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Special Communication |

Relationship Between Fasting Plasma Glucose and Glycosylated Hemoglobin:  Potential for False-Positive Diagnoses of Type 2 Diabetes Using New Diagnostic Criteria

Mayer B. Davidson, MD; David L. Schriger, MD, MPH; Anne L. Peters, MD; Brett Lorber, MPH
JAMA. 1999;281(13):1203-1210. doi:10.1001/jama.281.13.1203.
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Context New criteria for the diagnosis of type 2 diabetes mellitus have recently been introduced that lowered the diagnostic fasting plasma glucose (FPG) concentration from 7.8 to 7.0 mmol/L (140 to 126 mg/dL).

Objective To determine if individuals with diabetes diagnosed by the new FPG concentration criterion would have excessive glycosylation (elevated hemoglobin [HbA1c] levels).

Definitions We determined the distribution of HbA1c levels in individuals using 4 classifications: (1) normal by the new criterion (FPG concentration <6.1 mmol/L [110 mg/dL]); (2) impaired fasting glucose by the new criterion (FPG concentration of 6.1-6.9 mmol/L [110-125 mg/dL]); (3) diabetes diagnosed solely by the new FPG concentration criterion of 7.0 through 7.7 mmol/L (126-139 mg/dL); and (4) diabetes diagnosed by the previous FPG concentration criterion of 7.8 mmol/L (140 mg/dL) or higher.

Design Cross-sectional analysis of 2 large data sets (NHANES III and Meta-Analysis Research Group [MRG] on the Diagnosis of Diabetes Using Glycated Hemoglobin) that contained individuals in whom FPG concentrations, 2-hour glucose concentrations using an oral glucose tolerance test, and an HbA1c level were simultaneously measured. We cross-tabulated FPG concentrations (<6.1 mmol/L [110 mg/dL], 6.1-6.9 mmol/L [110-125 mg/dL], 7.0-7.7 mmol/L [126-139 mg/dL], and ≥7.8 mmol/L [140 mg/dL]) and HbA1c levels separated into 3 intervals: normal, less than the upper limit of normal (ULN); slightly elevated, ULN to ULN plus 1%; and high, higher than ULN plus 1%.

Results Among subjects with normal FPG concentrations, HbA1clevels in the NHANES III (and the MRG) data sets were normal in 97.3% (96.2%), slightly elevated in 2.7% (3.6%), and high in 0.1% (0.2%). Among individuals with impaired fasting glucose, HbA1c concentrations were normal in 86.7% (81.4%), slightly elevated in 13.1% (16.4%), and high in 0.2% (2.2%). Among diabetic patients diagnosed by the new FPG criterion only, HbA1c levels were normal in 60.9% (59.6%), slightly elevated in 35.8% (32.8%), and high in 3.4% (7.6%). In diabetic patients diagnosed by the former FPG criterion, HbA1c levels were normal in 18.6% (16.7%), slightly elevated in 32.5% (21.0%), and high in 48.9% (62.3%).

Conclusions About 60% of the new cohort of diabetic patients in both data sets have normal HbA1c levels. We believe that diabetes should not be diagnosed in those with FPG concentrations less than 7.8 mmol/L (140 mg/dL) unless excessive glycosylation is evident. Individuals without excessive glycosylation but with moderate elevations of FPG concentrations (6.1-7.7 mmol/L [110-139 mg/dL]) should be diagnosed as having impaired fasting glucose and treated with an appropriate diet and exercise. This diagnostic labeling achieves the goal of early intervention without subjecting these persons to the potentially negative insurance, employment, social, and psychological consequences of a diagnosis of diabetes mellitus.

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Figures

Figure 1. Distribution of Hemoglobin A1cLevels by Fasting Plasma Glucose
Graphic Jump Location
The third National Health and Nutrition Examination Survey (NHANES III) data set had 2836 subjects and the Meta-Analysis Research Group data set had 8917. To convert hemoglobin A1c from percentage of total hemoglobin to proportion of total hemoglobin, multiply by 0.01. See Table 1 for percentage of subjects in each strata of fasting plasma glucose concentrations and "Methods" for description of study population and for determination of upper limits of normal of hemoglobin A1c levels.
Figure 2. Distribution of Hemoglobin A1c Levels in Those Diagnosed as Having Diabetes Mellitus by New and Old Criteria
Graphic Jump Location
Diagnosis based on fasting plasma glucose concentrations of 7.0 to 7.7 mmol/L (126-139 mg/dL) and 2-hour glucose concentrations of less than 11.1 mmol/L (200 mg/dL) and by the old criteria (the cohort of patients diagnosed as having diabetes mellitus by 2-hour glucose concentrations of 11.1 mmol/L (200 mg/dL) or higher but with FPG concentrations <7.0 mmol/L [126 mg/dL]). To convert hemoglobin A1c from percentage of total hemoglobin to proportion of total hemoglobin, multiply by 0.01.
Figure 3. Prevalence of Retinopathy in Pima Indians
Graphic Jump Location
To convert hemoglobin A1c from percentage of total hemoglobin to proportion of total hemoglobin, multiply by 0.01. To convert glucose from milligrams per deciliter to millimoles per liter, multiply by 0.05551. Data adapted from McCance et al.51
Figure 4. Prevalence of Retinopathy in Egyptians
Graphic Jump Location
To convert hemoglobin A1c from percentage of total hemoglobin to proportion of total hemoglobin, multiply by 0.01. To convert glucose from milligrams per deciliter to millimoles per liter, multiply by 0.05551. Data adapted from Engelgau et al.52
Figure 5. Prevalence of Retinopathy in the Third National Health and Nutrition Examination Survey
Graphic Jump Location
Taken from unpublished data on 40- to 74-year-old participants (Katherine Flegal, PhD, written communication, July 1997). To convert hemoglobin A1c from percentage of total hemoglobin to proportion of total hemoglobin, multiply by 0.01. To convert glucose from milligrams per deciliter to millimoles per liter, multiply by 0.05551.
Figure 6. Alternative Approach to the Diagnosis of Diabetes Mellitus
Graphic Jump Location

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The American Medical Association is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians. The AMA designates this journal-based CME activity for a maximum of 1 AMA PRA Category 1 CreditTM per course. Physicians should claim only the credit commensurate with the extent of their participation in the activity. Physicians who complete the CME course and score at least 80% correct on the quiz are eligible for AMA PRA Category 1 CreditTM.
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