. Early forms of diabetic complications including nephropathy, neuropathy, and retinopathy can already be present 5.
Common Pitfall to Avoid
The most critical error is screening with fasting glucose alone. In stroke/TIA patients, for example, 52% had prediabetes and 27% had newly diagnosed diabetes when all three tests were used, but the majority would have been missed with fasting glucose alone 2. Always consider using OGTT or A1C in addition to fasting glucose, particularly in high-risk populations.