Tests for Type 2 diabetes – HbA1c vs Fasting Plasma Glucose

Published on 26 October 2021

Type 2 diabetes, a condition characterized by high blood glucose levels, is common in adults, though it also affects children. It is treated with tablets; insulin or other injections may be advised in some cases.

One of the two blood tests – the hemoglobin A1c (HbA1c) test or the fasting plasma glucose – is often used to test for type 2 diabetes. Given a choice, which one should you opt for? Here are some details about these tests which may help you decide which one could be the best for you.

About the HbA1c test

The HbA1c test, also called the A1c test, gives an idea about the average blood glucose levels over three months before the test. A high glucose level in the blood results in a high HbA1c percentage.

This blood test measures the glucose that attaches to the hemoglobin in the red blood cells. Since the life span of the red blood cells is around 3 months, it reflects the level of glucose levels in the blood over the previous three months.

About the fasting plasma glucose test

The fasting plasma glucose test reflects the glucose level when the blood sample is obtained. The blood sample is collected in the morning after an overnight fast. A change in diet, or stress or the presence of an illness on the day of the test could affect the reading.

Advantages of the HbA1c test over the fasting plasma glucose test

The HbA1c test has some advantages over the fasting plasma glucose test.

  • It does not require fasting before the test and may therefore be convenient to the patient
  • It can be done any time during the day, including during a routine visit to the clinic
  • The result is less likely to be affected by short-term changes in diet, stress or an illness, and may therefore give reliable results in these conditions as well

Conditions where the fasting plasma glucose level may be preferred over the HbA1c test

The HbA1c result may not be accurate in some conditions affecting the blood, which include the following:

  • Conditions that affect the life span of the red blood cells, for example, acute blood loss
  • Conditions that affect the hemoglobin such as sickle cell anemia and thalassemia
  • Iron deficiency, which may alter the structure of the hemoglobin molecule

Other conditions that may affect the HbA1c value include:

  • High urea, bilirubin or triglyceride levels
  • Chronic alcoholism
  • Ethnicity. The HbA1c result may be higher in African Americans, Hispanics, Asians as compared to Caucasians
  • Chronic Intake of medications such as salicylates and vitamin C
  • Patients with HIV infection

Test to Confirm Type 2 Diabetes

According to the American Diabetes Association, either of the above tests (or a 2-hour plasma glucose during 75-g oral glucose tolerance test) can be used to check for type 2 diabetes. If the test is abnormal, the same test or the alternate test should be repeated. If both the results are abnormal, a diagnosis of type 2 diabetes is confirmed. If one test is normal while the other is abnormal, the abnormal test should be repeated before the diagnosis of diabetes can be made.


Herman WH, Pajans SS. Hemoglobin A1c for the diagnosis of diabetes. https://www.mp.pl/paim/en/node/877/pdf

American Diabetes Association. 2. Classification and diagnosis of diabetes: Standards of Medical Care in Diabetes—2021. Diabetes Care 2021;44(Suppl. 1):S15−S33

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