How to Diagnose Diabetes?

By  , Expert Content
Aug 11, 2011

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How to Diagnose Diabetes

The diagnosis of diabetes based on clinical signs and symptoms can be difficult. If you have symptoms suggestive of diabetes, your doctor will recommend blood sugar level testing to confirm the diagnosis.

The tests which may be done to diagnose diabetes are:

  • Fasting plasma glucose (FPG)
  • Oral glucose tolerance test (OGTT)
  • Random Plasma Glucose Test


Fasting Plasma Glucose (FPG)


The level of blood glucose level is measured in the morning (after 8 hours of fasting) before you eat or drink anything during this test. The test can help to diagnose diabetes and pre-diabetes. Diagnoses based on fasting plasma glucose are as follows:

  • FPG <99 mg/dl is considered normal.
  • FPG between 100 to 125 mg/dl indicates pre-diabetes (or impaired fasting glucose).
  • FPG >126 mg/dl indicates diabetes.


Oral Glucose Tolerance Test (OGTT)


This test determines your blood glucose level after drinking 75 grams of glucose solution. Your doctor will recommend that you fast for at least 8 to 12 hours before doing this test. The test can help to diagnose diabetes and pre-diabetes. OGTT is more sensitive as compared to FPG test for diagnosis of pre-diabetes, but it is more cumbersome as compared to FPG.


Diagnoses based on OGTT are as follows:

  • Normal response: In people with normal glucose, the 2-hour glucose level remains lower than 140 mg/dl, and all values between 0 and 2 hours remain lower than 200 mg/dl.
  • Impaired glucose tolerance: If your fasting plasma glucose is less than 126 mg/dl and the 2-hour glucose level is between 140 and 199 mg/dl you have impaired glucose tolerance test.
  • Diabetes: If your fasting plasma glucose is greater than 126 mg/dl and/or the 2-hour glucose level is greater repeatedly (done on different days), you have diabetes.
  • Gestational diabetes: A woman is diagnosed with gestational diabetes if she has any two of the following after 100g OGTT; fasting plasma glucose > 95 mg/dl, 1-hour glucose > 180 mg/dl, 2-hour glucose level > 155 mg/dl, or a 3-hour glucose level > 140 mg/dl.


Random Plasma Glucose Test


This test measures your blood glucose at any time of the day irrespective of the time when you had your last meal or food. This test is usually done to diagnose diabetes but not pre-diabetes. If the random blood glucose level is > 200 mg/dL and you have symptoms of diabetes (such as increased urination, increased thirst and unexplained weight loss), it indicates that you may have diabetes. Your doctor may recommend FPG or OGTT to confirm the diagnosis.


Criteria for Diagnosis of Diabetes


If your blood sugar or blood glucose levels are elevated (FPG, OGTT, or RBS), your doctor will recommend repeat testing on a different day to confirm the results. According to experts, the criteria for diagnosing diabetes are satisfied if any of the following results are obtained on at least two different days:

  • Fasting blood glucose level: > is 126 mg/dL (7.0 mmol/L).
  • 2-hour oral glucose tolerance test level: > is 200 mg/dL (11.1 mmol/L).
  • Symptoms of diabetes along with random blood glucose test levels > is 200 mg/dL (11.1 mmol/L).


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