Complete Blood Count (CBC) Test: What You Need to Know

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. These cells carry oxygen, fight infection, and help your blood clot, so this test gives your care team a broad picture of your overall health.

The 17-Hydroxyprogesterone (17-OHP) test measures the amount of a steroid hormone called 17-hydroxyprogesterone in your blood. This hormone is produced by the adrenal glands and is a precursor to cortisol, an important stress hormone. The test is specifically performed on the first blood sample collected after receiving an injection of gonadotropin releasing hormone (GnRH), which stimulates the pituitary gland to release hormones that then affect the adrenal glands.

Your doctor may order a CBC as part of a routine checkup, to look into symptoms such as fatigue, weakness, fever, bruising, or unusual bleeding, to help diagnose conditions like anemia or infection, or to monitor how a treatment or medication is affecting your blood cells over time.

Your doctor may order this test to: (1) Screen for and diagnose congenital adrenal hyperplasia (CAH), a genetic condition affecting adrenal hormone production; (2) Evaluate adrenal gland function; (3) Help distinguish between different types of adrenal disorders; (4) Monitor treatment effectiveness for adrenal conditions.

Most people do not need any special preparation for a CBC. You can usually eat and drink normally and take your regular medications unless your doctor tells you otherwise. If the CBC is being done together with other blood tests that require fasting, you will be given specific instructions ahead of time.

Follow these instructions: (1) Fasting may be required - follow your doctor's instructions; (2) Inform your doctor about all medications you're taking, especially steroid medications; (3) The test involves an injection followed by blood draws at specific time points, so plan for multiple blood collections; (4) Tell your doctor if you have any bleeding disorders or are taking blood thinners.

During the test, a healthcare professional will wrap a band around your upper arm, clean the skin on the inside of your elbow, and insert a small needle into a vein to draw a sample of blood into a tube. The draw usually takes only a few minutes, and the sample is then sent to a laboratory for analysis.

The test typically involves: (1) A GnRH injection will be given by a healthcare professional; (2) Blood samples will be collected at specific intervals after the injection, usually 15, 30, 60, and 120 minutes; (3) Each blood draw takes just a few minutes; (4) You may experience mild discomfort from the needle injection.

A CBC is a very safe test. The most common effects are brief discomfort when the needle goes in and mild soreness or a small bruise at the puncture site afterward. Serious problems, such as heavy bleeding, infection, or feeling lightheaded, are rare, but tell your care team right away if you feel unwell.

The risks are minimal and include: (1) Minor bruising or soreness at the injection site; (2) Rare risk of infection; (3) Lightheadedness during or after blood draw; (4) Multiple needle sticks may cause more discomfort.