Complete Blood Count (CBC): What It Is, How It Is Done, and What Your Results Mean

A complete blood count, or CBC, is one of the most common blood tests. It measures the different cells that make up your blood, including red blood cells, white blood cells, and platelets, and helps your healthcare team evaluate your overall health.

The 17-Hydroxyprogesterone (17-OHP) After ACTH Stimulation Test is a specialized blood test that measures the level of 17-hydroxyprogesterone in your blood before and after an injection of adrenocorticotropic hormone (ACTH). ACTH is a hormone produced by the pituitary gland that tells your adrenal glands to produce cortisol and other steroid hormones. By measuring 17-OHP levels before and after ACTH stimulation, doctors can evaluate how well your adrenal glands are functioning and detect certain disorders.

The test is performed by drawing a small sample of blood, usually from a vein on the inside of your elbow or the back of your hand. A healthcare professional cleans the site, inserts a thin needle, collects the blood into a tube, and sends the sample to a laboratory for analysis. The procedure takes only a few minutes.

The test involves several steps: First, a healthcare professional will insert an intravenous (IV) line or draw blood from your arm to collect a baseline sample (before ACTH). Then, synthetic ACTH (cosyntropin) is injected into your muscle or vein. After 20-30 minutes, another blood sample is collected. The entire procedure usually takes about an hour. You may be asked to fast overnight and avoid certain medications before the test, as instructed by your healthcare provider.

You might need this test as part of a routine checkup, to help find the cause of symptoms such as fatigue, weakness, fever, or unusual bruising and bleeding, or to monitor a condition you already have, such as anemia, an infection, or a blood disorder, and to see how well your treatment is working.

Your doctor may order this test if you or your child have symptoms suggesting a adrenal gland problem, such as early puberty, irregular periods, excessive hair growth, or signs of hormonal imbalance. It is commonly used to diagnose and monitor congenital adrenal hyperplasia (CAH), a genetic condition affecting adrenal hormone production. The test can also help distinguish between adrenal and gonadal sources of hormonal abnormalities and assess adrenal insufficiency.

Your results show whether your blood cell counts fall within the normal reference ranges. Values that are too high or too low may point to conditions such as anemia, infection, inflammation, clotting problems, or certain blood disorders, but abnormal numbers alone do not confirm a diagnosis. Your doctor will interpret the results together with your symptoms, medical history, and any other tests you have had.

Results are interpreted by comparing the baseline (before ACTH) and stimulated (after ACTH) 17-OHP levels. Significantly elevated 17-OHP levels after ACTH stimulation typically indicate congenital adrenal hyperplasia (CAH). Mild to moderate elevations may suggest other forms of adrenal dysfunction. Normal responses indicate healthy adrenal gland function. Your healthcare provider will explain your specific results in the context of your symptoms, medical history, and other test findings.