Glutamic acid decarboxylase is an enzyme found in brain and pancreas that converts glutamic acid (glutamate) into GABA, an inhibitory neurotransmitter. The glutamic acid decarboxylase test is a test that looks for antibodies directed against the glutamic acid decarboxylase enzyme. While glutamic acid decarboxylase is a critical enzyme in brain, it is also found in beta cells in the pancreas. Some individuals develop autoantibodies against glutamic acid decarboxylase. When these antibodies are detectable in the blood, they may represent an autoimmune disease. The test is performed using radioimmunoassay. The absence or very low levels of glutamic acid decarboxylase autoantibodies are considered normal. Less than 10% of healthy individuals will have low levels of glutamic acid decarboxylase antibodies, suggesting that they may be present in people without disease.
Normal Ranges for Glutamic Acid Decarboxylase:
Less than or equal to 0.02 nmol/L
It is not possible to have glutamic acid decarboxylase antibody levels that are too low.
The two main types of diseases in which glutamic acid decarboxylase autoantibodies are present are diabetes and neurological disease. Approximately 4 out of 5 people with type I diabetes will have low levels of these autoantibodies. On the other hand, less than 5% of people with type II diabetes will have detectable amounts of these autoantibodies in their serum. The presence of these autoantibodies does not necessarily diagnose particular neurological diseases; however, a portion of people with various autoimmune neurological disorders will have low titers of glutamic acid decarboxylase antibodies. These antibodies may also predict a predisposition to certain autoimmune diseases such as autoimmune thyroid disease, pernicious anemia, Addison disease, and vitiligo.
Some specific causes of glutamic acid decarboxylase antibodies are:
- Type 1 diabetes
- Type 2 diabetes
- Myasthenia gravis
- Lambert Eaton syndrome
- Stiff person syndrome
- Autoimmune encephalitis
- Brainstem encephalitis
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