The goals with testing are to screen for high blood glucose levels (hyperglycemia), to detect and diagnose diabetes and pre-diabetes, to monitor and control glucose levels over time, and to detect and monitor complications.
Tests for screening
There are a few tests that may be used to screen for and/or diagnose diabetes and pre-diabetes. They may be used:
- As part of a regular physical
- When someone has symptoms suggesting diabetes
- When a person has a condition that is associated with diabetes
- When a person presents to the emergency room with an acute condition
- For pregnant women as a screen for gestational diabetes
Screening tests include:
- Fasting glucose (fasting blood glucose, FBG) – this test measures the level of glucose in the blood after an 8-12 hour fast.
- A1c (also called hemoglobin A1c or glycohemoglobin) – this test evaluates the average amount of glucose in the blood over the last 2 to 3 months and has been recommended more recently as another test to screen for diabetes. (For more on this test, see below.)
- For pregnant women, some professional organizations recommend a glucose challenge test (GCT) to screen for gestational diabetes at 24-28 weeks of pregnancy. This involves a fasting blood glucose followed by the woman drinking a standard glucose solution and another glucose test one hour after consuming the glucose. If results are abnormal, then a 3-hour oral glucose tolerance test (OGTT, GTT) is performed to help establish a diagnosis. However, the American Diabetes Association in 2011 adopted new guidelines that recommend the use of a 2-hour glucose tolerance test for screening and diagnosing gestational diabetes. See "Tests for diagnosis" below.
- Sometimes a random blood glucose level is used for screening when a fasting test is not possible, such as when a person is seriously ill.
- Sometimes random urine samples are tested for glucose, protein, and ketones during a physical. If glucose and/or protein or ketones are present on the indicator strip dipped in the urine sample, the person has a problem that needs to be addressed. This is a screening tool, but it is not sensitive enough for diagnosis or monitoring.
Tests for diagnosis
According to the American Diabetes Association (ADA), a fasting blood glucose, an oral glucose tolerance test, or an A1c test may be used to diagnose diabetes and pre-diabetes. Each test has advantages, disadvantages, and limitations.
- The FBG requires an 8-hour fast.
- The OGTT requires that the person have a fasting glucose test, followed by the person drinking a standard amount of glucose solution to "challenge" their system, followed by another glucose test 2 hours later.
- With the A1c, people don't have to fast for 8 hours or endure multiple blood samples being taken over several hours, but the test is not recommended for everyone. It should not be used for diabetes diagnosis in pregnant women, people who have had recent severe bleeding or blood transfusions, those with chronic kidney or liver disease, and people with blood disorders such as iron-deficiency anemia, vitamin B12 anemia, and hemoglobin variants. Also, only A1c tests that have been referenced to an accepted laboratory method (standardized) should be used for diagnostic or screening purposes. Currently, point-of-care tests, such as those that may be used at a doctor’s office or a patient’s bedside, are too variable for use in diagnosis but can be used to monitor treatment (lifestyle and drug therapies).
If the initial result from one of the above tests is abnormal, the test should be repeated on another day to confirm a diagnosis of diabetes.
- Gestational diabetes may be diagnosed using a glucose challenge test as a screen, followed by a 3-hour OGTT using a 100-gram glucose drink if the screen is abnormal, per recommendations of the American College of Obstetricians and Gynecologists (ACOG). If at least two of the glucose levels at fasting, 1 hour, 2 hour, or 3 hour are above a certain level, then a diagnosis of gestational diabetes is made. The ADA, however, recommends that a 2-hour glucose tolerance test using a 75-gram glucose drink be used to screen for and diagnose gestational diabetes. Only one of the values needs to be above a cutpoint for diagnosis.
- Diabetes autoantibodies – this test may help distinguish between type 1 and type 2 diabetes if the diagnosis is unclear. The presence of one or more of these antibodies indicates type 1 diabetes.
Tests for monitoring
- Glucose – diabetics must monitor their own blood glucose levels, often several times a day, to determine how far above or below normal their glucose is and, based on their doctor's instructions, what modifications they should make to their medications. This is usually done by placing a drop of blood (obtained by pricking your skin with a small lancet device), onto a glucose strip and then inserting the strip into a glucose meter, a small machine that provides a digital readout of the blood glucose level.
- A1c and estimated average glucose (eAG) – this is a test and a calculation that are ordered several times a year to monitor people with type 1 or type 2 diabetes. A1c is a measure of the average amount of glucose present in the blood over the last 2 to 3 months and helps the doctor to determine how well a treatment plan is working to control the person's blood glucose levels over time.
Occasionally other tests may be ordered to evaluate glucose levels over time. These may include:
- Fructosamine – evaluates average glucose levels over the past 2 to 3 weeks
- 1,5 Anhydroglucitol – a new test that detects high glucose levels in the past 1-2 weeks
Several other laboratory tests may be used to monitor diabetes, evaluate organ function, and detect emerging complications. These include:
- To monitor kidney function: Microalbumin (increased urinary albumin), Creatinine Clearance, eGFR, CMP, BUN, Creatinine, Cystatin C
- To monitor cholesterol and other lipids: Cholesterol, HDL cholesterol, LDL cholesterol, Triglycerides, Lipid profile
- To monitor insulin production: Insulin, C-Peptide
- Urine and/or blood ketone tests may be ordered to monitor people who present at the emergency room with symptoms suggesting acute hyperglycemia and to monitor those who are being treated for ketoacidosis. A build-up of ketones can occur whenever there is a decrease in the amount or effectiveness of insulin in the body.