There is no scientific or clinical definition of double diabetes, nor any gold standard for diagnosis. This may make diagnosis challenging. Physicians may suspect double diabetes if the patient has been diagnosed with type 1 diabetes and begins to show signs and symptoms of insulin resistance and type 2 diabetes, particularly obesity and acanthosis nigricans.

A physician suspecting double diabetes will typically begin by reviewing the patient’s medical history and performing a physical examination. Several laboratory tests may be conducted, such as:

• C-peptide test. This blood test can indicate hyperinsulinemia (excess insulin in the blood) and help distinguish type 2 diabetes from type 1.
  
  
• Autoantibody test. Antibodies are proteins released by the immune system that attack a perceived threat. The islet cell antibodies associated with type 1 diabetes are generally lacking in patients with type 2 diabetes. Patients who display the signs and symptoms of type 2 diabetes yet have these antibodies may have double diabetes.
  
  
• Glycohemoglobin test. The standard measure of long-term control of glucose (blood sugar), this blood test can suggest whether the effectiveness of a patient’s insulin therapy is declining, which could be due to insulin resistance.
  
  
• Genetic tests. These may reveal if a patient with type 1 diabetes has certain genes that have been linked to insulin resistance and type 2 diabetes. Scientists have found increasing numbers of such genes in recent years.
  
  
• Ketone test. This measures the amount of ketones, an acidic waste product, in a person’s urine or bloodstream. An excessive level of ketones (ketosis) is more common in people with type 1 diabetes.

A physician is likely to take a blood pressure reading and order a cholesterol test to see if an overweight patient has or is at risk for metabolic syndrome, which is associated with insulin resistance and double diabetes.