Complete blood count (CBC) test: what it checks and results

1. Introduction

A complete blood count, also called a CBC or full blood count, is a group of blood tests that checks how many cells you have in your blood and how large they are ^[1]. It is one of the most common blood tests. Doctors often include it in a routine check-up ^{[1, 3]}, and they also use it when you have symptoms such as tiredness, fever or unexplained bruising ^[2]. This article explains what the test measures, how it is done and how to think about the results. It is general information, and your own doctor decides what your numbers mean.

2. What a CBC measures

Your blood is a mix of fluid and cells. A CBC looks at three main kinds of cell ^[1]:

• Red blood cells carry oxygen from the lungs to the rest of the body.
• White blood cells help the body fight infection and disease. A standard CBC gives the total white cell count. A related test called a CBC with differential breaks the count into the five main types of white cell.
• Platelets are tiny cell pieces that help blood clot and stop bleeding.

The test also reports several other measures ^{[1, 2]}:

• Hemoglobin is an iron-rich protein inside red cells that carries oxygen.
• Hematocrit is the share of your blood volume that is made up of red cells.
• Mean corpuscular volume (MCV) is the average size of your red cells.

Reference ranges, the span of values seen in most healthy people, differ between men and women for red cell measures. They can also shift with altitude and vary a little between laboratories ^{[2, 3]}. This is why results should always be read against the range printed on your own report.

3. Why doctors order a CBC

A CBC can be part of a routine check-up. It is also used to look into symptoms and to follow known conditions. The MedlinePlus encyclopedia lists symptoms such as fatigue, fever and unexplained bruising or bleeding as common reasons to order the test ^[2]. Doctors also use it to monitor people who take medicines or have treatments that can affect blood cells, and people with long-term illnesses such as chronic kidney disease ^[2].

The NHS notes that a full blood count is one of the common tests used to check for iron deficiency anaemia, a condition caused by low iron ^{[4, 5]}. With that condition, people may notice tiredness, low energy, breathlessness, a pounding heartbeat, pale skin or headaches ^[5].

4. What happens during the test

Usually you do not need to prepare. If other tests are done on the same sample, you may be asked to avoid food and drink for several hours, and your provider will tell you if so ^[1].

A health professional may place a soft strap around your arm and uses a small needle to draw blood from a vein, usually in the arm ^{[1, 4]}. The blood goes into a tube or vial. You may feel a brief sting, and the blood draw typically takes less than five minutes ^[1]. If you are nervous about needles, you can tell the staff or bring someone with you ^[4].

The risks are very small. You may have slight pain or bruising where the needle went in, and this usually fades quickly ^[1]. The MedlinePlus encyclopedia also lists, as less common problems, excessive bleeding, light-headedness or fainting, a collection of blood under the skin called a hematoma, and infection at the puncture site ^[2].

5. Understanding your results

Results often come back within a few days to a few weeks, depending on where the test is done ^[4]. Your doctor should explain them to you. Here are some general patterns described by official sources ^{[1, 2, 3]}:

• Low red cell count, hemoglobin or hematocrit may point to anaemia (spelled anemia in the United States). Anaemia has many possible causes, including blood loss, low iron or other nutrients, long-term illness and problems in the bone marrow, where blood cells are made ^{[2, 3]}.
• High red cell measures can be linked to dehydration, low oxygen in the blood, smoking or certain medicines.
• High white cell count may be seen with an infection, inflammation or a reaction to a medicine.
• Low white cell count may be linked to immune or bone marrow conditions, among other causes.
• Abnormal platelet counts may point to a clotting or bleeding problem or a bone marrow problem.
• A low MCV, meaning small red cells, can be seen in some types of anaemia and in a blood condition called thalassaemia ^[3].

An abnormal number does not always mean you have a condition that needs treatment. MedlinePlus states that diet, activity level, medicines, menstrual periods and low fluid intake can all change results ^[1]. A CBC is also only one tool. Your doctor considers your history, symptoms and other tests, and you may need further tests to find the cause of a result ^{[1, 5]}. For example, if anaemia is suspected, the NHS says a doctor may order more tests, such as a stool test for blood, or refer you to a specialist if the cause is not clear ^[5].

6. Getting the most from your test

• Tell your doctor about the medicines you use, since medicines can affect results ^[1]. Always mention every treatment you take, including those from other systems of medicine.
• Drink fluids as usual unless told otherwise, because low fluid intake can change some numbers ^[1].
• Keep your reports so that you and your doctor can look back at them.
• Ask your doctor to explain anything you do not understand. The NHS says you can ask a professional to explain your results ^[4].

7. When to see a doctor

See a doctor if you have symptoms that might relate to your blood, such as ongoing tiredness, looking pale, breathlessness, fever, or bruising and bleeding that you cannot explain ^{[2, 5]}. Also contact your doctor if your report shows a result outside the reference range and no one has explained it. If you have had a test and have not heard back after a few weeks, contact your doctor's office ^[4].

Seek urgent medical help for heavy bleeding that will not stop, fainting, or severe breathlessness.

In an emergency, call your local emergency number (112 in India, 911 in the United States).

8. Conclusion

A complete blood count is a quick, low-risk test that counts red cells, white cells and platelets and measures related values such as hemoglobin. It helps doctors look for causes of symptoms and follow conditions and treatments. Because many everyday factors can shift the numbers, an abnormal result is a reason for a conversation with your doctor, not a reason to panic. Your doctor decides what, if anything, should happen next.