MCV or Mean corpuscular volume is an estimate of the average size of your red blood cells (RBCs). Red blood cells carry oxygen from lungs to each cell of your body. In MCV test, your doctor wants to check size of your RBCs. If they are too small or too large, it may mean that you have some disorder. MCV blood test is one of the components measured in the main test called the complete blood count (CBC) test. The CBC test is often done as a routine screening test or when prescribed by your physician to diagnose or monitor certain blood disorders. This test measures many different components of your blood, including RBC count.