Diagnosis and Screening Tests
There are no clear-cut guidelines for when to screen for diabetes. Some experts recommend that everyone over age 45 be tested regularly for diabetes, although others do not feel this is necessary in people with no symptoms or risk factors. In fact, early screening may identify some people with impaired glucose levels that would eventually normalize. Such people might be treated unnecessarily with medications that pose a risk for high blood sugar (hyperglycemia).
Still, given the risk for serious complications with diabetes and the potential value of early treatments, most experts recommend that all adults over 45 be screened and that younger adults be screened if they have one or more of the following conditions:
- A weight that is 20% more than ideal body weight
- Risk factors for heart disease, such as high blood pressure or unhealthy cholesterol levels -- especially for patients with low HDL ("good") cholesterol and high triglyceride levels
- History or presence of heart disease, stroke, or peripheral artery disease
- A close relative with diabetes
- A high-risk ethnic group background
- In women, having delivered a baby weighing over 9 pounds or having a history of gestational diabetes
- In women, polycystic ovary disease
Some experts recommend that children over age 10 should be tested for type 2 diabetes (even if they have no symptoms) if they are overweight and have at least two risk factors.
Testing for Diabetes
Fasting Plasma Glucose. The fasting plasma glucose (FPG) test is the standard test for diabetes. It is a simple blood test taken after 8 hours of fasting. Results indicate:
- FPG levels are considered normal up to 100 mg/dL (or 5.5 mmol/L).
- Levels between 100 - 125 mg/dL (5.5 - 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 test is 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 were tested in the morning.
People with FPG levels in the high end of the normal range (high 90s) may have increased risk for developing type 2 diabetes. Obesity further increases this risk. Patients with FPG levels in the upper 90s should exercise and lose weight to help lower their FPG levels.
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:
- The patient first has an FPG test.
- The patient has a blood test 2 hours later, after drinking a special glucose solution.
The following results suggest different conditions:
- OGTT levels are considered normal up to 140 mg/dL.
- Levels between 140 - 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.
The patient cannot eat for at least 8 hours prior to the FPG and OGTT tests.
Test for Glycosylated Hemoglobin. Tests for blood levels of glycosylated hemoglobin, also known as hemoglobin A1c (HbA1c), are not used for an initial diagnosis, but they are useful for determining the severity of diabetes. Some experts think this test can help predict complications in people who have FPG levels of 110 - 139, which are above normal but do not indicate full-blown diabetes.
The basis for this test as a diagnostic measurement in diabetes includes:
- Hemoglobin is a protein molecule found in red blood cells. When glucose binds to it, the hemoglobin becomes modified, a process called glycosylation.
- Glycosylation affects a number of proteins, and elevated levels of glycosylated hemoglobin are strongly associated with complications of diabetes.
- A glycosylated 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.
Food intake does not affect the test, 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% glycosylated hemoglobin indicate poor control of carbohydrates. High levels are also markers for kidney trouble.
Screening Tests for Complications
Screening for Heart Disease. All patients with diabetes should be tested for 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 mm Hg or lower. Other tests may be needed in patients with signs of heart disease.
Screening for Kidney Damage. The earliest manifestation of kidney damage is microalbuminuria, in which tiny amounts (30 - 300 mg per day) of protein called albumin are found in the urine. About 20% of type 2 patients show evidence of microalbuminuria upon diagnosis of diabetes. (However, not all people with type 2 diabetes eventually develop kidney disease.) Microalbuminuria typically shows up in patients with type 2 diabetes who have high blood pressure.
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 cleansing the blood.
Screening for Retinopathy. The American Diabetes Association recommends that patients with type 2 diabetes get an initial comprehensive eye exam by an ophthalmologist or optometrist shortly after they are diagnosed with diabetes, and once a year thereafter. (People at low risk may need follow-up exams only every 2 - 3 years.) The eye exam should include dilation to check for signs of retinal disease (retinopathy).
Screening for Neuropathy. All patients should be screened for nerve damage (neuropathy), including a comprehensive foot exam. Patients who lose sensation in their feet should have a foot exam every 3 - 6 months to check for ulcers or infections.
Screening for Thyroid Abnormalities. Thyroid function tests should be administered.
