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Blood Sugar Fasting

About the test
What is this test done for?
Fasting blood sugar is tested after overnight fasting and measures the amount of sugar (glucose) in the blood. It is performed to screen for pre-diabetes and diabetes. Glucose is the primary source of energy for all functions of the body and fluctuations in glucose levels can affect the way our body functions. Low glucose levels are termed hypoglycemia and can cause dizziness and fainting. High glucose levels are termed as hyperglycemia and are indicative of diabetes. The fasting blood sugar test can help diagnose diabetes and help take timely action to prevent complications like damage to the kidneys, eyes, heart, and nerves.
Why is it Done?
Fasting blood sugar is a routine screening test performed for the following reasons:
  • To check whether a person is suffering from pre-diabetes or diabetes if an individual has symptoms like
    • Frequent hunger, thirst, or urination (particularly at night)
    • Dizziness (frequent episodes)
    • Delay in the healing of wounds or pus formation at the site of the wound
    • Repeated infections
  • To adjust the dose of medicines/insulin in persons with diabetes
  • As a part of routine/annual checkup for those above 40 years
  • As a part of a fitness test before a surgery
  • To detect diabetes during pregnancy
When should it be performed?
The test is typically performed when diabetes is suspected in a person with:
  • A family history of diabetes
  • High risk of diabetes
  • Excess weight
  • Symptoms of diabetes
  • Pregnancy
The test is also done on a periodic basis to monitor the response to diabetes treatment (once every three months) and assess the need for switching to insulin injections if blood glucose levels are not controlled satisfactorily with oral drugs in persons with diabetes.
How is it done
Inform your doctor if you are taking any medications or have an allergy or any other medical condition before doing the test. You will need to fast for 8 hours overnight (no food, you may have water) before giving a blood sample from a vein in your arm the following morning. The test is performed using the glucose oxidase method (in laboratories) or with the help of electronic strips and glucometer device (in hospitals and at home).
Blood Sugar <70 mg/dL
  • Indicates very low levels of sugars also known as hypoglycemia, it can happen due to
    • Excessive dose of insulin or diabetes drugs
    • Starving for more than 12 hours with poor intake of dinner
Blood Sugar <110 mg/dL
  • It indicates a normal range of sugar
    • Non-diabetic person
    • Well controlled diabetic patient
Blood Sugar >110 mg/dL
  • It indicates high blood sugar levels that are seen in
    • Diabetic patients
    • People with less than 8 hours of fasting
    • Pre-diabetic patients
Blood Sugar >200 mg/dL
  • It indicates very high blood sugar levels, suggesting
    • Poorly controlled diabetes
    • If tested for first time, means the person has diabetes
Other Tests
Other tests that are done along with these to diagnose diabetes are
  • Blood sugar post-lunch
    • Blood sugar levels checked after 2 hours of lunch (heavy meals)
  • Blood glucose post-sugar (glucose tolerance test)
    • Blood sugar levels are evaluated after giving a certain amount of glucose solution
    • It helps in confirming diabetes (especially in pregnant women)
  • Blood sugar random
    • Blood sugar levels are performed during any time of the day, irrespective of meals (before or after food)
    • If levels are more than 200 mg/dL, it indicates diabetes
  • Glycosylated hemoglobin (HbA1c)
    • It helps to elucidate average sugar levels of the last 90 days
    • It helps in confirming diabetes as well as in monitoring the treatment of diabetes
  • Blood insulin levels (fasting and post-lunch)
    • This test is done for differentiating the type of diabetes, i.e., type 1 or type 2 diabetes
    • It suggests insulin levels of the blood
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