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How lab tests show you have Diabetes?

You just found out you have diabetes. If you’re like many people, the news makes your head spin. How does the doctor know? How do blood tests help diagnose the disease? Could the test results be wrong? Is more than one test needed? Should your family members have tests, too?  To understand how doctors diagnose diabetes, explore more below.

High Blood Glucose Levels

Doctors diagnose diabetes on the basis of too-high levels of glucose in the blood. If you have diabetes, your blood glucose levels rise because of the foods you eat. Foods have little effect on blood glucose in people without diabetes.

Normally, insulin, a hormone that is made in the pancreas, allows glucose to enter your body’s cells and be used as fuel. Insulin also makes the liver store excess glucose in the form of glycogen until it is needed. Insulin is the main tool your body uses to lower your blood glucose level.

People with diabetes can have insulin deficiency (they make too little insulin) or insulin resistance (their bodies don’t respond well to insulin). Some people have a combination of the two. In people with diabetes, insulin is not doing its job, so glucose can’t get into the cells to be used for energy.

Instead, the unused glucose builds up in the blood and, if high enough, passes through the kidneys. If a person’s blood glucose level rises high enough, the extra glucose causes frequent urination. This in turn leads to thirst, as the body tries to make up for the fluid lost in urination.

Blood Tests

Screening tests are performed on people who have no symptoms of diabetes. On the other hand, diagnostic tests are done to confirm a diagnosis that is already suspected from the patient’s symptoms.

Screening tests are fast, easy to perform, and cheap. (Health fairs often offer screening tests, for example.) Screening tests require as little as a drop of blood from your fingertip and take only a minute or two to complete.

Screening tests are useful for people who may be at risk for developing diabetes. People at risk include:

  • Blood relatives of people with diabetes.
  • Women with unexplained miscarriages or stillbirths or who have had babies weighing 9 pounds or more at birth.
  • People with a condition known as “impaired glucose tolerance” (see Oral Glucose Tolerance Test below).
  • People with high blood pressure or very high blood cholesterol or triglyceride levels.
  • People who are obese.

All pregnant women should be screened for gestational diabetes (diabetes that starts during pregnancy) between the 24th and 28th weeks.
For diagnostic tests, the doctor draws one or more samples of blood from a vein and sends them to a lab for analysis.

You should ask your doctor to explain your results to prevent any confusion. Several types of diagnostic tests are commonly used to diagnose diabetes.

Fasting Plasma Glucose Test

The fasting plasma (blood) glucose test is a reliable way to diagnose diabetes. After you have fasted overnight (10 hours), the doctor draws a single sample of your blood and sends it to the lab for analysis.

Normal fasting plasma glucose levels are less than 6.0 millimoles per litre (mmol/L). Fasting plasma glucose levels of more than 7.0 mmol/L on two or more tests indicate diabetes, and you need no other tests.

Random Plasma Glucose Test

If random blood glucose level is (more than 11.1 mmol/L) that there is no doubt the person has diabetes. A random plasma glucose between 7.8 mmol/L and 11.1 mmol/L taken after you’ve eaten warrants a further test called Oral Glucose Tolerance Test (OGTT).

Oral Glucose Tolerance Test (OGTT)

With the oral glucose tolerance test, you must fast overnight (at least 10 hours) and go to your doctor’s office or the lab in the morning.

First, your fasting plasma glucose is tested. After this test, you receive 75 grams of glucose. Usually, the glucose is in a sweet-tasting liquid that you drink. Blood samples are taken 1 hour and 2 hours to measure your blood glucose.

For the test to give reliable results, you must be in good health (not have any other illnesses, not even a cold). Also, you should be normally active (for example, not lying down as an in-patient in a hospital) and taking no medicines that could affect your blood glucose.

People with levels in between normal and diabetic may be classified as having impaired glucose tolerance (IGT).

Glucose tolerance tests may lead to one of the following diagnoses.

  • Normal response

A person is said to have a normal response when the fasting glucose level is less than 7.0 mmol/L and the 2-hour glucose level is less than 7.8 mmol/L.

  • Impaired glucose tolerance

A person is said to have IGT when the fasting plasma glucose is less than 7.0 mmol/L, the 2-hour glucose level is between 7.8 and 11.1 mmol/L.

  • Diabetes
    A person has diabetes when glucose tolerance test show that the blood glucose level at fasting is 7.0 mmol/L or more or the 2 hour level is more than 11.1 mmol/L.

The Blood Test With a Memory

Once you’ve been diagnosed with diabetes, your doctor may order another type of blood test called glycated haemoglobin or glycohaemoglobin (GHb). GHb lets the doctor take a backward look at your diabetes control.

Haemoglobin is the protein in red blood cells that carries oxygen. GHb forms when glucose in the blood attaches to the haemoglobin. Because blood cells stay in circulation for three to four months, GHb level is a good measure of a person’s average blood glucose level over the previous three to four months.

The HbA1c test requires only one sample of blood, which can be taken at any time of the day, even right after a meal.


After the diagnosis of diabetes, the doctor will take many factors into account to determine which type of diabetes you have.

In general, people with type 1 are diagnosed when they have had diabetic ketoacidosis (which sometimes lead to a coma) or have high levels of ketones in their urine.

People with type 2 are most often diagnosed in adults or obese. They usually do not have urine ketones. Of course, there are exceptions to these categories.

Whatever type you have, the goal of treatment is to get your blood glucose levels as close to normal as possible.

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