Definitions of FPG and IFG
Fasting Plasma Glucose (FPG) is a blood glucose measurement taken after at least 8 hours of no caloric intake, while Impaired Fasting Glucose (IFG) is a prediabetic state defined by FPG levels between 100-125 mg/dL (5.6-6.9 mmol/L) according to American Diabetes Association criteria, or 110-125 mg/dL (6.1-6.9 mmol/L) according to World Health Organization criteria. 1
Fasting Plasma Glucose (FPG)
FPG represents a standardized measurement of blood glucose concentration obtained from venous plasma after a minimum 8-hour fast (no caloric intake). 2, 1 This test reflects the body's ability to maintain adequate basal insulin secretion combined with sufficient hepatic insulin sensitivity to control hepatic glucose output. 3
Key technical specifications:
- Requires venous plasma sample (standardized measurement)
- Fasting defined as 8-14 hours without caloric intake
- Should be performed in the morning
- Measurements from whole blood are approximately 0.5 mmol/L (9 mg/dL) lower than plasma values 3
Impaired Fasting Glucose (IFG)
IFG represents an intermediate category of glucose metabolism—a prediabetic state where glucose levels are elevated above normal but below the diagnostic threshold for diabetes. 4, 1
Diagnostic criteria differ between organizations:
- American Diabetes Association (ADA): FPG 100-125 mg/dL (5.6-6.9 mmol/L) 2, 5, 1
- World Health Organization (WHO): FPG 110-125 mg/dL (6.1-6.9 mmol/L) 3, 6
This discrepancy exists because the ADA lowered the threshold from 6.1 to 5.6 mmol/L in 2003 to improve sensitivity for detecting individuals at risk of progression to diabetes. 3 The WHO retained the original 6.1 mmol/L cutoff.
Clinical Significance
IFG is not merely a laboratory finding but a risk state for:
- Future development of type 2 diabetes
- Cardiovascular disease
- Metabolic syndrome components (obesity, dyslipidemia, hypertension) 2, 4
IFG reflects the natural history of progression from normal glucose metabolism to type 2 diabetes. 3 Individuals with IFG often oscillate between different glycemic states, as expected when a continuous variable (plasma glucose) is dichotomized into categories. 3
Important Distinctions
IFG differs from Impaired Glucose Tolerance (IGT):
- IFG is diagnosed by fasting glucose measurement alone
- IGT requires a 2-hour oral glucose tolerance test (OGTT) showing 2-hour plasma glucose 140-199 mg/dL (7.8-11.0 mmol/L) 2, 3, 1
- An individual can have isolated IFG, isolated IGT, or both conditions simultaneously
- These conditions identify partially overlapping but distinct populations at risk for diabetes 3
Critical caveat: A normal FPG does not exclude IGT or even diabetes—approximately 30% of individuals with diabetes would be missed using FPG alone compared to OGTT. 3 The OGTT remains more sensitive for detecting glucose intolerance, though FPG is more convenient and reproducible for clinical practice. 2, 1