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7 Tests for Type 1 Diabetes: Sugar Levels, Antibody Tests, and More

Medically reviewed by Angelica Balingit, M.D.
Written by Emily Wagner, M.S.
Posted on February 7, 2024

Type 1 diabetes is a condition that causes high blood glucose (blood sugar) in both children and adults. The pancreas can’t make enough insulin, an important hormone that cells need to use sugar for energy. Without insulin, blood sugar levels continue to rise and can eventually become life-threatening.

Your child’s pediatrician or your doctor may suspect type 1 diabetes based on some telltale symptoms, such as:

  • Frequent urination and bed-wetting in children
  • Extreme thirst
  • Unintended weight loss
  • Fatigue and weakness
  • Blurry vision
  • Mood changes and irritability

If you or your child is experiencing these symptoms, it’s time to make an appointment for testing. Doctors may order a few different tests to make a type 1 diabetes diagnosis. Most are blood tests that check blood sugar levels, whereas others measure specialized proteins involved in type 1 diabetes. If type 1 diabetes is diagnosed, you’ll work closely with doctors to find the best treatment plan.

This article will describe the seven tests used to diagnose type 1 diabetes.

1. Fasting Blood Glucose Test

To look for signs of type 1 diabetes, your doctor will likely first run a fasting blood glucose test. Also known as fasting plasma glucose, this test measures your blood glucose level after you’ve fasted (not eaten) for at least eight hours. After you eat, your blood sugar levels spike. A fasting blood glucose test accurately measures your body’s base blood sugar levels without the influence of food.

Fasting plasma glucose testing measures blood glucose level after a person hasn’t eaten for at least eight hours.

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A fasting blood glucose test uses a small blood sample taken in a clinic. A normal fasting blood glucose level is 99 milligrams per deciliter (mg/dL) of blood or below. Your or your child may have diabetes if this test shows a blood glucose level of at least 126 mg/dL.

Doctors can also use fasting blood glucose levels to check for prediabetes. According to the Centers for Disease Control and Prevention (CDC), prediabetes is diagnosed when blood sugar levels are higher than normal — but not high enough to indicate diabetes. Prediabetes is diagnosed with a fasting blood glucose level of 100 mg/dL to 125 mg/dL.

2. Hemoglobin A1c Test

Doctors often test for types 1 and 2 diabetes with a hemoglobin A1c (HbA1c) test. This test specifically measures the amount of sugar attached to red blood cells, known as glycated hemoglobin. An HbA1c test reveals average blood glucose levels over the past two to three months. This is because red blood cells live for around three months and always have glucose attached.

Like a fasting blood glucose test, an A1c test uses a small blood sample collected through a needle in the arm. However, there’s no need to fast beforehand.

A1c test results are given as a percentage. The higher the blood glucose level, the higher the A1c. The CDC gives the following ranges for diagnosing prediabetes and diabetes using an A1c test:

  • Normal — Below 5.7 percent
  • Prediabetes — 5.7 percent to 6.4 percent
  • Diabetes — 6.5 percent or higher

It’s important to note that A1c testing can’t diagnose type 1 versus type 2 diabetes. It only lets people know that their average blood sugar level is higher than normal. A1c tests can also tell doctors how well type 1 diabetes is being managed. If your child’s A1c level is elevated, their pediatrician may recommend changing their treatment plan.

3. Random Blood Sugar Test

A random blood sugar test measures blood glucose levels at the time of testing. It can be taken anytime, regardless of when food was last eaten. If you or your child has type 1 diabetes, the test may show hyperglycemia or high blood sugar levels. According to Mayo Clinic, a random blood sugar level of 200 mg/dL or higher points to a type 1 diabetes diagnosis.

4. Oral Glucose Tolerance Test

Another way doctors diagnose diabetes is by performing an oral glucose tolerance test (OGTT). This test measures the body’s response to glucose. An OGTT is typically used to diagnose type 2 diabetes and gestational diabetes (diabetes during pregnancy), but it can also diagnose type 1 diabetes.

You (or your child) will fast overnight and have blood drawn before the OGTT test to measure your fasting blood sugar level. After you’ve downed a sugary drink, blood samples are taken one and two hours later to measure blood sugar levels again.

The CDC gives the following ranges for an oral glucose tolerance test:

  • Normal — 140 mg/dL or lower
  • Prediabetes — 140 mg/dL to 199 mg/dL
  • Diabetes — 200 mg/dL or higher

5. Antibody Testing

Type 1 diabetes is an autoimmune condition, meaning the immune system attacks the body’s healthy cells and tissues. Specifically, the immune system creates specialized proteins known as autoantibodies. These proteins mistake healthy cells for foreign invaders, attacking and destroying them. It’s important to note that autoantibodies are found only in people with type 1 diabetes and not in those with type 2 diabetes.

Autoantibodies are only found in people with type 1 diabetes and not in those with type 2.

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In type 1 diabetes, certain autoantibodies target and destroy insulin-producing cells in the pancreas, known as beta cells. Without these cells, the body can’t make enough insulin, and type 1 diabetes develops. Other autoantibodies can attack insulin and other chemicals related to insulin production in the pancreas.

Doctors can measure autoantibodies to diagnose type 1 diabetes. Four main autoantibodies can develop in children and adults with type 1 diabetes:

  • Insulin autoantibodies
  • Islet cell cytoplasmic autoantibodies
  • Insulinoma-associated-2 autoantibodies
  • Glutamic acid decarboxylase autoantibodies

Antibody testing can also tell doctors who’s at risk of developing type 1 diabetes. Autoantibodies can appear years before a type 1 diabetes diagnosis. A family history of type 1 diabetes — such as having a parent or sibling with the condition — increases the likelihood of developing it. You can ask your doctor or your child’s pediatrician for antibody testing if you’re concerned about the risk of type 1 diabetes.

Autoantibodies can appear years before a type 1 diabetes diagnosis.

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6. C-Peptide Test

When your body makes insulin, it also produces a byproduct protein known as C-peptide. In general, the pancreas makes equal amounts of insulin and C-peptide, which are released into the bloodstream. C-peptide proteins don’t affect blood sugar levels.

A C-peptide test can help show if a person’s pancreas is making insulin on its own. Since C-peptide proteins stay in the bloodstream longer than insulin, doctors can usually get an accurate measurement to diagnose type 1 diabetes.

A C-peptide test can be done with a blood or urine sample. The blood test collects a small sample of blood to measure C-peptide levels. The urine test is a bit more involved and takes around 24 hours. Your doctor will provide instructions for collecting urine samples properly.

A normal C-peptide level is between 0.5 to 2 nanograms per milliliter blood. A low C-peptide level may point to a type 1 diabetes diagnosis, but your doctor will need to confirm that result with blood glucose level testing.

C-peptide is used to diagnose type 1 diabetes because it shows whether the pancreas is making insulin. People with type 2 diabetes may also have a C-peptide test to see if their pancreas has stopped making insulin or monitor how effective their treatment is.

7. Test for Ketones

In some cases, type 1 diabetes isn’t diagnosed until a life-threatening complication known as diabetic ketoacidosis (DKA) occurs. The body’s cells normally use insulin to take up sugar for energy. Without enough insulin, the cells can’t use sugar, so they look for another source of energy. The body starts breaking down fat for fuel, which creates a byproduct known as ketones. Ketones build up in the bloodstream and eventually become toxic and cause DKA.

DKA may be the first sign of having type 1 diabetes. If you or your child begins experiencing any symptoms of DKA, it’s important to seek emergency care as soon as possible. Symptoms can include:

  • Extreme thirst
  • Stomach pain, nausea, and vomiting
  • Shortness of breath
  • Frequent urination
  • Confusion
  • Fruity-smelling breath
  • Fatigue and weakness

Your doctor will run blood tests to look for ketones, high blood sugar, and acid level to diagnose DKA. Treatment focuses on lowering blood sugar levels with insulin therapy and providing hydration. They may also run other blood sugar tests to confirm a type 1 diabetes diagnosis.

Talk With Others Who Understand

On myT1Dteam, the social network for people living with type 1 diabetes and their loved ones, members come together to ask questions, give advice, and share their stories with those who understand life with type 1 diabetes.

Are you or a family member living with type 1 diabetes? What tests were done leading up to the diagnosis? Share your experience in the comments below, or start a conversation by posting on your Activities page.

Posted on February 7, 2024
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Why Isn’t T1D Considered As A Possibility In Adult Patients Diagnosed With T2D When T2 Treatments Are Not Workin ?
February 15, 2024 by A myT1Dteam Member 4 answers
Angelica Balingit, M.D. is a specialist in internal medicine, board certified since 1996. Learn more about her here.
Emily Wagner, M.S. holds a Master of Science in biomedical sciences with a focus in pharmacology. She is passionate about immunology, cancer biology, and molecular biology. Learn more about her here.

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