Type 2 Diabetes Mellitus Differential Diagnoses

Updated: May 01, 2023
  • Author: Romesh Khardori, MD, PhD, FACP; Chief Editor: George T Griffing, MD  more...
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Diagnostic Considerations

Correctly determining whether a patient has type 1 or type 2 diabetes is important because patients with type 1 diabetes require continuous exogenous insulin for survival. In contrast, treatment of type 2 diabetes consists of lifestyle measures and a variety of other medications, with insulin introduced if those prove inadequate.

As previously stated, patients with type 2 diabetes mellitus can usually be differentiated from those with type 1 disease on the basis of history and physical examination findings and through simple laboratory tests. Patients with type 2 diabetes are generally obese, and may have acanthosis nigricans and/or hirsutism in conjunction with thick necks and chubby cheeks.

A patient whose diabetes has been controlled with diet or an oral antidiabetic agent for longer than several months generally has type 2 diabetes. A lean patient who has had diabetes since childhood, who has always been dependent on insulin, or who has a history of diabetic ketoacidosis (DKA) almost certainly has type 1 diabetes.

When dealing with patients with known diabetes in the emergency department, distinguishing the type of diabetes can be difficult in 2 groups: (1) patients who are treated with insulin and are young but clinically appear to have type 2 diabetes, and (2) older patients with late-onset of diabetes who nonetheless take insulin and seem to share characteristics of patients with type 1 diabetes. (This latter group is now said to have latent autoimmune diabetes of the adult [LADA]).

When in doubt, the emergency department patient should be treated with insulin and his or her glucose levels should be closely monitored. Some adolescents or young adults, mostly Hispanic or African American patients, who present with classic DKA are subsequently found to have type 2 diabetes.


Prediabetes often precedes overt type 2 diabetes. Prediabetes is defined by a fasting blood glucose level of 100-125 mg/dL or a 2-hour post-oral glucose tolerance test (post-OGTT) glucose level of 140-200 mg/dL. Persons with prediabetes are at increased risk for macrovascular disease, as well as diabetes. [1]

Often confused with prediabetes is the metabolic syndrome (also called syndrome X or the insulin-resistance syndrome). Metabolic syndrome, thought to be due to insulin resistance, can occur in patients with overtly normal glucose tolerance, prediabetes, or diabetes. It is diagnosed when a patient has at least 3 of the following 5 conditions:

  • Abdominal obesity

  • Elevated triglyceride level

  • Low level of high-density lipoprotein (HDL) cholesterol

  • Elevated blood pressure

  • Fasting glucose value of 100 mg/dL or higher

Eventually, clinically apparent insulin resistance develops. Unfortunately, insulin resistance is not possible to measure clinically, except in research settings. An elevated fasting blood glucose or triglyceride level may be the first indication of insulin resistance. Fasting insulin levels are generally increased at an earlier stage, but they are more directly related to insulin clearance than to insulin resistance. An effort to standardize insulin assays is under way and may allow for the use of fasting insulin levels to diagnose insulin resistance in the future.