Evidence Summary
Type 2 diabetes accounts for 90 to 95% of all diabetes.1,2 It typically develops in people over age 45 but can occur in younger adults and children. Certain factors increase the risk of developing type 2 diabetes including:
- Older age
- Overweight or obesity
- Lack of physical activity
- Prior diabetes while pregnant (gestational diabetes)
- High blood pressure
- High lipid levels
- Polycystic ovary syndrome
- Certain medications including some HIV medications, glucocorticoids, thiazide diuretics, and atypical antipsychotics.
- Family history of type 2 diabetes in a first degree relative
- Certain racial and ethnic groups including blacks, Native Americans, Asians, Hispanics, and Latinos
Most people with type 2 diabetes don’t have any symptoms at first and are identified with screening tests.1,2 Screening for type 2 diabetes is recommended every 3 years for people over age 45 with normal test results. People at risk for type 2 diabetes including younger adults and children 10 or older may be done sooner or more often.
Diagnosis of type 2 diabetes is based on the demonstration of hyperglycemia on one of the following tests.2 The American Diabetes Association recommends these criteria for diagnosis of diabetes: 
Although two abnormal blood glucose tests are diagnostic, the hemoglobin A1C test may have some advantages over those tests.2 It has greater preanalytical stability, represents the average blood glucose level over the last several months rather than a point in time that may be subject to outside factors such as stress, illness, or diet, and is easier to perform, as it does not require the patient to fast. However, people with certain conditions that interfere with the A1C test should be diagnosed using the plasma blood glucose criteria. These conditions include pregnancy, sickle cell disease, glucose-6-phosphate dehydrogenase deficiency, hemodialysis, HIV treatment, and anemia/hemoglobinopathies. Also, people with recent blood loss or who had a recent blood transfusion or erythropoietin therapy should use the plasma blood glucose criteria.
People diagnosed with diabetes who are not dependent on insulin treatment, respond to oral diabetes treatment, and meet the above profile are typically presumed to have type 2 diabetes.2,4 However, there are tests that can help differentiate between type 1 and type 2 in equivocal cases such as in younger or normal weight people without obvious risk factors. A panel of tests for islet autoantibodies which only occur in people with type 1 diabetes can provide diagnostic clarity.
Another test that can help differentiate between type 1 and type 2 diabetes is c-peptide level measurement, which measures pancreatic beta cell function.5 A fasting c-peptide level of less than 0.2 nmol/l usually indicates insufficient insulin production by the pancreas indicating type 1 diabetes or less commonly, very advanced type 2 diabetes. However, the various tests for c-peptide levels have drawbacks that make them a less popular option for diabetes testing. Aside from the UCPCR, c-peptide level tests are not recommended in the guidelines as a way to determine who may need genetic testing for monogenic diabetes.2
Without treatment, people with diabetes have a much higher risk of diabetes-related complications such as:1,6,7
- Hyperglycemic and hypoglycemic crises requiring emergency help or hospital treatment
- Cardiovascular disease (heart attack, stroke, ischemic heart disease)
- Kidney disease and failure
- Vision loss and blindness
- Neurological disease
- Sexual dysfunction
- Dental problems
- Foot ulcers and amputations
References
- Vijan S. Type 2 diabetes. Ann Intern Med. Nov 5 2019;171(9):Itc65-itc80. doi:10.7326/aitc201911050
- 2. Classification and diagnosis of diabetes: standards of medical care in diabetes-2021. Diabetes Care. Jan 2021;44(Suppl 1):S15-s33. doi:10.2337/dc21-S002
- Mouri M, Badireddy M. Hyperglycemia. StatPearls. Treasure Island, FL: StatPearls Publishing; 2022.
- Butler AE, Misselbrook D. Distinguishing between type 1 and type 2 diabetes. BMJ. 2020;370:m2998. doi:10.1136/bmj.m2998
- Leighton E, Sainsbury CA, Jones GC. A Practical Review of C-Peptide Testing in Diabetes. Diabetes Ther. Jun 2017;8(3):475-487. doi:10.1007/s13300-017-0265-4
- National Diabetes Statistics Report (Centers for Disease Control and Prevention, U.S. Dept of Health and Human Services) (2020).
- National Institute of Diabetes and Digestive and Kidney Diseases. What is diabetes? NIH. Accessed January 16, 2020. https://www.niddk.nih.gov/health-information/diabetes/overview/what-is-diabetes
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