Complete blood count (CBC): how to read your results

Until the late 1940s, counting blood cells took patience. A lab technician diluted a drop of blood, placed it on a slide etched with a grid and counted the cells under a microscope, square by square. It was slow, and the answer could vary quite a bit from one count to the next.

American engineer Wallace Coulter knew the problem well. According to historian Marshall Don Graham, who worked as a technical adviser to Coulter's company, the atomic bombings of Hiroshima and Nagasaki in 1945 convinced Coulter that red blood cells would have to be counted quickly across entire populations, to track the recovery of people exposed to radiation. In April 1947, he and his brother bought a house in Chicago and began experimenting in the basement. The breakthrough came in October 1948: each cell passing through a tiny hole between two electrodes briefly changed the electric current, so counting those "blips" meant counting the cells. It is known as the Coulter principle, and it underlies today's automated blood cell counters.

Thanks to that idea, the complete blood count could be automated and became a routine test. The report arrives full of abbreviations and numbers, and it is easy to get lost in it. So what does a CBC actually measure, and how do you read the report without jumping to the wrong conclusion?

What is a complete blood count?

It is a set of measurements taken from a single blood sample. In Brazil it is called a hemograma; elsewhere it is also known as a full blood count. The report is usually divided into three parts:

• Red cell panel: red blood cells, which carry oxygen from the lungs to the rest of the body, and hemoglobin, the iron-rich protein that does the carrying.
• White cell panel: white blood cells, the immune cells that fight infection.
• Platelet count: platelets, which help blood clot and stop bleeding.

Today the counting is done by automated analyzers. When something falls outside the expected range, the lab may look at a blood smear under the microscope to check the shape of the cells.

What is a complete blood count used for?

It is part of routine check-ups, and it is also ordered to investigate symptoms such as fatigue, signs of infection, bruising or bleeding. It is also used to monitor conditions that affect blood cells, such as chronic kidney disease, and treatments that can lower blood counts, such as chemotherapy.

One important point: a CBC rarely gives a diagnosis on its own. It shows that something has changed and helps the doctor decide which tests to order next.

Do you need to fast for a CBC?

Usually not. A CBC needs no special preparation. Fasting only comes into play when other tests are drawn at the same visit, such as blood sugar or cholesterol and other blood fats. So follow the instructions on your test order and from the lab. It also helps to tell them if you take any medication, because some drugs can change the result.

The blood is drawn from a vein in the arm. It takes a few minutes and the risk is very low. It may hurt a little, or leave a small bruise at the spot, which usually fades quickly.

How to read a CBC: what do red blood cells and hemoglobin show?

This is the part that raises the most questions. The most common items are:

• Red blood cells (erythrocytes): the number of red cells, in millions per microliter.
• Hemoglobin: how much of that protein is in the blood, in grams per deciliter (g/dL). It is the number most often used to define anemia.
• Hematocrit: the percentage of blood volume taken up by red blood cells.
• MCV (mean corpuscular volume): the average size of the red cells. It helps tell types of anemia apart, with cells smaller or larger than normal.
• MCH and MCHC: the average amount and concentration of hemoglobin inside each red cell.
• RDW: shows how much the red cells vary in size. It is useful for investigating the cause of anemia and conditions such as thalassemia.

Just to give a sense of scale, MedlinePlus, from the U.S. National Library of Medicine, lists common adult ranges of 13 to 18 g/dL of hemoglobin in men and 12 to 16 g/dL in women, and an MCV of 80 to 100 fL. It also warns that these numbers vary between labs and even with altitude. Children and pregnant women have their own ranges. What counts is the reference range printed on your own report, which reflects the lab's method as well as your age and sex.

Low hemoglobin and hematocrit usually point to anemia. Brazil's Ministry of Health, citing the World Health Organization, describes anemia as hemoglobin below the normal level and estimates that about 90% of cases come from a lack of iron. But there are other causes: blood loss, very heavy periods over a long time, a lack of vitamin B12 or folate, chronic diseases and problems in the bone marrow, where blood is made. High values, on the other hand, can show up with smoking, testosterone use, living at high altitude and some diseases of the heart, lungs or blood itself.

How to read a CBC: what do white blood cells and platelets show?

White blood cells appear as a total (MedlinePlus cites a common range of 4.500 to 11.000 per microliter) and, in a CBC with differential, split into five types: neutrophils, lymphocytes, monocytes, eosinophils and basophils. Each type has its own role in the body's defenses, which is why the report lists the percentage and the absolute number for each one.

High white cell counts, known as leukocytosis, are common in infections. They can also appear with intense physical or emotional stress, corticosteroid use, burns and inflammatory diseases. Low counts, known as leukopenia, can come from viral infections, some medications, autoimmune diseases or bone marrow problems. One detail that confuses people: because the differential is a percentage, when one type rises a lot the others seem to fall, even though they have not actually changed. That is why the absolute number matters too.

Platelets are usually between 150.000 and 400.000 per microliter in adults, again keeping in mind that the range varies between labs. Low platelets can cause bleeding that takes a long time to stop, nosebleeds, bruises without any bump and tiny red dots on the skin. High platelets can appear after bleeding, with iron deficiency, in some cancers and in blood disorders.

Does a result outside the range mean disease?

Not always. MedlinePlus notes that a value outside the range does not necessarily mean a condition that needs treatment. Diet, physical activity, menstruation, not drinking enough water, medications and even altitude all shift the numbers. An asterisk on the report is an invitation to talk to your doctor, not a diagnosis.

What the doctor looks at is the whole picture: which items changed together, by how much, whether there are symptoms and what earlier tests showed. Keeping your old results helps a lot with that comparison. And if the CBC comes back abnormal, the doctor may order other tests, such as iron or vitamin B12 levels, or a review of the blood smear.

When to see a doctor

Take the result to the doctor who ordered the test, even if everything looks normal. Seek care soon if your CBC came back abnormal and you have a persistent fever, severe fatigue, shortness of breath, paleness, unexplained bruises or tiny red dots on the skin, or bleeding that takes a long time to stop.

Go to an emergency room or call SAMU, Brazil's emergency medical line, at 192, in case of heavy bleeding that will not stop, blood in vomit or black stools, fainting, severe shortness of breath or chest pain. For follow-up, a general practitioner or family doctor is usually the first step; abnormal results that need closer investigation are referred to a hematologist, the specialist in blood diseases.

The bottom line: a quick test that deserves a calm reading

Wallace Coulter wanted a way to count the cells of many people, quickly and precisely. He succeeded, and that is why a CBC is done fast and automatically today. But speed does not change the essentials: these are numbers that only make sense alongside your history, your symptoms and your earlier tests.

Understanding what each line of the report means can help you arrive at the appointment with better questions. The interpretation, though, remains the doctor's job.

Frequently asked questions

Does a CBC detect cancer?

It can show changes that raise suspicion of some blood cancers, such as leukemia, but it cannot confirm or rule out cancer on its own. The diagnosis depends on other tests.

Does a CBC show iron deficiency?

It can suggest it, with low hemoglobin and small red cells, but confirmation comes from specific iron tests ordered by the doctor.

How long do the results take?

The time varies by lab. Ask when your report will be ready at the time of the blood draw.

Is one abnormal CBC reason to worry?

A small, isolated change is often repeated or followed over time before any conclusion is drawn. The doctor decides, looking at the whole picture.

More from RaisLife: who can donate blood and how donation works, cancer prevention and early diagnosis and whether a blood test can diagnose autism.

Sources

• MedlinePlus (National Library of Medicine). Complete Blood Count (CBC). medlineplus.gov
• MedlinePlus Medical Encyclopedia. CBC blood test (reference values and causes of abnormal results). medlineplus.gov
• MedlinePlus Medical Encyclopedia. Blood differential test. medlineplus.gov
• MedlinePlus. RDW (Red Cell Distribution Width). medlineplus.gov
• MedlinePlus. Platelet Tests. medlineplus.gov
• NHS. Blood tests. nhs.uk
• NHS. Iron deficiency anaemia. nhs.uk
• Ministério da Saúde (Brazil's Health Ministry), Biblioteca Virtual em Saúde. Anemia. bvsms.saude.gov.br
• Graham MD. The Coulter principle: imaginary origins. Cytometry A, 2013. pmc.ncbi.nlm.nih.gov
• Green R, Wachsmann-Hogiu S. Development, history, and future of automated cell counters. Clin Lab Med, 2015. pubmed.ncbi.nlm.nih.gov

Text by Rai, RaisLife's virtual reporter (artificial intelligence), with automated editorial review.