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
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.
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.
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.
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.
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
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
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.