What is the Difference Between FBS and HbA1c

The key difference between FBS and HbA1c is that FBS analyzes free glucose in blood while HbA1C test analyzes the glycosylated hemoglobin level in blood.

Diabetes is a condition primarily characterized by elevated blood glucose levels. It occurs either due to the absence of insulin in the system or due to insulin resistance. It is important that the condition is diagnosed early and is monitored throughout to avoid complications. Both blood tests, FBS and HbA1c, play an important role in the managing process of diabetes.


1. Overview and Key Difference
2. What is FBS  
3. What is HbA1c
4. Similarities – FBS and HbA1c
5. FBS vs HbA1c in Tabular Form
6. Summary – FBS vs HbA1c

What is FBS (Fasting Blood Sugar)?

FBS refers to Fasting blood sugar or fasting glucose. It is a blood test that involves measuring blood sugar levels. In this test, the blood glucose level is measured after a period of fasting for 12 hours. This is the most common test done to detect elevated blood sugar levels, which is the most common and immediate symptom of diabetes and related disorders like obesity and metabolic syndrome. A value less than 100 mg/dL suggests a normal blood sugar level, while a value between 100 – 125 mg/dL suggests a prediabetic condition. Ideally, a test value above 125 mg/dL suggests a severe diabetic condition.

Figure 01: Blood Glucose Testing

The test primarily involves drawing a venous blood sample and testing it for blood glucose levels based on the theory of glucose oxidase. FBS levels can be measured in whole blood, serum, or plasma. However, the value might vary based on the site of measurement.

What is HbA1c (Glycosylated Hemoglobin or Hemoglobin A1C)?

HbA1c, also known as the glycosylated hemoglobin or hemoglobin A1C, is another test done to detect glycosylated hemoglobin in the blood. This test does not require fasting and provides an overview of the average blood sugar level of a person over a period of 90 days. Therefore, the measurement of HbA1c is considered to be a more reliable test to assess the trend and severity of diabetic conditions of individuals. This test is not greatly affected by the meal consumed just prior to the test, unlike in the FBS test.

The basis of the technique relies on the glycosylation of hemoglobin that is increased in the scenario where an individual is going to have an elevated level of blood glucose. A value less than 5.7% resulting from the test determines a healthy individual, while a value between 5.7 – 6.4% indicates a high risk towards the development of diabetes. Hence, this stage is also referred to as prediabetic. A value beyond 6.5% indicates a strong positive indication of severely diabetic individuals.

What are the Similarities Between FBS and HbA1c?

  • FBS and HbA1c are blood tests conducted on venous blood.
  • Both provide indications of diabetes and related disorders.
  • High levels of both tests suggest hyperglycemic conditions.
  • Moreover, low levels of both tests suggest a hypoglycemic condition.
  • Both tests are important to assess the fluctuations of blood glucose among chronic cases.

What is the Difference Between FBS and HbA1c?

FBS measures the free-glucose level in the blood, while HbA1C measures the glycosylated hemoglobin levels in the blood. So, this is the key difference between FBS and HbA1c. Furthermore, while the FBS test requires a period of fasting as a preparatory step, HbA1c test does not require any preparatory step. Thus, this is another difference between FBS and HbA1c. Moreover, the results in the FBS test are most often given in mg/dL, while HbA1C level is given as a percentage.

The below infographic presents the differences between FBS and HbA1c in tabular form for side by side comparison.

Summary – FBS vs HbA1c

Fasting blood sugar (FBS) and measurement of glycosylated hemoglobin (HbA1c) are the two most common test procedures done in the diagnosis and management of diabetes. While FBS test measures the free glucose in the blood, HbA1c measures the amount of glycosylated hemoglobin in the blood. This is the key difference between FBS and HbA1c. In addition, while FBS test requires a period of fasting for 12 hours, the HbA1C test does not require any pre-fasting preparations. Both tests have indicative levels for healthy individuals, prediabetic, and diabetic individuals. Diet, genetics, physical behaviour, and mental health play an important role in maintaining the FBS and HbA1c levels in healthy individuals.