Glucose Fasting

Also known as: Fasting Blood Glucose (FBG), Blood Sugar, Fasting Blood Sugar (FBS), Fasting Plasma Glucose (FPG)

Testing Basics

Why do you need to get tested?

This test is ordered by the doctor to screen his or her patient for diabetes. The test also aids in the diagnosis of the disease. It is also used to detect high blood glucose, which is also known as hyperglycemia, and low blood glucose, which is also known as hypoglycemia. The test is also helpful in the evaluation of blood glucose levels over time to identify whether the treatment has been fruitful in controlling diabetes or not.

When do you need to get tested?

The test is ordered by the doctor when the patient has risk factors for diabetes. According to the American Diabetes Association, anyone who is 45 years old or older must get tested for the disease. The test is also ordered if the patient has signs and symptoms of high or low blood glucose. On the other hand, if a patient has diabetes, he or she may need to check his or her glucose level multiple times throughout the day.

Which kind of sample is required?

Screening and diagnosis require a different type of sample in comparison to the monitoring. For screening and diagnosis, a blood sample is drawn from a vein in the arm. It must also be noted that blood samples from finger sticks are not suitable for diagnosing diabetes. On the other hand, a drop of blood from a fingerstick is tested by a glucometer (a home glucose monitor). In some cases, people with diabetes may need to use a continuous glucose monitoring device. The device involves a small sensor wire being inserted beneath the skin to measure blood glucose at regular intervals.

Do you need to prepare for the test?

For screening and diagnosis, patients are recommended to fast for at least 8 hours or overnight before the fasting blood draw. Fasting means that you cannot eat anything, nor can you drink anything except water. In some cases, testing is done randomly when the patient has not fasted, for example, during a general health exam as part of routine screening. People who have diabetes need to check their blood sugar levels at regular intervals while fasting and after meals to ensure that diabetes is being managed properly. The doctor often sets instructions for timed, random, and post-meal glucose tests, which must be followed by the patient diligently and faithfully.

What is being tested in Glucose, Fasting Blood (FBG)?

This test is used to measure the level of glucose in the blood and chart a treatment plan accordingly. Glucose is also known as blood sugar. It is a type of sugar found in the blood. Glucose is the main source of energy for the cells in the body, providing life to the various processes that take place to keep the body functional. It also functions as a short-term energy source for the brain and the nervous system.

Glucose must be present in the blood in a steady supply. Blood sugar levels must also be maintained at a constant rate. When food is being digested by the body, the carbohydrates you have had – such as pasta, rice, bread, tortillas – are broken down primarily into glucose and other nutrients in the body. These by-products are absorbed by the digestive tract, diffusing into the blood, and circulating throughout the body. After we have had a meal, the levels of glucose in the blood rise slightly.

In response to this, insulin, a hormone made by the pancreas, is released into the blood, aiding in the transport of glucose into the body’s cells, where it is used as a source of energy. The amount of insulin released by the pancreas is contingent on the size and content of the meal. When glucose moves into the cells and is broken down, blood sugar levels drop, and the pancreas ceases insulin response. When the glucose-insulin feedback system functions as normal, the glucose levels in the blood remain stable and steady. However, if the feedback system malfunctions, the amount of glucose found in the blood rises above normal.

In this case, the body responds by attempting to store balance through the release of more insulin. Diabetes is a condition that results from this imbalance. There are several types of diabetes. Type 1 diabetes occurs because the body is unable to produce an adequate amount of insulin to control blood sugar levels. In type 1 diabetes, beta cells – responsible for the production of insulin – have been damaged and destroyed by the immune system.

On the other hand, in type 2 diabetes, the root cause is a combination of insulin resistance whereby the body does not respond properly to insulin and a decline in the production of insulin on a relative scale. Sometimes women develop a third type of diabetes known as gestational diabetes. This condition involves the development of high blood glucose in pregnancy. Serious changes in blood glucose levels, very high or very low, can be fatal, causing organ failure, brain damage, coma, and sometimes death. If a condition of hyperglycemia persists or diabetes goes untreated or is poorly controlled, organs are damaged progressively.

How is the test used?

The fasting blood draw is used to screen for and diagnose prediabetes in people who have signs and symptoms. Sometimes there are no visible signs or symptoms of the disease, and a fasting blood draw is used to screen and help determine the state of affairs. Screening is useful in administering treatment promptly before the condition derails and leads to severe complications. The fasting blood draw may also be used in conjunction with other tests like the hemoglobin A1c (HbA1c) to confirm the diagnosis of diabetes.

When is the test ordered?

The test is ordered when an individual is 45 or older or if a patient is at risk for the disease. People that may be at risk include those who:

Have a history of cardiovascular disease
Are overweight, obese, or physically inactive
Have a close relative with diabetes, such as a parent, sibling, aunt, or uncle
Delivered a baby weighing more than 9 pounds
Have a history of gestational diabetes
Have polycystic ovary syndrome
Belong to a high-risk race or ethnicity, such as Latino, Black, Asian or, Pacific Islander
Have high blood pressure (hypertension) (≥140/90 mmHg) or are taking medication for high blood pressure
Have a low HDL cholesterol level (less than 35 mg/dL or 0.90 mmol/L) and/or a high triglyceride level
Have prediabetes identified by previous testing
Have a history of cardiovascular disease (CVD)

The test may also be ordered when the patient shows signs and symptoms of abnormal blood sugar levels. Symptoms of hyperglycemia include:

Fatigue
Blurred vision
Increased thirst coupled with frequent urination
Wounds and infections which heal slowly

Symptoms of hypoglycemia include:

Hunger
Anxiety
Trembling
Blurred vision
Confusion
Sweating

What does the test result mean?

High blood sugar levels commonly indicate diabetes but may also be linked to other conditions which lead to elevated glucose in the blood. These include:

Acromegaly
Acute stress (response to trauma, heart attack, and stroke, for instance)
Chronic kidney disease
Cushing syndrome
Overactive thyroid (hyperthyroidism)
Pancreatic cancer
Pancreatitis

On the other hand, hypoglycemia or low blood sugar first impacts the nervous system leading to sweating, palpitations, hunger anxiety, and then impacts the brain, causing confusion, blurred vision, even leading to coma and death. Hypoglycemia is also linked to conditions like

Adrenal insufficiency
Drinking excessive alcohol
Severe liver disease
Hypopituitarism
An underactive thyroid (hypothyroidism)
Severe infections
Severe heart failure
Chronic kidney (renal) failure
Insulin overdose
Tumors that produce insulin (insulinomas)
Starvation
The deliberate use of glucose-lowering products

Related Tests: Hemoglobin A1c, Continuous Glucose Monitoring, Glucose Tolerance Test, Glucose Tests for Gestational Diabetes, Urinalysis, Insulin, C-peptide, Islet Autoantibodies in Diabetes, Urine Albumin and Albumin to Creatinine Ratio, Comprehensive Metabolic Panel test (CMP), Basic Metabolic Panel blood test (BMP), Fructosamine

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