How Is Diabetes Diagnosed And Assessed?

Diabetes is diagnosed and assessed using blood tests in combination with other tests when needed. For some individuals, the symptoms combined with the blood tests are all that is required for the diagnosis. Other concurrent health issues may be present with the diabetes which requires additional tests. These are typically tests designed to assess the damage to other tissues of the body. Neurological testing, kidney tests, cardiovascular tests, tests for ketoacidosis and eye tests can all be required to assess the overall health and well being of the patient. The additional tests may be completed at the time of the initial diagnosis or they may occur later as other symptoms and health issues become apparent.

A1C Test (Glycated Hemoglobin)

The A1C test is commonly used to test for the diagnosis of diabetes and prediabetes. This test was formally recommended to diagnose diabetes by the American Diabetes Association and the International Diabetes Federation as well as other national diabetes associations around the world in 2009. Prior to this, it was used to monitor the glucose control of those individuals who were already diagnosed with diabetes. The major benefit of A1C testing is that it is able to determine the average blood glucose levels for two to three months prior to testing.

The A1C test works by determining the level of glucose that is located on the hemoglobin. The traces of glucose from several weeks and up to three months then forms a pattern of blood glucose levels that are considered more accurate than a fasting glucose test or a random blood sugar test, which is only a snap shot in time. There are some individuals that cannot use A1C test including those with blood disorders such as sickle cell hemoglobin, anemia, hemolysis (breakdown of red blood cells), blood transfusions, kidney or liver disease or those that are pregnant.

The A1C test is generally considered a good option for younger patients but may have limitations with older patients. In a study of 2, 107 older patients with a mean age of 69.4 years it was found that the cut point of 6.5% did not diagnose diabetes  in a significant number of the patients in the study that were identified as having diabetes or prediabetes on an oral glucose tolerance test. Researchers in this study indicated that this may result in delayed diagnosis until symptoms become identified. 1

This test may also become more difficult in older populations because of the natural increase in glycemic levels, even in non-diabetic people, as they age. In studies that compared A1C levels in adults less than 40 years of age and those over 70 without diabetes, there were significant changes in the average A1C levels. Researchers of this study reported that further studies on age-specific diagnostic and treatment criteria may be required for elderly patients using the A1C test. 2

Random Blood Sugar Tests

Random blood sugar tests give doctors a snapshot in time of the blood glucose levels in the body. Obviously there are several factors that can influence the glucose levels including the time of last meal, health issues and concurrent medical conditions. However a blood sugar reading of more than 200mg/dL is a strong indicator of diabetes. In most cases, this type of test is completed when other physical symptoms are present or as a routine screening test on an annual physical examination and check-up.

Oral Glucose Tolerance Test (OGTT)

Oral glucose tolerance tests are typically used when diabetes or prediabetes is suspected. This test actually measures the body’s ability to regulate blood glucose levels in a controlled setting. Patients will be required to fast overnight and then return to their doctor’s office. A blood test will be taken to obtain a fasting blood glucose level. The patient then drinks a highly sweetened drink and the blood test is repeated in two hours. The same rating scale is used as with a random blood test with levels at the two hour mark of more than 200 mg/dL indicating diabetes. Because of its difficulty to administer, it has fallen out of favor with most doctors with a few exceptions.

One such exception is in pregnancy. This test is often used to determine gestational diabetes. With pregnant women the test can last up to three hours and may involve multiple blood samples.

Fasting Plasma Glucose Test (FPG)

Similar to the OGTT the fasting plasma glucose test requires at least an eight hour fast prior to the blood test. In general people with a fasting blood glucose level of higher than or equal to 126 mg/dL are considered to be diabetic. It is essential with this test, as with the OGTT, that no foods are consumed prior to the test for the indicated time period. Smokers will also have to refrain from smoking. Consuming any types of food, including chewing gum, can elevate readings and cause inaccurate results. Any abnormal results would need to be confirmed with a repeat test on another day.

While not as accurate for all patients, the oral glucose tolerance test and the fasting plasma glucose test are seen as critical for specific populations. This is because these tests are quick and easy to perform in any medical setting and can be used, especially in the diagnosis of prediabetes, to help prevent further complications such as major cardiovascular disease complications which include hypertension and microvascular complications.3

Other Tests

Other tests include the dilated eye exam, which allows the physician to see in to the retina and determine if elevated blood sugar levels have led to microvascular damage in the eye. Tests of the feet to determine if any neurological damage has occurred are also considered important in situations where type 2 or type 1 diabetes is suspected.

Kidney tests and lipid profiles of the blood are also used to determine if diabetes is present and if there are any concurrent health conditions that need to be treated. Assessment of the entire person is essential since diabetes type 1 and type 2 can lead to various health complications that cannot be determined by a simple blood test. Going through the entire battery of tests is often not necessary.  For most patients, the tests ordered will be based on the symptoms exhibited, the type of diabetes determined and the actual blood glucose readings for the few weeks to months immediately following the diagnosis.


1 Kramer, C. K., Araneta, M. R., & Barrett-Connor, E. (2009). A1C and Diabetes Diagnosis: The Rancho Bernardo Study. Diabetes Care , 101-103.

2 Pani, L. N., Korenda, L., Meigs, J., et al. (2008). Effect of Aging on A1C Levels in Individuals Without Diabetes. Diabetes , 1991-1996.

3 Nathan, D. M., Davidson, M. B., DeFronzo, R. A., et al. (2007). Impaired Fasting Glucose and Impaired Glucose Tolerance. Diabetes Care , 753-759.

This article was originally published July 12, 2012 and last revision and update of it was 9/10/2015.