The Fasting Blood Sugar (FBS) test measures the blood glucose/sugar levels in a blood sample following a fasting period of 8 hours . The FBS is a fundamental test for the diagnosis and monitoring of prediabetes and diabetes. Diabetes is a condition associated with consistently elevated blood sugar levels due to the insufficient production or ineffective use of insulin hormone by the body. Prediabetes is a stage before diabetes, also known as borderline diabetes, where the blood sugar levels are slightly elevated but not high enough to be diagnosed as diabetes. The test is also done to check for gestational diabetes – diabetes that occurs in pregnant women due to the effect of pregnancy hormones.

The FBS test provides a precise evaluation of how effectively the body regulates blood glucose levels in the absence of recent food intake. The test helps to evaluate and manage impaired glucose metabolism and diabetes in a person.

Preparing for an FBS test requires adherence to specific guidelines to ensure accurate results. They are as stated below:

Prerequisites

It is imperative to not consume any food or drink, except water, for a minimum of 8-12 hours before the FBS test. Since food affects sugar levels, testing in a fasting state gives a more accurate picture of blood sugar levels.

The best time to get tested

The FBS test is usually scheduled in the morning before breakfast. This timing ensures that the individual has undergone overnight fasting for accurate test results.

Who can do the FBS test

Individuals with symptoms of diabetes (such as increased thirst, frequent urination, unexplained weight loss, and increased hunger) or who have risk factors for diabetes such as obesity or a family history of diabetes are eligible for the FBS test. Besides being a screening test to check for diabetes, the FBS test is done along with Postprandial Blood Sugar (PPBS) to monitor treatment efficacy in individuals with diabetes. The test is also done as part of routine health assessments to monitor blood glucose levels and to check for gestational diabetes in pregnant women.

Cautions before taking the FBS test

Before undergoing the FBS test, it is essential to inform your healthcare provider about any medications you have been taking. Certain medications can potentially influence blood sugar levels, thereby affecting the accuracy of the test results.

An FBS test is recommended in the following instances:

• Routine Monitoring

The FBS test is ordered for routine monitoring of blood glucose levels, often during annual physical examinations, to track changes in blood glucose levels over time.

• Screening During Pregnancy

FBS testing is done between the 24th and 28th weeks of pregnancy to screen for gestational diabetes as a part of a Glucose Tolerance test to ensure that pregnancy hormones are not adversely affecting blood sugar levels.

• Symptom Assessment and Risk Factor Evaluation

Your healthcare provider may order the FBS test if you have symptoms of diabetes, such as increased thirst, frequent urination, blurred vision, fatigue, unexplained weight loss, numbness/tingling in the hands and feet, or sores that don’t heal. The test may also be ordered if you have risk factors for diabetes, such as obesity, high blood pressure, age 45 years or older, sedentary lifestyle, history of stroke or heart disease, or a history of gestational diabetes.

• Monitoring of Prediabetes

An FBS test may be ordered to monitor your sugar levels if you have previously had a glucose level that was higher than normal or are prediabetic. Regular FBS testing may be done to check if the recommended lifestyle changes reverse prediabetes and prevent diabetes.

• Monitoring in Individuals with Diabetes

Regular testing for FBS is essential to track blood sugar levels in those with diabetes. This helps assess treatment efficacy and adjust medication doses if needed.

• Diagnosing Conditions Other Than Diabetes That Cause Low Glucose Levels

The FBS test aids in identifying conditions other than diabetes that can cause hypoglycaemia (low blood sugar levels). These include liver diseases like viral hepatitis, liver cancer or cirrhosis; chronic kidney disease; and hormonal disorders such as reduced hormone production by the pituitary and adrenal glands. Diabetes medications and prolonged fasting can also cause low glucose levels.

• Diagnosing Conditions Other Than Diabetes That Can Cause High Blood Glucose Levels

The test can be used to diagnose conditions other than diabetes that can cause elevated blood glucose levels, like acromegaly, Cushing syndrome, hyperthyroidism, and pancreatic disorders.

The results of the FBS test can be interpreted as per the table below:

Interpretation of Abnormal Results

Low blood glucose levels, when blood sugar levels fall below 70mg/dL, may be due to an overdose of insulin, insulinomas (tumours in the pancreas that produce insulin), starvation, adrenal insufficiency, (underactive adrenal glands), drinking excessive alcohol, severe liver disease, hypopituitarism (underactive pituitary function), hypothyroidism (low thyroid function), and severe infections.

High blood glucose levels most frequently indicate diabetes, but many other diseases and conditions can also cause elevated blood glucose, such as acromegaly (excess growth hormone), acute stress (response to trauma, heart attack, and stroke for instance), Cushing syndrome (excess cortisol hormone), hyperthyroidism (increased thyroid function), pancreatic cancer, and pancreatitis (inflammation of pancreas).