What is the Difference Between FBS and HbA1c?

Diabetic patients or individuals prone to diabetes need to monitor their blood sugar levels at regular intervals. They will have to focus on changing their lifestyles and go with the proper medications prescribed by their physicians. In the same aspect, they will also have to learn the meaning of the various blood sugar tests done.

In this context, two main blood sugar tests are conducted by clinicians. These two tests are:

  • Fasting blood sugar (FBS) test
  • Glycosylated haemoglobin (HbA1c) test

Let us discuss what these tests are and what their differences are.

Fasting blood sugar test: An introduction

As the name suggests, this is a blood test conducted by drawing a blood sample from an individual to check the blood sugar level during a fasting period that ranges up to 12 hours. This is why the individual is asked not to eat something right after waking up before the sample is drawn. The sample is generally drawn from a vein in the upper extremity. It can be measured in serum, whole blood or from the plasma extracted from the blood sample.

This test is conducted to check the level of blood sugar present in the bloodstream. The result is then compared to the normal range to infer. According to the standards, a blood sugar level of less than 100 mg/dl is considered to be normal. When it is 100-125, it is considered to be pre-diabetic. When the FBS test result is higher than 125, it is called diabetic.

Glycosylated haemoglobin test: An introduction

It is a blood test where the percentage of haemoglobin that has gone through glycosylation is counted. Haemoglobin, in the long run, combines with free glucose molecules in the blood and forms glycosylated haemoglobin. It is also called HbA1c. The test conducted to measure it is called the HbA1c test.

No fasting is required to calculate the percentage of this combined haemoglobin in blood as it takes 3 months to form. Eating will not affect its level immediately. Its normal level is considered to be 5.7%. Less than 5.7% is considered to be normal. A result between 5.7% and 6.4% is considered to be pre-diabetic. Any value above 6.5% is considered to be severely diabetic.

Differences between FBS and HbA1c test

Now that we have been introduced to these two important blood sugar tests, let us find out the basic differences for proper knowledge.

  • Sample withdrawal time

The FBS blood sample is taken before the patient or individual has consumed anything. It is done to check the blood sugar level within 10-12 hours of a meal.

On the other hand, the HbA1c level is not dependent on whether the individual has consumed something or not as haemoglobin takes a few months to form HbA1c.

  • Measuring unit

FBS is measured in milligrams per decilitres (mg/dl).

HbA1c is measured in percentage. It depicts the percentage of this pigment of the red blood cells that have combined with blood sugar.

  • Testing intervals

An FBS test can be done frequently within a gap of a few weeks or a month.

The HbA1c test is generally done at an interval of 3-4 months as recommended by medical experts.

  • Diet

The test results for FBS can alter with the diet. A high-carbohydrate diet can alter the results. The same individual will have a different count when tested within a few days due to the differences in the meals consumed on previous nights.

The percentage of HbA1c will not alter as it takes at least 90 days to form. A change in the meal in a few days will not affect the value. Hence, the result of an HbA1c test will not depend on a short-term diet.

Similarities of HbA1c and FBS test

  • Both of these tests are conducted by drawing a blood sample.
  • Both of them are done to check whether a patient is prediabetic or diabetic or normal.
  • The higher levels indicated in the results conclude that a patient is diabetic or hyperglycaemic.
  • The lower levels of these tests indicate that a patient is hypoglycaemic.
  • Both these tests are conducted at regular intervals to check the fluctuations in blood sugar levels over a period of time, especially in chronic cases.


Hence, the above discussion clearly suggests what these two blood sugar tests are. The procedure of drawing blood is the same. In fact, the sample drawn for FBS can be used for calculating the level of HbA1c but not vice versa. It is because of the timing an individual and the clinician have to follow for drawing an FBS blood sample.

Both tests are often prescribed by doctors to determine whether a patient is diabetic or not. The result of an FBS test may vary due to the food consumed the previous night. To make sure that the patient is diabetic or non-diabetic, a parameter of HbA1 is added to the test profile.  As the HbA1c forms in 90 days, it can easily determine whether a patient is non-diabetic or not. This is all about the tests, their differences and similarities you should know.


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