In general, autoantibody testing is not required to make a diagnosis of autoimmune type 1 diabetes. In the general population, there is no benefit to autoantibody screening, but it can be useful in patients at high risk of diabetes (e.g. siblings of known type 1 diabetics and offspring of diabetic parents).

Diabetes-related (islet) autoantibody testing is primarily ordered to help distinguish between autoimmune type 1 diabetes and diabetes due to other causes (e.g., diabetes resulting from obesity and insulin resistance). If ICA, GADA, or IA-2A are present in an individual with diabetes, the diagnosis of type 1 diabetes has been established. IAA testing must be performed before insulin therapy is initiated. Like ICA, GADA and IA-2A, if IAA are present in a person with diabetes who is not insulin-treated, type 1 diabetes is the cause.

It is generally recommended that testing be done either for ICA or for the GAD/IA-2A combination since this approach is more cost-effective. The autoantibody profile is usually different between children and adults. IAA is usually the first marker to appear in young children at risk of diabetes. As the disease evolves, this may disappear and ICA, GADA and IA-2A become more important. IA-2A is less commonly positive at the onset of type 1 diabetes than either GADA or ICA. Whereas about 50% of children with new-onset type 1 diabetes will be IAA positive, IAA positivity is not common in adults. Thus in younger children at risk for diabetes, testing for IAA may also be requested.

In research settings where investigators want to predict the development of type 1 diabetes, islet autoantibody testing can be carried out. The more islet autoantibodies that a nondiabetic person has in their blood stream, the higher their risk for later developing type 1 diabetes.

A combination of these autoantibodies may be ordered when a patient is newly diagnosed with diabetes and the doctor suspects that the condition may be due to an autoimmune process. IAA testing as a marker of autoimmune type 1 diabetes must be performed before insulin injections are begun. Insulin injections of either human or animal insulin can stimulate insulin antibodies that will give a positive result in the IAA assay. The IAA assay does not distinguish autoantibodies from antibodies that form in response to insulin injections.

One or more of the autoantibodies may be ordered on the siblings of a patient diagnosed with type 1 diabetes or on the offspring of diabetic parents. This may be done initially and then again at intervals recommended by the doctor in a research setting.

Normally, nondiabetic individuals in the general population will not have any of these islet autoantibodies. However, when islet autoantibodies are detected in individuals in the general population or siblings of affected patients, there may be an increased risk for type 1 diabetes since false positives are known to occur. Many of these islet autoantibody-positive individuals will never develop diabetes, however. When type 1 diabetes does not develop, the level of the islet autoantibody in the blood is usually low and the islet autoantibody may be transient.

Some patients who do have type 1 diabetes will never develop detectible amounts of islet autoantibodies, although this is rare. The majority of people (95% or more) with new-onset type 1 diabetes will have at least one islet autoantibody. Therefore, if one or more islet autoantibody (e.g., ICA, GADA, IA-2A, and/or IAA) are present in a patient with symptoms of diabetes, the diagnosis of type 1 diabetes is confirmed.

In nondiabetic individuals who are positive for one or more islet autoantibodies, there is an increased risk for type 1 diabetes as mentioned above. The more islet autoantibodies that are present, the higher is the individual’s risk for developing type 1 diabetes. However not everybody with islet autoantibodies will develop type 1 diabetes. If a non-diabetic individual with one or more islet autoantibodies has a low insulin response to the intravenous injection of glucose, their risk for type 1 diabetes can be very high. In first degree relatives of patients with type 1 diabetes who have ICA and have a low insulin response to the intravenous injection of glucose, the 5-year risk of developing type 1 diabetes is approximately 60%. Because there are no effective therapies to prevent type 1 diabetes, general population screening for islet autoantibodies or testing first degree relatives of patients with type 1 diabetes is not recommended.

It is up to the doctor and patient to decide together which islet autoantibodies to test for at any given time. Because GADA and IA-2A assays are automated, these tests are generally more available than ICA testing, which is labor-intensive and requires considerable expertise in interpretation.

Islet autoantibodies may also be seen in patients with other autoimmune endocrine disorders such as Hashimoto thyroiditis or autoimmune Addison disease.