Fasting Blood Glucose Test: What You Need to Know Before, During, and After

What is this test? A fasting blood glucose test measures the amount of sugar (glucose) in a sample of your blood after you have not eaten for at least 8 hours. Glucose is your body's main source of energy, and this test shows how well your body is managing the amount of sugar in your bloodstream.

The 17-Hydroxyprogesterone (17-OHP) challenge test measures the level of 17-hydroxyprogesterone in your blood before and after stimulation with adrenocorticotropic hormone (ACTH). ACTH is a hormone produced by the pituitary gland that tells your adrenal glands to make cortisol and other hormones. By measuring 17-OHP levels 10 minutes after ACTH administration, doctors can assess how well your adrenal glands are functioning and detect certain hormonal disorders.

Why is this test performed? Doctors use this test to screen for diabetes or prediabetes, to monitor blood sugar control if you have already been diagnosed with diabetes, and to help investigate symptoms such as increased thirst, frequent urination, unexplained fatigue, or blurred vision.

This test is primarily used to: (1) Diagnose congenital adrenal hyperplasia (CAH), a genetic condition affecting adrenal hormone production, (2) Evaluate adrenal gland function, (3) Distinguish between different types of adrenal disorders, and (4) Monitor treatment effectiveness in patients with adrenal conditions. It is particularly useful for detecting 21-hydroxylase deficiency, the most common form of CAH.

How to prepare for the test: You will need to fast for at least 8 hours before the test, which usually means having nothing to eat or drink except water after midnight. Drinking plain water is allowed and encouraged. Keep taking your regular medications unless your doctor gives you different instructions, and tell your care team about any supplements you use.

Your doctor will provide specific instructions. Generally, you may need to fast (not eat or drink) for 8-12 hours before the test. Inform your doctor about all medications you're taking, as certain drugs like corticosteroids, sex hormones, or anti-seizure medications may affect results. Tell your doctor if you have any known adrenal conditions or have taken steroid medications recently. Wear loose-fitting clothing to allow easy access to your arm for blood draws.

What happens during the test? A trained technician will clean a small spot on your arm, wrap a band around your upper arm, and draw a small sample of blood from a vein using a thin needle. The blood draw usually takes less than five minutes, and the sample is then sent to a laboratory for analysis. You can return to your normal activities and eat afterward.

The test typically takes about 30-60 minutes. First, a healthcare professional will draw a baseline blood sample from your arm. Then, you will receive an injection of synthetic ACTH (cosyntropin). After exactly 10 minutes, another blood sample will be drawn. The injection may cause temporary flushing, increased heart rate, or a warm sensation. These effects are normal and usually subside quickly.

Understanding your results: A normal fasting glucose level is typically between 70 and 99 mg/dL. A result of 100 to 125 mg/dL suggests prediabetes, while a reading of 126 mg/dL or higher on more than one test may indicate diabetes. Your doctor will interpret your numbers together with your medical history, so be sure to ask what your specific results mean for your health.

Your results will show 17-OHP levels in the baseline and post-stimulation samples. Normal responses vary by age and sex. Significantly elevated 17-OHP levels after ACTH stimulation typically indicate congenital adrenal hyperplasia. Mild to moderate elevations may suggest other adrenal issues. Your doctor will interpret your results in context with your symptoms, medical history, and other test findings.

Are there any risks? This test is very safe. You may feel a brief sting when the needle goes in, and mild bruising or soreness at the puncture site is common. Some people feel briefly lightheaded during or after a blood draw. Serious problems are extremely rare, but contact your doctor if you notice ongoing bleeding, worsening pain, or signs of infection where the needle was inserted.

The risks are minimal and similar to routine blood tests. You may experience slight pain, bruising, or bleeding at the injection site. Some people feel dizzy or faint during or after blood draws. The ACTH injection is generally safe, though rare allergic reactions are possible. Contact your healthcare provider if you experience severe pain, persistent bleeding, signs of infection at the injection site, or unusual symptoms after the test.