Fasting Plasma Glucose. The fasting plasma glucose (FPG) test is the standard test for diagnosing diabetes. It is a simple blood test taken after 8 hours of fasting. In general, results indicate the following:
- FPG levels are considered normal up to 100 mg/dL (or 5.5 mmol/L).
- Levels between 100 and 125 mg/dL (5.5 to 7.0 mmol/L) are referred to as impaired fasting glucose or pre-diabetes. These levels are considered to be risk factors for type 2 diabetes and its complications.
- Diabetes is diagnosed when FPG levels are 126 mg/dL (7.0 mmol/L) or higher.
The FPG test is not always reliable, so a repeat test is recommended if the initial test suggests the presence of diabetes, or if the tests are normal in people who have symptoms or risk factors for diabetes. For example, people who take the test in the afternoon and show normal results may actually have abnormal levels that would be revealed if they are tested in the morning.
Glucose Tolerance Test. The oral glucose tolerance test (OGTT) is more complex than the FPG and may overdiagnose diabetes in people who do not have it. Some experts recommend it as a follow-up after FPG, if the latter test results are normal but the patient has symptoms or risk factors of diabetes. The test uses the following procedures:
- It first uses an FPG test
- A blood test is then taken 2 hours later after drinking a special glucose solution
The following results suggest different conditions:
- OGTT levels are normal up to 140 mg/dL
- Levels between 140 mg/dL and 199 mg/dL are referred to as impaired glucose tolerance or pre-diabetes
- Diabetes is diagnosed when OGTT levels are 200 mg/dL or higher
Both the FPG and OGTT tests require that the patient not eat for at least 8 hours prior to the test.
The oral glucose tolerance test is used to diagnose diabetes. The first portion of the test involves drinking a special glucose solution. Blood is then taken several hours later to test for the level of glucose in the blood. Patients who have diabetes will have higher than normal levels of glucose in their blood.
Test for Glycated Hemoglobin. Another test examines blood levels glycated hemoglobin, also known as hemoglobin A1c (HbA1c). Measuring glycated hemoglobin is not currently used for an initial diagnosis, but it may be useful for determining the severity of diabetes.
The basis for its use as a diagnostic measurement in diabetes is as follows:
- Hemoglobin is a protein molecule found in red blood cells. When glucose binds to it, the hemoglobin becomes modified, a process called glycation.
- Glycation affects a number of proteins, and elevated levels of glycolated hemoglobin is strongly associated with complications of diabetes.
- A glycated hemoglobin level of 1% above normal range identifies diabetes in 98% of patients. Normal HbA1c levels do not necessarily rule out diabetes, but if diabetes is present and levels are normal, the risk for complications is low.
The test is not affected by food intake so it can be taken at any time. A home test has been developed that might make it easier to measure HbA1c. In general, measurements suggest the following:
- Normal HbA1c levels should be below 7%.
- Levels of 11 - 12% glycolated hemoglobin indicate poor control of carbohydrates. High levels are also markers for kidney trouble.
Testing for Insulin Resistance. Investigators hope that some day a simple test for insulin resistance will be available that will be able to identify people at risk for diabetes. Some research suggests that measuring insulin and triglyceride levels during a fasting period may predict a person's sensitivity to insulin.
Autoantibody Tests
Type 1 diabetes is characterized by the presence of a variety of antibodies called autoantibodies that attack the islet cells. These antibodies are referred to as autoantibodies, because they attack the body's own cells -- not a foreign invader. Blood tests that test for these autoantibodies can help differentiate between type 1 and type 2 diabetes.
Screening Tests for Complications
Screening for Heart Disease. All patients with diabetes should be tested for high blood pressure (hypertension) and unhealthy cholesterol and lipid levels and given an electrocardiogram. For cholesterol, people with diabetes should aim for LDL levels below 100 mg/dL, HDL levels over 50 mg/dL, and triglyceride levels below 150 mg/dL. Blood pressure goals should be 130/80 mmHg or lower. Other tests may be needed in patients with signs of heart disease.
High blood pressure is strongly associated with diabetic nephropathy (kidney disease). In fact, patients with type 2 diabetes who show signs of microalbuminuria typically already have hypertension. Type 1 diabetes patients with microalbuminuria, on the other hand, usually have normal blood pressure readings in the doctor's office. A 2002 study using home monitors, however, found that in type 1 patients, high systolic blood pressure during sleep often occurs before development of nephropathy. (Systolic pressure is the first and higher number in a blood pressure reading.) Home blood pressure monitoring, may help identify type 1 patients at risk for kidney damage.
Screening for Kidney Damage. The earliest manifestation of kidney disease is microalbuminuria, in which tiny amounts (30 to 300 mg per day) of protein called albumin are found in the urine. Microalbuminuria is also a marker for other complications involving blood vessel abnormalities, including heart attack and stroke.
The American Diabetes Association recommends that people with diabetes receive an annual microalbuminuria urine test. Patients should also have their blood creatinine tested at least once a year. Creatinine is a waste product that is removed from the blood by the kidneys. High levels of creatinine may indicate kidney damage. A doctor uses the results from a creatinine blood test to calculate the glomerular filtration rate (GFR). The GFR is an indicator of kidney function; it estimates how well the kidneys are cleaning the blood.
Screening for Thyroid Abnormalities. Thyroid function tests should be administered.
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The New York Times | Health Guide
Reviewed By: Elizabeth H. Holt, MD, PhD, Assistant Professor of Medicine, Section of Endocrinology and Metabolism, Yale University. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.
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