There are many types of diabetes mellitus. In general there are four types of diabetes. They are type 1 diabetes, type 2 diabetes, other specific types of diabetes, and gestational diabetes.

American Diabetes Association.

Type 1 diabetes results from β-cell destruction, usually leading to absolute insulin deficiency. Type 2 diabetes results from a progressive insulin secretory defect on the background of insulin resistance. Other specific types of diabetes due to other causes, e.g. genetic defects in β-cell function, genetic defects in insulin action, diseases of the exocrine pancreas etc. And the gestational diabetes mellitus is diagnosed during pregnancy that is not clearly overt diabetes.

There also are three criterias for the diagnosis of diabetes. They are the fasting plasma glucose (FPG) [≥126 mg/dL (7.0 mmol/L)] which is defined as no caloric intake for at least 8 hours, the 2-h value in the 75-g oral glucose tolerance test (OGTT) [≥200 mg/dL (11.1 mmol/L).] which uses a glucose load containing the equivalent of 75 g anhydrous glucose dissolved in water, and the A1C test [≥6.5%].

But note that all the three criterias above should be repeated for confirmation before the diagnosis is clear, unless that the patient has classic symptoms of hyperglycemia or hyperglycemic crisis, and simultaneously his or her random plasma glucose ≥200 mg/dL (11.1 mmol/L). It is that the diagnosis of diabetes shall be clear.

A1C assay is a good way for the diagnosis of diabetes.It has many advantages compared to the FPG and OGTT, including greater convenience, evidence to suggest greater preanalytical stability, and less day-to-day perturbations during periods of stress and illness. But A1C assay is not perfect. It costs more, and the there is incomplete correlation between A1C and average glucose in certain individuals.

Also A1C inaccurately reflects glycemia with certain anemias and hemoglobinopathies. For example in conditions with abnormal red cell turnover, such as pregnancy, recent blood loss or transfusion, or some anemias, the diagnosis of diabetes must employ glucose criteria exclusively rather than A1C.

As we discussed above there are four different types mothod to diagnose diabetes and the test should be repeated to rule out laboratory error. Unless the diagnosis is clear on clinical grounds, such as a patient with a hyperglycemic crisis or classic symptoms of hyperglycemia and a random plasma glucose ≥200 mg/dL.

It is preferable that the same test be repeated for confirmation, since there will be a greater likelihood of concurrence in this case. For example, if the A1C is 7.0% and a repeat result is 6.8%, the diagnosis of diabetes is confirmed. However, if two different tests are both above the diagnostic thresholds, the diagnosis of diabetes is also confirmed. On the other hand, if two different test are available in an individual and the results are discordant, the test whose result is above the diagnostic cut point should be repeated, and the diagnosis is made on the basis of the confirmed test. That is, if a patient meets the diabetes criterion of the A1C (two results ≥6.5%) but not the FPG (≤126 mg/dL or 7.0 mmol/L), or vice versa, that person should be considered to have diabetes.

Also it is possible that when a test whose result was above the diagnostic threshold is repeated, the second value will be below the diagnostic cut point. This is least likely for A1C, somewhat more likely for FPG, and most likely for the 2-h PG. Barring a laboratory error, such patients are likely to have test results near the margins of the threshold for a diagnosis. The health care professional might opt to follow the patient closely and repeat the testing in 3-6 months.