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Diagnosing diabetes as soon as possible is important, as early intervention is necessary for the prevention of long-term complications. What can you expect from medical and physical examinations?

Diabetes can be managed well, as long as it is diagnosed early on and treatment is initiated as soon as possible. The longer diabetes goes undetected, the higher a patient's risk of complications. Therefore, diagnosing diabetes early on is extremely important.

So, how is diabetes diagnosed? There are several different blood tests that can help diagnose diabetes.

Fasting plasma glucose test

A fasting plasma glucose test measures your blood glucose levels after at least eight hours of fasting. As a fasting plasma glucose test is most reliable when conducted in the morning, patients will often be asked to fast overnight and come in for the test first thing in the morning.

The results are categorized as follows:

  • Plasma glucose levels of 99 mg/dL or below are normal.
  • Plasma glucose levels of between 100 and 125 mg/dL point to a condition called prediabetes which means that your blood glucose levels are higher than normal but not high enough to warrant a diagnosis of diabetes. Prediabetes can eventually progress to type 2 diabetes.
  • Plasma glucose levels of 126 mg/dL or higher indicate diabetes, but you will be asked to repeat the test again on another day.

Oral glucose tolerance test

An oral glucose tolerance test evaluates your blood sugar levels after you have fasted for at least eight hours, and then again two hours after you have been given a beverage that contains 75 grams of glucose dissolved in water. Studies have shown that the oral glucose tolerance test is a more sensitive test for diagnosing prediabetes than the fasting plasma glucose test. However, it is much more inconvenient to do. The results are categorized as follows:

  • Plasma glucose levels of 139 mg/dL or below two hours after the test are normal.
  • Plasma glucose of between 140 and 199 mg/dL two hours after the test indicate prediabetes
  • Plasma glucose levels of 200 mg/dL or higher after two hours mean you most likely have diabetes. You will be asked to repeat the test to confirm the result on a different day.

In the case of gestational diabetes (pregnancy diabetes), an oral glucose tolerance test is often used to diagnose the disease. As opposed to a regular oral glucose tolerance test, for patients with gestational diabetes, the blood sugar levels are checked four times during the test. If results indicate that your blood sugar levels are higher than normal two times during the test, you will be diagnosed with gestational diabetes.

These are the steps through which gestational diabetes is diagnosed through an oral glucose tolerance test:

  1. Plasma glucose levels are checked during fasting. If the plasma glucose levels are 95 mg/dL or higher, this means you may have gestational diabetes.
  2. Plasma glucose levels are checked one hour after drinking the glucose beverage. If the plasma glucose levels are 180 mg/dL or higher then you may have pregnancy diabetes.
  3. Plasma glucose levels are checked again two hours after drinking the beverage. If the plasma glucose levels are 155 mg/dL or higher during this time, you may have gestational diabetes.
  4. Plasma glucose levels are checked once more three hours after you drink the beverage. If the plasma glucose levels are 140 mg/dL or higher, you guessed it, you may have gestational diabetes.

Random plasma glucose test

A random plasma glucose test is conducted when your blood sugar levels are checked randomly and without taking into account what your last meal was. The blood sugar levels at this time, as well as other symptoms that suggest diabetes, are combined to give a diagnosis of diabetes.

Most times, a diagnosis of diabetes through random plasma glucose test is later confirmed through other tests such as the fasting plasma glucose test or the oral glucose tolerance test that are conducted on separate days. Essentially, if you have a random blood glucose level of 200 mg/dL or more, as well as other symptoms of diabetes such as excessive thirst and hunger, blurred vision, fatigue and frequent urination, you will be asked to come in again and do other testing.

Hemoglobin A1c test

A hemoglobin A1c test is a test that is able to measure the level of glucose in your blood over the last several months. Recent guidelines have suggested that the hemoglobin A1c test should be used as a screening for both prediabetes and diabetes. Hemoglobin A1c levels of between 5.7 and 6.4 percent suggest prediabetes and hemoglobin A1c levels of 6.5 percent or higher point to a diagnosis of diabetes.

Zinc transporter 8 autoantibody (ZnT8Ab) test

Your doctor may suggest a zinc transporter 8 autoantibody (ZnT8Ab) test once you have already been diagnosed with diabetes in order to figure out if it is type 1 or type 2 diabetes. Patients with type 1 diabetes will have antibodies to the zinc transporter 8. Thus, this test can identify which patients have type 1 diabetes so they can start treatment promptly. 

  • American Diabetes Association. "Diagnosis and classification of diabetes mellitus." Diabetes care 33.Supplement 1 (2010): S62-S69.
  • American Diabetes Association. "Screening for diabetes." Diabetes care 25.suppl 1 (2002): s21-s24.
  • Carpenter, Marshall W., and Donald R. Coustan. "Criteria for screening tests for gestational diabetes." American journal of obstetrics and gynecology 144.7 (1982): 768-773.
  • Photo courtesy of SteayHealth

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