DIAGNOSING DIABETES

You just found out that you have diabetes. If you're like many people, the news makes your head spin. How does the doctor know? How do blood tests help diagnose the disease? Could the test results be wrong? Is more than one test needed? Should your family members have tests, too?

HIGH BLOOD GLUCOSE LEVELS

Doctors diagnose diabetes on the basis of high blood glucose levels. If you have diabetes, your blood glucose levels rise because of the foods you eat. Foods have little effect on blood glucose levels of people who do not have diabetes. Normally, insulin, a hormone made in the pancreas, allows blood glucose to enter cells where it is used as fuel to make energy. Insulin is the main tool your body uses to lower blood glucose levels. People with diabetes can have insulin deficiency (which means they make too little insulin), or insulin resistance (which means they respond well to insulin). In people with diabetes, insulin is not doing its job, so their body's glucose can't get into cells to be used to make energy. Instead, their unused glucose builds up in the bloodstream, and passes through the kidneys if the level is high enough. This extra glucose causes frequent urination, which in turn leads to excessive thirst (this is the body's way of making up for the fluid lost through frequent urination).

WHAT URINE TESTS MEASURE

High urine glucose levels give doctors a clue that something is wrong. But urine tests are not a good way to diagnose diabetes. Urine tests are not as accurate as blood tests. And the level of blood glucose needed to make glucose appear in the urine differs from person to person. Your blood glucose levels may be abnormally high, but glucose may not appear in the urine. So, in diagnosing diabetes, doctors measure blood glucose.

BLOOD TESTS

The goal of blood test is to find out whether or not you have an abnormally high level of glucose in your blood. Different types of diagnostic tests are used--fasting plasma glucose, random plasma glucose and oral glucose tolerance tests. When you get your test results, ask your doctor to explain them to you. Comparing your test results with those of your friends or family may confuse or alarm you. You may not have had the same type of test, so your results could have a completely different meaning.

Fasting Plasma Glucose Test

The fasting plasma glucose test is the preferred way to diagnose diabetes. It is easy to perform and convenient. After you have fasted overnight (at least 8 hours), a single sample of your blood is drawn at the doctor's office and sent to the laboratory for analysis.

Normal fasting plasma glucose levels are less than 110 mg/dl or 6.0 mM. Fasting plasma glucose levels of more than 126 mg/dl or 7.0 mM on two or more tests on different days indicate diabetes.

Random Plasma Glucose Test

Sometimes, random blood samples (taken shortly after eating or drinking) may be used to test for diabetes when symptoms are present. A blood glucose level higher than 200 mg/dl or 11.1 mM indicates diabetes, but it must be reconfirmed on another day with a fasting plasma glucose or an oral glucose test.

Oral Glucose Tolerance Test (OGTT)

With the oral glucose tolerance test, you must fast overnight (at least 8 but not more than 16 hours) and go to your doctor's office or the laboratory in the morning. First, your fasting plasma glucose is tested. After this test, you receive 75 grams of glucose. Usually, the glucose is in a sweet-tasting liquid that you drink. Blood samples are taken two more times to measure your blood glucose. For the test to give reliable results, you must be in good health (not have any other illnesses, not even a cold). Also, you should be normally active (for example, not lying down as an inpatient in a hospital) and taking no medicines that could affect your blood glucose. For 3 days before the test, you should have eaten a diet high in carbohydrates (150-200 grams per day). The morning of the test, you should not smoke or drink coffee. The oral glucose tolerance test measures blood glucose levels three times over a period of 2 hours. In a person without diabetes, the glucose levels rise and then fall quickly. In someone with diabetes, glucose levels rise higher than normal and fail to come back down as fast. 

People with glucose levels between normal and diabetic have impaired glucose tolerance (IGT). People with IGT do not have diabetes. Each year, only 3-10% of people whose test results show IGT actually develop diabetes. And with retesting, as many as half of the people with IGT have normal oral glucose tolerance test results. Weight loss and exercise may help people with IGT return their glucose levels to normal.

A normal OGTT response. A person is said to have a normal response when the 2-hour glucose level is less than 140 mg/dl or 7.8 mM, and all values between 0 and 2 hours are less than 200 mg/dl or 11.1 mM.

Impaired glucose tolerance. A person is said to have IGT when the fasting plasma glucose is less than 126 mg/dl or 7.0 mM, and the 2-hour glucose level is between 140 and 200 mg/dl or 7.8 mM and 11.1 mM.

Diabetes. A person has diabetes if the fasting result is 126 mg/dL or 7.0 mM and/or the 2-hour reading is 200 mg/dL or 11.1 mM, or higher.