Measurement of Endogenous Insulin Secretion
The results for tests to measure endogenous insulin secretion may be low in type 2 diabetes patients with glucose toxicity. If in doubt, measure the following after glycemic control has been restored for several weeks:
- Fasting insulin level – if the patient is not on exogenous insulin
- C-peptide (the other half of pro-insulin) – this is useful even if the patient is taking insulin injections.
Autoantibody Tests
Positive antibody tests denote an autoimmune process, but negative tests do not rule it out:
- IA-2 (Insulinoma-associated protein-2)
- GAD-65 (Glutamic acid decarboxylate-65)
- Other antibody tests have been used in research and clinical settings – e.g., ZnT8 (Zinc Transporter 8), thyroid peroxidase antibodies, insulin autoantibodies
Other Lab Tests and Exams
Although some overweight type 1 diabetes patients may have some signs of insulin resistance, in general, they will not have the usual type 2 diabetes measurements at diagnosis. Gauging the degree of insulin deficiency versus insulin resistance with the following tests can be helpful:
- Lipids – Most type 2 diabetes patients have the typical low HDL/high triglyceride pattern.
- Blood pressure – Type 2 diabetes patients often have some degree of hypertension at time of diabetes diagnosis.
- Ketones – Although patients with type 2 diabetes can present with ketonuria and even diabetic ketoacidosis (DKA), generally these only occur at very high glucose levels or with a serious concurrent illness or infection. More often, it is patients with type 1 diabetes who present with significant ketosis and who are more profoundly acidotic with DKA.
- Microvascular complications – Many type 2 diabetes patients already have some degree of retinopathy, microalbuminuria, or neuropathy at the time of diabetes diagnosis. This is seldom true of patients with type 1 diabetes.
- Weight loss – The degree and speed of weight loss before diagnosis is usually more rapid in patients with type 1 diabetes than with type 2 diabetes.
Even taking the results of these tests into consideration, there still will be a few patients whose type of new-onset diabetes is not initially clear; over time, however, the diagnosis will become apparent. In the meantime, if there is concern that the patient may become acidotic if taken off insulin or if insulin is needed for glycemic control, insulin therapy should be continued, at least until it is established that it is no longer necessary.