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Glucose, Plasma

The American Diabetes Association (ADA) published new Standards of Medical Care in Diabetes in 2011 (Diabetes Care 2011;34: Supplement 1, S11). Historically, the diagnosis of diabetes was based on fasting plasma glucose and 2 hour plasma glucose level after a 75 gram oral glucose tolerance test (OGTT). Despite being the gold standard, measurement of blood glucose is less accurate and precise than most physicians realize. Blood glucose measurements are also subject to several limitations including:

  • 8-12 hour fasting specimen requirement
  • Diurnal variation requiring morning collection to capture peak levels
  • Large biological variation of 5-8%
  • Nonstandardized instrument methods with >12% bias
  • Glycolysis after collection, even in sodium fluoride tubes

 Today, the diagnostic criteria for diabetes include one of the following:

  • HbA1c >6.5%
  • Fasting plasma glucose of >126 mg/dL (7.0 mmol/L). Fasting is defined as no caloric intake for at least 8 hours.
  • Two hour plasma glucose >200 mg/dL (11.1 mmol/L) during an OGTT with a 75 gram glucose load performed according to the World Health Organization (WHO)
  • Random plasma glucose of >200 mg/dL in an individual with symptoms of diabetes such as polyuria, polydipsia, and unexplained weight loss;

The Expert Committee on Diagnosis and Classification of Diabetes Mellitus recommends that testing for diabetes be considered in all adults age 45 and older. If results are normal, testing should be repeated at three year intervals. An abnormal result should be confirmed on a different day by any one of the recommended tests. Physicians should consider testing people at a younger age who are at risk of diabetes, including those who:

  • are overweight or obese (BMI >25 kg/m2)
  • have a first degree relative with diabetes
  • are a member of a high risk ethnic group
  • have a history of gestational diabetes or delivered a baby weighing >9 lb.
  • have hypertension (>=140/90)
  • have HDL cholesterol </=35 mg/dL and/or triglyceride level >/=250 mg/dL
  • have polycystic ovarian disease
  • have other conditions associated with insulin resistance such as acanthosis nigricans
  • have a history of cardiovascular disease
  • have a history of HbA1c >5.7%, impaired fasting glucose or impaired glucose tolerance

The Expert Committee on Diagnosis and Classification of Diabetes Mellitus also recognizes an intermediate group of individuals whose glucose or HbA1c levels are higher than normal but do not meet the diagnostic criteria for diabetes. People with fasting glucose levels between 100-125 mg/dL (5.6-6.9 mmol/L) are defined as having impaired fasting glucose (IFG). Individuals who have a 2 hour glucose level between140-199 mg/dL (7.8-11.0 mmol/L) are defined as having impaired glucose tolerance (IGT). Individuals with any of these intermediate laboratory values are referred to as having prediabetes. Patients with HbA1c of 5.7 to 6.4% are also included in this category. People with prediabetes are at much higher risk of developing diabetes and cardiovascular disease. These patients often have abdominal obesity, dyslipidemia with low HDL cholesterol and high triglycerides and hypertension.






4.0 – 5.6%

5.7 – 6.4%

6.5% or >

Fasting plasma glucose

<100 mg/dL

100 – 125 mg/dL

126 mg/dL or >

2 hour glucose

<140 mg/dL

140 – 199 mg/dL

200 mg/dL or >

Specimen requirement is one lavender top (EDTA) tube of blood for HbA1c and one grey top tube of blood for glucose.

For more information, see Diabetes Mellitus Diagnosis

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