Diagnostic Criteria for Impaired Fasting Glucose
Impaired fasting glucose (IFG) is diagnosed when fasting plasma glucose is between 100-125 mg/dL (5.6-6.9 mmol/L), with fasting defined as no caloric intake for at least 8 hours. 1, 2
Specific Diagnostic Thresholds
The current standard criteria for IFG are:
- Fasting Plasma Glucose (FPG): 100-125 mg/dL (5.6-6.9 mmol/L)
This represents a category of increased risk for diabetes, formerly termed "prediabetes." The lower threshold of 100 mg/dL was established by the American Diabetes Association in 2003, lowering it from the previous WHO cutoff of 110 mg/dL (6.1 mmol/L) 1, 3, 2.
Complete Prediabetes Classification
IFG is one of three ways to identify prediabetes. The complete criteria include 2:
- FPG: 100-125 mg/dL (5.6-6.9 mmol/L) = IFG
- 2-hour plasma glucose on 75-g OGTT: 140-199 mg/dL (7.8-11.0 mmol/L) = Impaired Glucose Tolerance (IGT)
- A1C: 5.7-6.4% = Increased diabetes risk
Key Clinical Considerations
Risk is continuous across these ranges, becoming disproportionately greater at the higher ends 1, 2. Individuals can have isolated IFG, isolated IGT, or both conditions simultaneously.
Important Caveats
The WHO still uses 110 mg/dL (6.1 mmol/L) as the lower cutoff for IFG, creating international discrepancy 3. However, the ADA criteria (100 mg/dL threshold) are more widely adopted in clinical practice and identify more at-risk individuals who would benefit from intervention 1, 2.
IFG and IGT identify different populations: Only about 18-30% of individuals with IFG also have IGT 4. Those with isolated IFG tend to have lower insulin levels and somewhat better cardiovascular risk profiles compared to those with IGT or combined IFG&IGT 4.
Ethnic Considerations
Prevalence patterns differ by ethnicity: Caucasian populations show higher rates of isolated IFG, while Asian populations demonstrate higher rates of isolated IGT and combined IFG&IGT 5. This suggests the 100 mg/dL threshold may be particularly important for identifying at-risk Asian individuals who might otherwise be missed.
Clinical Implications
IFG is not merely a laboratory finding but a risk factor for both future diabetes and cardiovascular disease 6, 7. Individuals identified with IFG should have other cardiovascular risk factors assessed and treated appropriately 1, 3. The diagnosis warrants counseling on effective risk reduction strategies including weight loss and exercise 8.
Confirmation is not required for the diagnosis of prediabetes categories, unlike diabetes diagnosis which requires confirmatory testing 2.