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Diabetes Diagnosis

Jessica Evert, MD, edited by Benjamin McDonald, MD

blood sampleYour doctor relies on tests of your blood and urine to diagnose diabetes. Several tests are commonly used:

Fasting Plasma Glucose. This test measures baseline blood sugar concentration by testing blood samples taken after prolonged fasting (not eating for eight or more hours), often first thing in the morning. This is the first and most common screening test a physician will use to diagnose diabetes.  The fasting plasma glucose test should be done as part of any yearly check-up in adults.  The result of the test is compared to charts that show sugar ranges for people known to be healthy, pre-diabetic and diabetic. 99 milligrams of sugar per deciliter of blood or less is considered normal, 100 to 125 mg/dl is considered pre-diabetic, and anything above 125 mg/dl is considered to fall within the diabetic range. The test should be repeated on another day if the result is abnormal to make sure that the findings are meaningful and not simply due to chance.

Oral Glucose Tolerance. The Fasting Plasma Glucose blood test looks at resting (baseline) sugar levels, but doesn't provide information about how blood sugar might swing in the short term after food has been eaten. The Oral Glucose Tolerance Test captures this information by first collecting a fasting blood sample (as in the Fasting Plasma Glucose test), and then having the patient eat a known amount of sugar (75 grams), and collecting a blood sample from them after two hours have passed. The normal blood sugar range for the post-sugar sample in response to this test is below 140 mg/dl. Pre-diabetic persons' blood sugar concentrations fall between 140 and 199 mg/dl, and diabetic persons' blood sugar concentrations are 200 mg/dl or greater. This test is less commonly administered these days as it is time-consuming and inconvenient. However, this test is commonly used to test pregnant women for gestational diabetes and may also be used in high risk patients.

Hemoglobin A1C. This is a newer test that measures the relative amount of a particular hemoglobin molecule in a person's blood. The level of this molecule in a person's blood is related to the amount of glucose their blood contains. A hemoglobin A1C greater than or equal to 6.5% is diagnostic for diabetes. This test is gaining favor as an initial screening test for diabetes as it does not require the patient to fast the night before the test. In fact, this test is also used to monitor the blood sugar control of people who have been previously diagnosed with diabetes. It gives a good representation of a person's blood glucose control over the past two to three months.

Random Plasma Glucose. Very high blood sugar concentrations are always suggestive of diabetes. In Random Plasma Glucose testing, blood samples are taken without regard for meals and fasting, and the results are compared against charts that indicate when glucose levels are simply too high to be normal. This test is only used in patients who exhibit some of the classic symptoms of hyperglycemia.  A result greater than or equal to 200 mg/dl suggests diabetes.  Repetitions of this test are useful for confirming that a problem exists.

There are also a group of tests that physicians will use to monitor their patients who have already been diagnosed with diabetes.  The purpose of this set of tests is to check for complications of diabetes and to learn whether the diabetes is improving or getting worse.

  • Urinalysis: For this test the patient gives a sample of their urine in the physician's office. This test is important in those known to have diabetes because it can determine the effect of diabetes on the kidneys.
  • Hemoglobin A1C: This test was discussed above.
  • Blood pressure: This should be done at least once a year at the yearly check-up. Many people with diabetes are also encouraged to purchase a home blood pressure unit so that they may check their pressure regularly.
  • Eye Exam: Any person with diabetes should have an eye exam at least once a year. This can help catch diabetic retinopathy early when it may still be treatable.
  • Cholesterol testing: Individuals with diabetes should have their cholesterol checked at least once a year during their check-up. As discussed previously, diabetes and high cholesterol often go hand in hand and can combine to cause serious heart and vascular problems.