Blood tests are a standard part of routine and preventive healthclick. A doctor will often order a blood test before or following a physical examination. A doctor may also order blood tests to evaluate specific conditions.

Many blood tests don't require any special preparations. For some, you may need to fast (not eat any food) for 8 to 12 hours before the test. Your doctor will let you know how to prepare for blood tests.

During a blood test, a small sample of blood is taken from your body. It's usually drawn from a vein in your arm using a needle. A finger prick also might be used.

Complete blood count

A complete blood count (CBC) measures a variety of the blood’s components, such as:

  1. red blood cells
  2. white blood cells
  3. hemoglobin
  4. platelets
  5. mean corpuscular volume (MCV) — the average size of a person’s red blood cells
  6. hematocrit — how much space red blood cells take up in the blood