What tests are used to diagnose prediabetes?

To diagnose prediabetes, doctors rely on several blood tests that measure blood sugar levels. The most common tests are the Fasting Plasma Glucose (FPG) test and the Hemoglobin A1C test [1]. These tests help determine if your blood sugar levels are higher than normal but not high enough to diagnose type 2 diabetes [1].

The numerical thresholds presented here pertain to standard non-pregnancy testing; gestational diabetes involves different criteria and procedures. In the absence of clear symptoms, results indicating diabetes are confirmed through retesting or by using another test as directed by the physician. Additionally, the accuracy of the A1C test can be affected by pregnancy and certain blood conditions, so a single figure is not sufficient for making a diagnostic decision [4].

Fasting Plasma Glucose (FPG) Test

This test measures blood sugar levels at a single point in time. It requires fasting for at least 8 hours before blood is drawn, meaning you must abstain from food and drink, except for sips of water [2]. Results are given in milligrams per deciliter (mg/dL) [1]. A blood sugar level of 99 mg/dL or lower is considered normal. Prediabetes is diagnosed if the results are between 100 and 125 mg/dL. If the result is 126 mg/dL or higher, it may indicate diabetes, and the doctor will confirm the diagnosis and determine its type [1].

Hemoglobin A1C Test

The A1C test, also known as the glycated hemoglobin test, provides the average blood sugar levels over the past three months [1]. This test does not require fasting, and you can eat and drink normally before it [2]. Results are given as a percentage; the higher the percentage, the higher the blood sugar levels [1]. Levels below 5.7% are considered normal. Prediabetes is diagnosed if results are between 5.7% and 6.4%. If the result is 6.5% or higher, type 2 diabetes is diagnosed [1].

Oral Glucose Tolerance Test (OGTT)

This test is used to diagnose prediabetes, type 2 diabetes, and gestational diabetes [2]. It requires fasting for at least 8 hours before the test. An initial blood sample is taken to measure the fasting blood sugar level, then the patient is asked to drink a sugary liquid. Another blood sample is taken two hours later to measure the blood sugar level [2]. Prediabetes is diagnosed if results are between 140 and 199 mg/dL two hours after drinking the sugary liquid. If it is 200 mg/dL or higher, it may indicate diabetes and requires interpretation and confirmation based on the condition [2].

When Should Tests Be Conducted?

Experts recommend routine type 2 diabetes testing if you are 35 years or older, or if you are overweight or obese and have at least one other risk factor, such as a family history of diabetes [2]. If diabetes test results are normal, you should repeat testing every 3 years. If you are diagnosed with prediabetes, you should be tested for type 2 diabetes annually [2]. These tests can help in the early detection of prediabetes, allowing you to take preventive steps to delay or prevent the development of type 2 diabetes. For more information on how to reverse prediabetes, you can visit Prediabetes: Can the condition be reversed and type 2 diabetes prevented? (Arabic)

Medical references

• Prediabetes
• Diabetes Tests & Diagnosis
• The A1C Test & Diabetes - NIDDK