How Prediabetes Is Diagnosed

Prediabetes is characterized by elevated blood sugar levels that are not yet high enough to fulfill a diagnosis of type 2 diabetes.

Even though prediabetes is a common condition, most affected people are unaware they have it because there are often no apparent symptoms. Nevertheless, since prediabetes is a stepping stone to diabetes and serious complications, getting screened is essential if you are at risk.

This article will review the three blood tests used to detect prediabetes. It will also reveal how clues from your medical history and physical exam may draw suspicion for a diagnosis of prediabetes.

A lab professional prepares blood samples for testing

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Self-Checks/At-Home Testing

An estimated 96 million adults in the United States, or approximately 38% of the population, have prediabetes.

While not an exhaustive list, possible symptoms and warning signs of elevated blood sugar levels (hyperglycemia) include:

  • Excessive thirst (polydipsia)
  • Frequent urge to urinate (polyuria)
  • Blurry vision
  • Unusual tiredness
  • Sores or cuts that won't heal
  • Numbness and tingling of the feet, toes, hands, and fingers

Regardless of symptoms, it's crucial to see a healthcare provider for periodic check-ups where your blood sugar levels can be checked. Prediabetes or diabetes cannot be diagnosed at home, even with a home blood glucose monitoring kit or device.

Screening for Prediabetes

The American Diabetes Association advises adults without risk factors to begin screening for prediabetes at age 35. 

Screening for prediabetes is also recommended for children who fulfill all of these three criteria:

  • Have entered puberty or are at least 10 years old
  • Have overweight or obesity
  • Have at least one other risk factor for type 2 diabetes (e.g., family history of diabetes)

Physical Examination

There is no single telltale physical sign of prediabetes.

That said, your physical examination may reveal some warning clues, such as:

  • High blood pressure
  • Large waistline
  • Excess weight
  • Acanthosis nigricans (dark patches of velvety skin on the back of your neck, armpit, or groin)
  • Skin tags (fleshy, small skin growths often found on the neck, eyelids, and underarms)

As your healthcare provider examines you, they will likely inquire about your medical history, as certain conditions are associated with prediabetes.

Some of these conditions include:

Labs and Tests

Based on screening recommendations or symptoms, your healthcare provider can confirm a diagnosis of prediabetes using one of three tests:

  • Hemoglobin A1c: This blood test reveals your average blood sugar levels for the past three months. It measures the amount of glucose stuck to hemoglobin A (a protein in your blood cells that carries oxygen). An A1C level between 5.7% and 6.4% indicates a diagnosis of prediabetes.
  • Fasting plasma glucose (FPG): This blood test is usually performed in the morning before breakfast. It measures your blood sugar level when fasting, which means not having anything to eat or drink (except water) for at least eight hours before the test. An FPG level from 100 to 125 milligrams per deciliter (mg/dL) indicates prediabetes.
  • Oral glucose tolerance test (OGTT): This test involves checking your blood glucose levels before and two hours after you drink a special sugary drink. It evaluates how your body processes sugar. Levels between 140 and 199 milligrams per deciliter indicate prediabetes.
Summary of Prediabetes Diagnostic Values
  Hemoglobin A1C Fasting Plasma Glucose (FPG) Oral Glucose Tolerance Test (OGTT)
 Normal Below 5.7% Below 100 mg/dL Below 139 mg/dl
 Prediabetes 5.7% to 6.4% 100 to 125 mg/dL 140 to 199 mg/dL
 Diabetes 6.5% or higher 126 mg/dL or higher  200 mg/dL or higher

Differential Diagnoses

Besides prediabetes, other conditions may cause elevated blood sugar levels and/or symptoms of hyperglycemia.

Type 2 Diabetes

Prediabetes that is not managed well can lead to type 2 diabetes, a disease of insulin resistance.

Insulin is a hormone made by your pancreas that moves glucose into your cells, where it's converted into energy. Insulin resistance occurs when your cells stop responding to insulin, causing blood sugar levels to rise.

The diagnosis of type 2 diabetes is made by fulfilling any of the following criteria:

  • Hemoglobin A1C of 6.5% or higher
  • Fasting plasma glucose level higher than 125 milligrams per deciliter
  • Glucose of 200 milligrams per deciliter or higher on a two-hour oral glucose tolerance test
  • Random glucose of 200 milligrams per deciliter or higher in the presence of symptoms of hyperglycemia

In the absence of symptoms, a diagnosis of diabetes must be confirmed by repeating the same positive test.

Progression Into Type 2 Diabetes

Around 5% to 10% of people with prediabetes develop type 2 diabetes yearly.

Type 1 Diabetes

Type 1 diabetes is an autoimmune disease of insulin deficiency and is much less common than prediabetes and type 2 diabetes. With type 1 diabetes, a person's immune system attacks and destroys the pancreatic cells that make insulin.

Type 1 diabetes is usually differentiated from type 2 diabetes by the person's symptoms and medical history. As an example, people with type 1 diabetes are classically younger and have a leaner body habitus than those with type 2 diabetes. They may also have a personal or family history of autoimmune disease.

Antibody blood tests, like the glutamic acid decarboxylase (GAD) antibody, can also help in the differentiation process.

Stress Hyperglycemia

Stress hyperglycemia may occur in critically ill adults without known diabetes. This transient state of elevated blood sugar levels stems from increased levels of cortisol (stress hormone) and catecholamines (hormones produced by the adrenal gland). People who survive stress hyperglycemia are at an increased risk of developing diabetes.

Summary

Prediabetes is a common condition where blood sugar levels are elevated but not high enough to meet the threshold for a diagnosis of diabetes. Specific symptoms and findings from a physical exam may point toward a diagnosis of prediabetes.

However, only a healthcare provider can confirm the diagnosis using one of three blood tests—a hemoglobin A1c, fasting blood glucose, or oral glucose tolerance test. 

A Word From Verywell

If you are diagnosed with prediabetes, know that this is a wake-up call to change your lifestyle habits. Prediabetes can be reversed with healthy habits including daily exercise and eating a nutritious diet rich in fruits and vegetables.

Losing excess weight, stopping smoking, and getting enough sleep are also essential to maintaining normal blood sugar levels.

Frequently Asked Questions

  • Is prediabetes a formal diagnosis?

    While there are established criteria to diagnose prediabetes, experts debate whether prediabetes should be categorized as a disease versus a risk factor (for diabetes and cardiovascular disease). 

  • Can prediabetes be reversed?

    Yes, prediabetes can be reversed. However, this can most effectively be done by engaging in healthy lifestyle habits, such as eating nutritiously, losing excess weight, and staying physically active.

  • What are the warning signs of prediabetes?

    Most people with prediabetes have no symptoms. If they do, they may include excessive thirst, frequent urge to urinate, blurry vision, extreme tiredness, nerve tingling, and slow-healing sores or cuts.

10 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Colleen Doherty, MD
 Colleen Doherty, MD, is a board-certified internist living with multiple sclerosis.