CBC Blood Test: What It Measures, RBC Count, Hemoglobin, and MCV

A CBC blood test (complete blood count) is a group of tests that measures the number and size of the cells in your blood, including red blood cells, white blood cells, platelets, hemoglobin, hematocrit, and mean corpuscular volume (MCV).¹ It is one of the most common blood tests, often done at a routine checkup, and it is frequently the first place a clue about anemia or a vitamin problem shows up.¹² This page explains each part of the CBC, why it is ordered, how to read results, and why large red blood cells lead clinicians to check folate and vitamin B12.

What does a CBC blood test measure?

A complete blood count is also called a full blood count or blood cell count.¹ It reports several separate measurements from one blood sample.

There are five major types of white blood cells. A standard CBC reports the total white cell count, and a separate test called a CBC with differential counts each type.¹

Why is a CBC ordered?

A CBC is often part of a routine checkup.¹ It is also used to:

• Help diagnose blood diseases, infections, immune system disorders, and other conditions ¹
• Monitor a condition or treatment that can affect blood counts, such as anemia, immune system disorders, or blood cancers ¹
• Check for changes in an existing blood disorder ¹

Clinicians may also order one when you have symptoms that suggest a low red blood cell count, such as shortness of breath, weakness or fatigue, headache, dizziness, an irregular heartbeat, or pale skin.² Treatments that can affect blood counts, such as chemotherapy, and long-term conditions such as chronic kidney disease are other common reasons.²

What happens during a CBC, and how do you prepare?

A health care professional takes a blood sample from a vein in your arm with a small needle. The draw usually takes less than five minutes, and you may feel a brief sting.¹ Risks are minimal: some slight pain or bruising at the site that usually fades quickly.¹

Usually no special preparation is needed for a CBC. If other tests are being run on the same sample, you may need to fast for several hours, and the person who ordered the test will tell you.¹ If you are arranging testing yourself, our guide to ordering a blood test online explains how direct-to-consumer lab orders work.

RBC blood test: what a red blood cell count shows

The red blood cell (RBC) count measures how many red blood cells, also called erythrocytes, are in your blood.² Red blood cells are made in the bone marrow and contain hemoglobin, which carries oxygen to every cell.² An RBC count that is higher or lower than normal is often the first sign of an illness, which can allow treatment before symptoms appear.²

Living at a high altitude and misuse of certain drugs for athletic performance, such as anabolic steroids, can also raise the red cell count.² If your count is outside the range, follow-up tests may include a reticulocyte count (immature red blood cells), iron tests, or a vitamin B test.²

Hemoglobin and hematocrit

Hemoglobin and hematocrit are often reported together (sometimes called "H and H").³⁴ They describe how much oxygen-carrying capacity your blood has, in two different ways.

Hemoglobin

A hemoglobin test measures the iron-rich protein that carries oxygen in red blood cells.³ It is commonly used to check for anemia, a condition in which the body does not make enough healthy red blood cells.³ It is not the same as the hemoglobin A1C test, which checks average blood sugar.³

Low hemoglobin may be a sign of different types of anemia, thalassemia, iron deficiency, liver disease, cancer, blood loss (including heavy menstrual periods), or a lack of nutrients such as iron or vitamin B12.³ High hemoglobin may be linked to lung disease, heart disease, polycythemia vera, or sleep apnea.³ The test cannot tell you the cause of an abnormal level, but it helps show how severe a problem may be.³

Hematocrit

Hematocrit is reported as a percentage. A result of 42, for example, means 42% of your blood is red blood cells, and the rest is white blood cells, platelets, and plasma.⁴ Normal hematocrit varies with sex, age, smoking, and the altitude where you live.⁴

A low hematocrit can reflect blood loss, anemia, or certain bone marrow conditions and cancers.⁴ A high hematocrit can reflect the body making too many red blood cells or too little plasma, and dehydration is the most common cause of a high result.⁴ Pregnancy can lower hematocrit because the body holds more fluid than usual.⁴

MCV: what red blood cell size tells you

The MCV (mean corpuscular volume) is the average size of your red blood cells.⁵ It is especially useful when anemia is present, because different types of anemia change red cell size in different ways, and the MCV helps identify which type you may have.⁵

An MCV result alone cannot diagnose a disease. Your clinician combines it with your other results and your medical history.⁵ For the small-cell pattern and how iron stores are measured, see our page on the ferritin blood test.

High MCV, folate, and vitamin B12

Folate is needed to make DNA, and red blood cell production depends on it. When folate-dependent DNA formation is impaired, the result can be megaloblastic anemia, with large, immature red blood cells.⁶ Vitamin B12 and folate are both needed to make healthy red blood cells.⁷

That is why a high MCV (macrocytosis) sends clinicians to check both nutrients. Macrocytosis should prompt evaluation of folate and B12, not automatic folic acid treatment.⁸ The reason is safety: large amounts of folate can correct the anemia of vitamin B12 deficiency without treating the underlying nerve damage.⁹⁶

A typical follow-up workup reads the CBC alongside red cell indices, a blood smear, vitamin B12, a folate test, and your history of diet, alcohol use, medicines, and possible malabsorption.¹¹⁸ Our folate blood test guide explains serum and red blood cell folate, and folic acid and vitamin B12 explains how the two vitamins interact. If testing confirms low folate, folic acid deficiency covers causes and treatment.

Routine prevention is a separate question

A CBC result does not change the everyday prevention advice. CDC recommends 400 mcg of folic acid every day for everyone who could become pregnant, from supplements, fortified foods, or both.¹²¹³ That prevention dose is different from treating diagnosed anemia, which is a clinician's decision.¹⁰¹⁴

How to read your CBC results

Every value on a CBC report is compared with a reference range. That range is not universal: it can vary by laboratory and by factors such as age, sex, and race.³ Hematocrit ranges also shift with smoking and altitude.⁴ Use the range printed on your own report rather than a number found online.

A value outside the range does not always mean you have a condition that needs treatment. Diet, activity level, medicines, having a menstrual period, not drinking enough water, and other factors can affect results.¹ A CBC is one tool among several, and your clinician will consider your history, symptoms, and other tests before making a diagnosis.¹

Questions worth asking about your report:

• Which values are flagged, and are they slightly or far outside the range?
• If I have anemia, is my MCV small, normal, or large?
• Do I need iron tests, vitamin B12, or folate testing next?
• Could my medicines, recent illness, menstrual cycle, or hydration explain the result?

When to talk to a clinician

Bring a list of your medicines and supplements, including any multivitamin or prenatal vitamin with folic acid, so the results can be read in context.¹¹¹⁵

This article is for general education and is not medical advice. Doses and needs differ from person to person, so talk with your clinician or pharmacist about your situation. Read our editorial policy and medical disclaimer.