Complete Blood Count (CBC) Test: What It Is, Why It's Done, and What Your Results Mean

What is this test? A complete blood count, or CBC, is a common blood test that measures the different cells in your blood, including red blood cells, white blood cells, and platelets, giving your care team a broad picture of your overall health.

The Corticotropin (ACTH) Stimulation Test with Glucagon measures the level of adrenocorticotropic hormone (ACTH) in your blood 2 hours after receiving an injection of glucagon. ACTH is a hormone produced by the pituitary gland that tells your adrenal glands to release cortisol, which helps your body respond to stress and maintain blood pressure, blood sugar, and immune function.

Why is it performed? Doctors order a CBC to check for a wide range of conditions such as anemia, infection, and blood disorders, to investigate symptoms like fatigue, weakness, fever, or unusual bruising, and to monitor an existing condition or how well a treatment is working.

This test helps doctors evaluate how well your pituitary and adrenal glands are working. It is used to diagnose conditions such as Addison's disease (adrenal insufficiency), Cushing's syndrome (excess cortisol), hypopituitarism (pituitary gland dysfunction), and other disorders affecting hormone production. The glucagon injection stimulates the pituitary gland to release ACTH, and this test measures whether that response is normal.

How to prepare: In most cases, no special preparation is needed and you can eat and drink normally before your appointment. If the CBC is ordered together with other tests, such as a fasting glucose or lipid panel, you may be asked not to eat for several hours beforehand. Tell your care team about any medicines or supplements you take.

Your doctor will provide specific instructions. Generally, you may need to fast overnight before the test. Certain medications, especially steroids, hormone therapies, and some diabetes medications, may need to be paused. Tell your doctor about all medications and supplements you are taking. The test is usually performed in the morning because hormone levels follow a daily rhythm.

What happens during the test: A nurse or phlebotomist will wrap a band around your upper arm, clean the skin at the inside of your elbow, and draw a small sample of blood into a tube using a thin needle. The draw usually takes only a few minutes, and you may feel a quick pinch. Afterward, brief pressure and a small bandage are applied to the site.

A healthcare provider will first draw a baseline blood sample. Then, glucagon will be injected either under the skin or into a muscle. After exactly 2 hours, another blood sample will be drawn to measure your ACTH levels. You may experience temporary side effects from glucagon such as nausea, vomiting, flushing, or dizziness.

Understanding your results: Your report will list each component of your blood count next to a reference range, and values outside that range will be flagged. High or low results can point to conditions such as anemia, infection, inflammation, or clotting problems, but a single abnormal value does not confirm a diagnosis. Your provider will interpret the numbers together with your symptoms and history, so review the report with them before drawing conclusions.

Your results will show your ACTH level 2 hours after glucagon administration. Abnormal results may indicate problems with your pituitary gland, adrenal glands, or hypothalamus. Your doctor will interpret your results alongside other tests and your clinical symptoms to make a diagnosis. Reference ranges vary by laboratory.