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Glucose Challenge Test

Interpreting Results
Glucose Challenge Test

Overview of GCT

What is GCT?

In the Glucose Challenge Test (GCT) - Obstetrical, blood glucose level is measured one hour after the administration of 50 gms of glucose solution. It is done to screen for and diagnose gestational diabetes (diabetes developing during pregnancy).

Why is GCT done?

The Glucose Challenge Test is performed to:

  • Screen for high blood sugar in presence of risk factors for diabetes in pregnancy
  • Detect gestational diabetes

What does GCT Measure?

Glucose Challenge Test (GCT) - Obstetrical measures blood glucose level one hour after the administration of 50 gms of glucose solution.

Glucose is a simple sugar or monosaccharide which is the main source of energy for all the cells of the body, and the only energy source for the nervous system. Carbohydrates consumed in the diet are broken down in the body to their simplest form, glucose. It is absorbed by the intestines and transported by the blood to various organs. Glucose is subsequently utilized by cells of these organs to produce energy wherever necessary, and the excess is stored either as glycogen in the liver for short-term storage or in fat tissues as triglycerides for long-term storage. The uptake, utilization, and storage of glucose after it is absorbed in the intestines are facilitated by the hormone, insulin which is secreted by the pancreas. Insulin influences the transport of glucose to the organs requiring it, like the heart, brain, working muscles, etc. It also directs storage of excess glucose. The action of insulin reduces sugar levels in the blood.

After every meal, sugar levels increase in blood and insulin is secreted as a response to reduce sugar levels until it becomes normal. If glucose levels fall too low in blood, another pancreatic hormone called Glucagon is released. Glucagon directs the liver to convert stored glycogen into glucose and release it into the blood. The insulin and glucagon hormones create a feedback mechanism to keep blood glucose levels within the normal range. Imbalance in their activity can cause an excess or shortage of blood sugar.

The glucose challenge test measures the levels of glucose in the blood, one hour after the administration of a sugar solution (usually 50 gm glucose dissolved in about 300 ml water). This helps to determine whether the body is able to utilize or store glucose efficiently. Excess sugar in blood indicates it is not being utilized or stored. It is usually performed as a part of the panel of tests performed during pregnancy and can be performed at any time during pregnancy. It is also necessary to repeat the test at 24 to 28 weeks to detect gestational diabetes.

Preparation for GCT

  • Overnight fasting is not required.
  • Intake of glucose solution (50 gms of glucose dissolved in 300 ml of water) is required one hour before the blood sample collection.

Sample Type for GCT

The sample type collected for Glucose Challenge Test is: Blood

Interpreting GCT results


Normal range: Less than 130 to 140 mg/dL (7.2 to 7.8 mmol/L)

Gestational diabetes:

A cut-off value of 140 mg/dL (7.8 mmol/L) identifies 80%  of women with Gestational diabetes.

A cut-off value of 130 mg/dl (7.2 mmol/L) identifies 90%  of women with Gestational diabetes.

Blood sugar levels higher than the normal range indicates increased blood sugar due to the inability of the body to process it.

GCT is a screening test for Gestational diabetes and continuously raised blood sugar levels in subsequent tests indicate a strong possibility of gestational diabetes. Further tests like 2-hour or 3-hour Glucose Tolerance Tests (GTT) are performed to diagnose Gestational diabetes.

Answers to Patient Concerns & Frequently Asked Questions (FAQs) about GCT

Frequently Asked Questions about Glucose Challenge Test

Q. Is there any risk associated with this test?
There is no risk associated with the test. However, since this test involves a needle prick to withdraw the blood sample, in very rare cases, a patient may experience increased bleeding, hematoma formation (blood collection under the skin), bruising or infection at the site of needle prick.
Q. Is there any preparation required before the test?
Inform the doctor about the medications you may be taking. No other specific preparations are usually required before this test.
Q. How is GCT performed?
This test is performed on a blood sample. A syringe with a fine needle is used to withdraw blood from a blood vessel in your arm. The healthcare provider will tie an elastic band around your arm to make the blood vessels swell with blood and may ask you to make a tight fist. Then he will clean the elbow area over the blood vessel with an antiseptic solution. After that, he will insert the needle into your blood vessel. You will feel a tiny pinprick during the procedure. Blood sample once collected will then be sent to the laboratory. For the GCT that can be performed at any time of day and does not require fasting, you consume a sugar solution of a known concentration (usually 50 gm glucose dissolved in about 300 ml water), and your blood is drawn to measure glucose levels after one hour.
Q. What additional tests can be prescribed by your doctor in case GCT result is not normal?
Additional tests that may be prescribed in case of abnormal Glucose Challenge test result are: · HbA1c Test · 2 Hour and 3 Hour Glucose tolerance Tests · Postprandial Glucose Test · Urine Glucose Test · Lipid Profile Test
Q. What is Gestational Diabetes?
Gestational Diabetes is a condition where blood sugar levels in a non-diabetic woman rise to higher than normal levels, during pregnancy. This is caused primarily by hormones secreted during pregnancy which cause insulin resistance by inhibiting cellular signaling mechanisms, thereby preventing cells from effectively taking up and utilizing glucose from blood, leading to rise in blood glucose levels. The condition is usually resolved and blood sugar levels return to normal after the delivery of the child . However, in some cases, gestational diabetes carries a high risk of developing Type 2 Diabetes in the woman. Gestational diabetes also carries a risk of complications during pregnancy and childbirth if blood sugar levels are not managed correctly.
Q. How can Gestational Diabetes affect the baby?
Gestational Diabetes can have a pronounced negative impact on the health of the newborn. Some conditions that may affect the newborn if the mother suffers from unmanaged gestational diabetes include: · Persistent hypoglycemia (low blood sugar levels) in the newborn · Hypocalcemia (low blood calcium levels) · Hypomagnesemia (low blood magnesium levels) · Increased birth weight · Jaundice · Polycythemia (high red blood cell mass) · Incomplete development of respiratory system · Birth defects
Q. What are the risk factors for Gestational Diabetes?
The risk factors for Gestational diabetes are: · Being overweight or obese before pregnancy together with low physical activity · Having a blood relative already diagnosed with Diabetes · Having been diagnosed with gestational Diabetes previously or having delivered a baby weighing more than 4kg · Suffering from Polycystic Ovarian Syndrome (PCOS) · Being diagnosed with hypertension · Having delivered a stillborn child or one with birth defects · Being diagnosed with Prediabetes before pregnancy · Belonging to a high risk ethnic group like Native American, Pacific Islander, African origin, or South Asian origin
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