Typically, diabetes is diagnosed using four commonly employed tests. More than one of these tests are often used to diagnose diabetes in combination with clinical signs and symptoms. The following is a brief description of the four tests:
- Random or casual plasma glucose test: Measuring plasma glucose levels randomly during the day can be done conveniently and quickly without fasting. Usually, a glucose level result of equal or greater than 200 mg/dL may indicate a person could have diabetes. However, random testing or other testing such as fasting plasma glucose and/or oral glucose tolerance test (OGTT) are usually required to confirm the diagnosis.
- Fasting plasma glucose test: Measuring plasma glucose after at least an 8-hour fast can help to diagnose diabetes. Fasting plasma glucose levels between 60 and 99 mg/dL are normal. Diabetes is typically confirmed when 2 separate fasting plasma glucose levels are equal to or greater than 126 mg/dL.
- Oral glucose tolerance test (OGTT): The OGTT is usually performed to confirm the diagnosis of diabetes or used if a suspected diabetic individual has a normal or slightly elevated fasting plasma glucose level. The OGTT is performed after at least an 8-hour fast and after the individual drinks a glucose solution of 75 gms. Blood is drawn before drinking the solution (fasting) and 2 hours after drinking. The test determines how well the body can metabolize the ingested glucose. (In special cases, a clinician may request a plasma glucose test be performed every 30 minutes for 2 hours in adults and children. The OGTT may vary for pregnant patients, and a higher dose of glucose solution with plasma glucose testing every hour for up to 3 hours may be used.)
- For most individuals, an OGTT plasma glucose result 2 hours after drinking the glucose solution of less than 140 mg/dL is considered normal. A 2-hour OGTT value of 140 to 199 mg/dL may suggest prediabetes, whereas a 2-hour value equal to or greater than 200 mg/dL is suggestive of diabetes.
- For pregnant patients, an OGTT plasma glucose at 1 hour greater than 180 mg/dL, 2 hours greater than 155 mg/dL, or 3 hours greater than 140 mg/dL is considered abnormal and could suggest gestational diabetes.
- Glycosylated hemoglobin or Hemoglobin A1c test: The A1c test (also called hemoglobin A1c, HbA1c, glycated hemoglobin, or glycohemoglobin test) measures the percentage of hemoglobin proteins coated with glucose. As a component of the red blood cell, hemoglobin attaches or binds to glucose (glycated). The higher the glucose level in the bloodstream, the more glucose will attach to the hemoglobin. The A1c test measures the amount of hemoglobin with attached glucose over the past 3 months and expresses the results as a percentage. The higher the rate of A1c, the higher the blood glucose levels have been over the past 3 months.
- If the A1c test is below 5.7%, it is considered normal.
- If it is in the 5.7–6.4% range, it indicates prediabetes.
- If it is 6.5% or above, it indicates diabetes.
