Diabetes mellitus type 2 laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Seyedmahdi Pahlavani, M.D. [2]

Overview

Laboratory findings of diabetes mellitus type 2 are diagnostic for this disease. Diabetes may be diagnosed based on plasma glucose criteria, either the fasting plasma glucose (FPG) or the 2-h plasma glucose (2-h PG) value after a 75-g oral glucose tolerance test (OGTT) or A1C criteria. All of these measurements are equally appropriate in diagnosis.

Laboratory Findings

• Diabetes may be diagnosed based on plasma glucose criteria, either the fasting plasma glucose (FPG) or the 2-hour plasma glucose (2-h PG) value after a 75-g oral glucose tolerance test (OGTT) or A1C criteria. All of them are equally appropriate for diagnosis. Two tests that are commonly used to determine whether you have pre-diabetes or diabetes are the fasting plasma glucose test (FPG) or the oral glucose tolerance test (OGTT).
• The blood glucose levels measured after these tests determine whether you have a normal metabolism, or whether you have prediabetes or diabetes.
• If your blood glucose level is abnormal following the FPG, you have impaired fasting glucose (IFG); if your blood glucose level is abnormal following the OGTT, you have impaired glucose tolerance (IGT).

       FPG                         OGTT

Fasting and 2-Hour Plasma Glucose

• The FPG and 2-hour PG may be used to diagnose diabetes.

Hb A1C

• The A1C test should be performed using a method that is certified by the NGSP and standardized or traceable to the Diabetes Control and Complications Trial (DCCT) reference assay.

Advantages of HbA1C

• HbA1C is more closely associated with both microvascular and macrovascular complications.
• Greater availability and standardization of the procedure to measure HbA1C has helped in its wider use.
• Relatively less preanalytical and analytical variability.
• Physicians are familiar with the method.
• Convenient for patient as no fasting required.
• Minimal variability due to stress and other illnesses.

Disadvantages of HbA1C

• Expensive method
• Still not widely available in developing countries
• Incorrect estimates in patients with certain forms of anemia and hemoglobinopathies.
  • In patients with sickle cell anemia (abnormal hemoglobin, but normal red cell turnover) an A1C assay without interference from abnormal hemoglobin should be used
  • In patients with anemia from hemolysis and iron deficiency (abnormal red cell turnover) the diagnosis of diabetes should be done based on glucose criteria exclusively.
• Glycation rates differ with race. A recent epidemiological study found higher HbA1c rates in African American population than whites for matched levels of fasting blood glucose. However this population was also found to have higher rates of fructosamine and glycated albumin along with lower 1,5 anhydroglucitol which was concordant with the higher HbA1c levels. Further, racial differences in the rates of hemoglobin glycation and erythrocyte turnover was not enough to explain the higher HbA1c levels. Differences in post-prandial glucose levels in different races is controversial and warrants further studies.

American Diabetes Association Diabetes Diagnostic Criteria 2017 (DO NOT EDIT)

Criteria for the diagnosis of diabetes
FPG ≥126 mg/dL (7.0 mmol/L). Fasting is defined as no caloric intake for at least 8 hours.
OR
2-hour Plasma Glucose (PG) ≥200 mg/dL (11.1 mmol/L) during an OGTT. The test should be performed as described by the WHO, using a glucose load containing the equivalent of 75 g anhydrous glucose dissolved in water.
OR
A1C ≥6.5% (48 mmol/mol).
OR
In a patient with classic symptoms of hyperglycemia or hyperglycemic crisis, a random plasma glucose ≥200 mg/dL (11.1 mmol/L).

The abnormal test should be repeated to rule out laboratory error, unless the diagnosis of diabetes is evident from the clinical presentation. The same test should be preferably repeated to confirm the diagnosis. If two different tests show a discordant result, then the test with abnormal result should be preferably repeated and diagnosis be made based on the results of that particular test. For example, if a patient has a normal fasting blood glucose and two abnormal HbA1c's, then the diagnosis of diabetes can be confirmed based on the two abnormal HbA1c's.

• Most of the patients with higher variability in their tests for diagnosing diabetes, have borderline abnormal results on repeat testing. Such patients may be followed up in 3 to 6 months for repeat testing.

Impaired Glucose Tests/Pre-Diabetes

• In 1997 and 2003, the International Expert Committee on Diagnosis and Classification of diabetes mellitus recognized an intermediate group of patients who did not meet the diagnostic criteria for diabetes mellitus and at the same time had a borderline abnormal glucose test result. These individuals were termed as "prediabetics".
• The patients with impaired fasting glucose have higher risks of developing diabetes and cardiovascular disease.
• The glucose level used to diagnose impaired glucose tests are:

Categories of increased risk for diabetes (prediabetes)
FPG 100 mg/dL (5.6 mmol/L) to 125 mg/dL (6.9 mmol/L) (IFG)
OR
2-hour PG in the 75-g OGTT 140 mg/dL (7.8 mmol/L) to 199 mg/dL (11.0 mmol/L) (IGT)
OR
A1C 5.726.4% (39247 mmol/mol)

References

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