Diagnosis of Diabetes Mellitus

This text will address the blood tests performed for the diagnosis of diabetes mellitus and for the monitoring of blood glucose levels. We will talk, among others, about fasting glycemia, postprandial glycemia, glycated hemoglobin and fructosamine.

Diabetes mellitus test
Diabetes mellitus test

For the past two decades there has been scientific evidence that strict control of blood glucose levels, called glycemia, can prevent complications of diabetes, such as diabetic nephropathy (renal damage), diabetic retinopathy (eye injury), and diabetic neuropathy ( injury of peripheral nerves).

Therefore, in addition to early diagnosis of diabetes, continuous follow-up of these patients' glucose levels is required. We currently have some options for quantifying blood glucose. Let's talk specifically about each one below.

Fasting glycemia

Examination of fasting blood glucose is a classic way to diagnose diabetes mellitus. We fast, in this case, the absence of calorie consumption for at least 8 hours.

When fasting, blood glucose levels should be below 100 mg/dL. This is the normal value and desired for all.

When fasting glycemia is between 100 and 125 mg/dL, we say that this patient has altered fasting glycemia, also called non-diabetic hyperglycemia or, more accurately, pre-diabetes. The term pre-diabetes can be employed based on the fact that 1 in 4 patients with altered fasting glycemia will develop criteria for diabetes mellitus within 3 to 5 years. If the patient has other risk factors such as obesity and family history, the risk is even greater.

When fasting blood glucose is above 126 mg/dL in at least 2 blood tests collected at different times, we have criteria for the diagnosis of diabetes.

Fasting glycemia is currently used only for diagnosis. In patients with diabetes mellitus already under treatment, its use is more limited, because it only gives us the value of blood glucose at the time of collection and it is not possible to know how it was in the previous days. For the follow-up of diabetes the best exam currently is glycated hemoglobin, explained in details below.

Glucose without fasting

The best test for the diagnosis of diabetes is the measurement of fasting blood glucose for at least 8 hours. However, eventually the patient can dose his blood glucose without fasting, and sometimes this value can be useful.

When we feed, in a matter of minutes, our bloodstream gets a load of glucose, rapidly raising our blood glucose. After a feeding, fatally our blood glucose will be above 126 mg/dL, which obviously does not indicate diabetes. However, just as the blood receives a glucose bath, our pancreas also releases a load of insulin so that this glucose can be harnessed by our body. Thus, our glycemia remains more or less controlled, not exceeding 200 mg/dL at any time, returning to normal values after about 3 hours. Therefore, any glycemia that is above 200 mg/dL, even after a meal, is indicative of diabetes. If the patient has symptoms of diabetes, then the diagnosis can be closed even without requesting fasting blood glucose for confirmation.

Postprandial glycemia 2H

The major problem with non-fasting blood glucose levels is lack of standardization. Each individual consumes a different amount of calories and the test is done with different interval times between the last meal. To avoid confusion, there is an examination called postprandial blood glucose that works as follows: Patients go to the laboratory and collect a blood sample to evaluate fasting blood glucose. After this collection, the laboratory provides a drink with a fixed amount of glucose (75 g) and at the end of 2 hours, a new blood sample will be collected to gauge your blood glucose.

This test is used to evaluate your insulin secretion after a glucose load. Normal postprandial glycemia is one that, after 2 hours, is below 140 mg/dL.

Values between 140 and 199 mg/dL indicate glucose intolerance and are a sign that your body is not coping properly with glucose elevation after meals. Usually indicates resistance to insulin action. It is also considered a pre-diabetes stage, even if fasting blood glucose is below 100 mg/dL.

Values above 200 mg/dL are indicative of diabetes.

Oral glucose tolerance test

The oral glucose tolerance test (OGTT) is a modified version of postprandial glycemia, used to diagnose diabetes that develops in pregnancy, called gestational diabetes. It is usually performed between the 24th and 28th weeks of gestation.

The test is done as follows. A first blood sample is taken on an empty stomach. You are then offered a drink with 100g of glucose. New blood samples are collected after 1, 2, and 3 hours. Gestational diabetes is diagnosed when the results exceed two or more of the following values:
  • Fasting glycemia greater than 95 mg/dL
  • 1-hour glycemia greater than 180 mg/dL
  • 2-hour glycemia greater than 155 mg/dL
  • 3-hour glycemia greater than 140 mg/dL

This type of examination only has value in pregnant women.

Glycated hemoglobin

In contrast to the above tests, which are mainly used for the diagnosis of diabetes mellitus, glycosylated hemoglobin, also called glycated hemoglobin, hemoglobin A1c or HbA1c, is an exam used to evaluate glycemic control in patients with a diagnosis of diabetes.

Glycated hemoglobin is an extremely useful test because it serves to assess the state of glycemia in the last 2-3 months. When we titrate fasting blood glucose in diabetic patients, its outcome only indicates how diabetes is controlled in the last few hours. For example, a patient spends the last 3 months without a diet and using diabetes medications irregularly, but 24-48 hours before the tests decides to take the medicines correctly. When he is dosing fasting blood glucose it is possible that he is in or near normality giving the false idea that his diabetes is well controlled. However, if glycated hemoglobin is also dosed, it will be clearly altered, indicating that, in fact, diabetes is not being treated as it was supposed to be.

But how does glycated hemoglobin work?

Hemoglobin is the main protein in our red blood cells. When the blood glucose level is high, part of the hemoglobin begins to bind to this excess circulating glucose, turning into glycosylated hemoglobin, that is, hemoglobin linked to glucose. Since red blood cells have a life of 3 to 4 months, this is the time when each one is exposed to blood glucose, causing the glycated hemoglobin to be a mirror of average glycemia in the last 3 months.

Normal values of glycated hemoglobin for people without diabetes are between 4% and 6%. Well-controlled diabetes is one that has values below 7%. Levels above 7% are associated with an increased risk of complications such as cardiovascular, renal, peripheral and eye diseases.

From the values of glycosylated hemoglobin it is possible to estimate the average rate of glucose in the last 3 months:

HbA1c - Mean glycemia (range):
  • 5% - 97 (76-120)
  • 6% - 126 (100-152)
  • 7% - 154 (123-185)
  • 8% - 183 (147-217)
  • 9% - 212 (170 -249)
  • 10% - 240 (193-282)
  • 11% - 269 (217-314)
  • 12% - 298 (240-347)

Although not yet universally accepted, there are already many doctors who use glycated hemoglobin also for the diagnosis of diabetes mellitus. Two different tests with HbA1c values greater than 6.5% would be enough to close the diagnosis. Patients with glycated hemoglobin between 5.7% and 6.4% would be in the pre-diabetes group.


Other proteins besides hemoglobin undergo glycosylation, that is, binding with glucose. Fructosamine is the name we give to this protein-glucose complex, the main protein being albumin.

The dosage of fructosamine gives us an estimate of glycemia in the last 4 to 6 weeks, as the average life of an albumin is only 1 month, so it is not as good as glycosylated hemoglobin. However, fructosamine may be very useful in patients with anemia, in the use of erythropoietin, hemoglobin diseases or chronic renal failure, situations that may cause errors in the measurement of glycated hemoglobin.

The normal value of fructosamine varies from one laboratory to another.

Capillary glycemia

Capillary glycemia is the exam where we evaluate the glycemia of the moment through a small drop of blood and a device to read the blood glucose concentration.

This is an excellent and practical way of evaluating the blood glucose variation more than once a day, making it possible to make timely adjustments to the dose and schedule of antidiabetic medicines, especially insulin.

Capillary glycemia should not be used to screen for diabetes in the healthy population. Its relation to the blood glucose results by blood laboratory analysis are not so correct since several factors can lead to a wrong reading, such as a not properly clean hand, bad storage of the strips, dirt on the device, very cold hands, etc. In addition, glycemia in the capillaries of the fingers is usually a little higher than the blood glucose in the veins.

Therefore, capillary glycemia is for the follow-up of diabetes, but not for its diagnosis.

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