Last Updated on Oct 13, 2017

What are Blood Sugar Tests?

Blood glucose tests are used in the diagnosis of diabetes, a condition if left uncontrolled could affect virtually any organ in the body. Blood glucose tests help to differentiate a patient as normal, prediabetic, or diabetic.

Prediabetes is a condition, where the patient has higher than normal blood sugar levels, but lower than the diabetic range. Adequate precautions at this stage could prevent the worsening of the condition. In addition, blood glucose tests can also help to identify hypoglycemia or low blood sugar levels.

Tests for Blood Sugar

Blood glucose tests are used for the following purposes:

  • Diagnosis of diabetes, especially in a patient with clinical features of diabetes
  • Screening for diabetes, which is normally done in individuals at 45 years of age or above. It should be started earlier if a person is overweight or obese and has one additional risk factor for developing diabetes
  • Monitoring the control of diabetes once it is diagnosed
  • Adjusting the dosage of medication. Hospitalized diabetic patients may require testing even 2 to 3 times a day to maintain the blood sugar under control.

How is a Blood Sugar Test Done?

There are several ways to test for diabetes based on the blood glucose. All the tests require a sample of blood. Some tests are carried out in the fasting state, some after a specific time following eating, and some at random.

Some of these methods are as follows:

Hemoglobin A1C level: The hemoglobin A1C or glycated hemoglobin is a blood test that reflects the percentage of hemoglobin that is attached to glucose. The salient features of the test are:

  • The test is useful to find out the blood glucose status for the last three months, since the lifespan of the red blood cells is three months.
  • The test can be done at any time and does not require fasting
  • The results are not affected by acute conditions like stress and illnesses
  • It can be used to predict some of the complications of diabetes
  • Depending on the result, the condition can be classified as:
    • Normal: less than 5.7%
    • Prediabetes: 5.7%-6.4%
    • Diabetes: More than 6.4%
  • The disadvantage of this test is that it is not as sensitive as other blood glucose tests and is expensive
Blood Sugar Chart

Fasting Plasma Glucose Level: The fasting plasma glucose levels indicate the blood glucose levels in the fasting state:

  • The blood sample is obtained in the morning after overnight fasting for 8 hours
  • The test is available in most laboratories and is relatively cheap
  • Depending on the result, the condition can be diagnosed as:
    • Normal - 80-100 mg/dL
    • Impaired fasting glucose or prediabetes - 100-125 mg/dL
    • Diabetes - ≥ 126 mg/dL

The reference values may vary slightly among laboratories. It is therefore necessary to correlate your result with the reference value provided by the particular laboratory.

Oral glucose tolerance test: The oral glucose tolerance test is a sensitive method of testing for diabetes. Its salient features are:

  • The blood glucose levels are measured after overnight fasting and 2 hours after a standard glucose drink (postprandial)
  • The test is often used to detect gestational diabetes, that is diabetes that first appears in pregnancy
Gestational Diabetes
  • If the postprandial result is abnormal, the condition can be diagnosed as:
    • Prediabetes or impaired glucose tolerance (IGT) - 140-199 mg/dL at 2 hours
    • Diabetes - ≥ 200 mg/dL at 2 hours
  • The test is sensitive. However, it is cumbersome to perform.

Random plasma glucose levels: Random plasma glucose levels measure blood glucose at any time of the day in a non-fasting state.

  • The test is not very sensitive and is usually not preferred
  • It is particularly useful to confirm diabetes in a patient already suffering from the symptoms, that is, polyuria, polydipsia, polyphagia, and unexplained weight loss
  • A reading of 200 mg/dL with this test confirms the presence of diabetes

Devices are also available that monitor blood glucose levels on a continuous basis. These are necessary for individuals who use insulin pumps.

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