Blood Sugar Testing

Diabetes mellitus is defined by elevated blood sugars. These elevated blood sugars are measured with a number of blood tests.


There are a number of tests used to measure if blood sugars are elevated. The most direct and accurate blood testing is whole blood glucose testing and requires sending blood to a large laboratory for testing the concentration of glucose in whole blood. There are also tests of blood sugar that are done after a patient has been given either a 75 gram or 100gram glucose load. The hemoglobinA1c is a single blood test that indirectly measures a three month average on a patient's blood sugar. A similar test is called fructosamine that can give a 2 week average on blood sugars. A home glucometer can be inaccurate by up to 20-30% so it cannot be used for diagnosis, but rather for trends.

The hemoglobinA1c is a test that should be run every 3 months approximately as it reflects a 3 month average and correlates well with the likelihood of long term complications of diabetes mellitus. The bone marrow makes red blood cells. Normally these red blood cells (hemoglobin) turn over every three months and the bone marrow makes a new supply. Blood sugar attaches in a specific way to the hemoglobin. We can measure the percentage of sugar on the hemoglobin and therefore determine a three month average. The hemoglobinA1c is reported as a percent and the chart shows with what average blood sugars that corresponds.

Normal glucose metabolism

Patients without diabetes mellitus have fasting blood sugars less than 100 mg/dl. Usually the hemoglobinA1c is less than 5.7% and a blood glucose obtained two hours have ingesting a 75gram glucose load is less than 140 mg/dl.

Pre-diabetes (pre-type 2)

Patients with "pre-diabetes" either have impaired glucose tolerance or impaired fasting glucose. Impaired glucose tolerance (IGT) is usually the first defect in early diabetes mellitus type 2 and is an elevated blood glucose level in the range of 140 to 199 mg/dl which occurs two hours after ingesting a 75 gram of glucose load. Impaired fasting glucose (IFG) is defined as a fasting blood sugar greater than 100 mg/dl. Either IFG or IGT may be diagnosed with a hemoglobin A1c that is between 5.7% to 6.4%.

Diabetes Mellitus type 1 or 2

Patients with diabetes mellitus type 1 or diabetes mellitus type 2 have fasting blood sugars that are greater than 125 mg/dl, a random blood sugar greater than 200 mg/dl, a blood sugar greater between 140 and 199 two hours after a 75 gram glucose challenge, or a hemoglobinA1c greater 6.5% or greater.

Gestational Diabetes Mellitus

Patients with gestational diabetes mellitus should be tested at some point during the first trimester and potentially again in the third trimester if they are high risk. This is tested with a 100gram glucose challenge test: Two or more values must be met or exceeded for a diagnosis of diabetes to be made. The test should be done in the morning after a 8 to 14 hour fast.

  • fasting blood sugars greater than 95 mg/dl, and

  • 1 hour value >180mg/dl

  • 2 hour value > 155 mg/dl

  • 3 hour value >140 mg/dl

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