Tests to Diagnose Type 1 Diabetes

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Tests to Diagnose Type 1 Diabetes

Type 1 diabetes is often diagnosed suddenly and quickly, based on certain signs and symptoms. Sometimes, though, diagnosing type 1 diabetes isn’t that clear cut. In this case, your provider might order other tests. Learn more about diagnosing type 1 diabetes and what the various tests mean.

Classic symptoms of type 1 diabetes

Frequently, a person who has undiagnosed type 1 diabetes will feel ill, and have symptoms including:

  • Increased (and sometimes extreme) thirst
  • Increased hunger
  • Frequent urination
  • Weight loss
  • Fatigue and weakness
  • Blurry vision
  • Nausea and vomiting
  • Stomach pain
  • Wounds or cuts that aren’t healing
    Yeast infections

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Blood tests for diagnosing diabetes

Both type 1 and type 2 diabetes can be diagnosed using blood tests. These include:

  • A random plasma glucose (RPG) test, which measures blood sugar at a single point in time
  • A fasting blood glucose test, which is done after the person fasts for at least eight hours (usually overnight)
  • An oral glucose tolerance test (OGTT), which involves fasting overnight, having a blood test, then drinking a sugary drink and having the blood sugar checked over the course of two hours
  • An A1C test, which is a blood test that shows the average blood sugar over the past three months

Here are blood test results that indicate diabetes:

  • Random plasma glucose: 200 mg/dl or above
  • Fasting blood glucose: 126 mg/dl or above
  • OGTT: 200 mg/dl or above
  • A1C: 6.5% or above

Diagnosis of diabetes requires two abnormal screening test results, either from the same sample or in two separate test samples,” according to the American Diabetes Association, unless there is a clear clinical diagnosis with classic symptoms of hyperglycemia (high blood sugar).

It’s important to note that these blood test results don’t distinguish between type 1 and type 2 diabetes.

Another potential indicator of type 1 diabetes is the presence of ketones in the urine. A lack of insulin causes fat to be used for energy, and this process forms substances called ketones. However, people with type 2 diabetes may also have ketones, although it’s not as common as it is with type 1 diabetes.

C-peptide test

The C-peptide test measures the level of C-peptide in the blood or urine. C-peptide is a substance made in the pancreas, along with insulin, and both C-peptide and insulin are released from the pancreas at about the same time and in equal amounts. So, in the case of type 1 diabetes, the pancreas makes little or no insulin, and therefore, makes little or no C-peptide. With type 2 diabetes, the body makes insulin, but doesn’t use it well. C-peptide levels can be higher than normal in someone with type 2 diabetes.

While a low level of C-peptide can indicate type 1 diabetes, it may also be a sign of Addison’s disease (a disorder of the adrenal glands) or liver disease.

C-peptide is not used to diagnose diabetes; rather, it gives information about the type of diabetes a person has.


Autoantibodies are antibodies (proteins made by the immune system to protect against bacteria or viruses that cause illness or disease) that attack healthy tissues and cells by mistake. The presence of certain autoantibodies is common in someone who is suspected of having type 1 diabetes, but not type 2 diabetes.

The autoantibodies that are often measured to diagnose type 1 diabetes include:

GAD (glutamic acid decarboxylase)

This blood test measures whether the body is producing a type of antibody that destroys a type of enzyme in the beta cells. GAD autoantibodies are present in about 70% to 80% of people with new-onset type 1 diabetes.

IAA (insulin autoantibodies)

This test looks to see if the body has produced antibodies against insulin. IAA are present in about 50% of children with new-onset type 1, but is uncommon in adults.

ICA (islet cell cytoplasmic autoantibodies)

This test measures antibodies targeted at islet cell proteins. ICA are present in about 70% to 80% of people with new-onset type 1 diabetes.

IA2A (insulinoma-associated-2-autoantibodies)

This test checks for antibodies directed at beta cell antigens. IA2A are present in about 60% of people with new-onset type 1 diabetes.

ZnT8A (zinc transporter-8 autoantibodies)

This test checks for antibodies targeted against a protein that pumps zinc into insulin-containing granules in beta cells. ZnT8A are present in about 60% of people with new onset type 1 diabetes, and may also help diagnose latent autoimmune diabetes of adulthood (LADA).

Remember that autoantibodies can be found in people with other autoimmune endocrine disorders, so their presence alone does not necessarily indicate type 1 diabetes.

If you have symptoms that you think could indicate type 1 diabetes, talk with your health care provider about having autoantibodies checked. This may also be something to consider having done to predict the risk of getting type 1 diabetes, especially if you have a first-degree relative who has type 1 diabetes.

Want to learn more about type 1 diabetes? Read “Type 1 Diabetes Questions and Answers,” “Six Type 1 Diabetes Symptoms You Need to Know” and see our type 1 diabetes videos.

Amy Campbell, MS, RD, LDN, CDCES

Amy Campbell, MS, RD, LDN, CDCES

Amy Campbell, MS, RD, LDN, CDCES on social media

A Registered Dietitian and Certified Diabetes Educator at Good Measures, LLC, where she is a CDE manager for a virtual diabetes program. Campbell is the author of Staying Healthy with Diabetes: Nutrition & Meal Planning, a co-author of 16 Myths of a Diabetic Diet, and has written for  publications including Diabetes Self-Management, Diabetes Spectrum, Clinical Diabetes, the Diabetes Research & Wellness Foundation’s newsletter, DiabeticConnect.com, and CDiabetes.com

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