Diagnosis and tests

Complete blood count (CBC)

On the road to a leukemia diagnosis, almost everything starts with an ordinary blood test — and during treatment it remains the test you will have most often. This topic explains, calmly and in plain words, what a complete blood count measures, what changes can appear in leukemia and what to keep in mind when you read the numbers.

Last updated: 7 October 2026~7 min read

A translucent glowing drop of blood on a dark navy background, with red disc-shaped red cells, a few pale white cells and tiny golden platelets floating inside. — **Figure 1.**Concept: a small blood sample tells a lot about how the bone marrow is working.

01Short answer

A complete blood count (CBC, also called a full blood count) counts the three main groups of blood cells: red blood cells that carry oxygen (together with hemoglobin and hematocrit), white blood cells that fight infection, and platelets that stop bleeding. A white cell differential shows the five types of white cells separately — neutrophils, lymphocytes, monocytes, eosinophils and basophils. In leukemia the white count can be very high, low or normal; anemia and low platelets are common, and sometimes immature cells called blasts, which are not normally found in the blood, appear. These changes are a reason to suspect leukemia, but the diagnosis is usually confirmed not by a blood test alone but by a bone marrow examination and specialised tests. During treatment the CBC is repeated often to track the risk of infection and bleeding, the need for transfusions and the recovery of the bone marrow. Always look at your results together with your hematologist.

02Simple explanation

Your blood is made of fluid with cells floating in it. A complete blood count finds out how many of these cells there are and how big they are, using a sample taken in a few minutes. The blood is usually drawn from a vein in your arm, and most of the time no special preparation is needed.

Red blood cells carry oxygen around the body. When the protein inside them — hemoglobin — drops, anemia develops: you may feel tired, look pale, get short of breath or notice your heart racing.

White blood cells protect the body from infection. Among them, neutrophils are the first line of defence against bacteria. When neutrophils are low, even a common infection can become serious, which is why doctors pay special attention to this number during treatment.

Platelets act like a “patch” when a blood vessel is damaged and stop the bleeding. When they are low, you may bruise easily, see tiny red spots on the skin, or have bleeding gums or nosebleeds.

One number outside the normal range does not automatically mean illness: dehydration, a recent infection, medicines and other factors can all affect the result. Numbers should always be read together and with your doctor.

03Scientific explanation

What a CBC includes: according to MedlinePlus, the test measures red cells, white cells, platelets, hemoglobin, hematocrit (the share of blood made up of red cells) and MCV (the average size of red cells). The white cell differential is a separate test that gives the percentage or absolute number of the five main types of white cells. Modern analysers count cells automatically; when the machine flags unusual cells, the sample is checked under the microscope on a blood smear.

Typical findings in leukemia: according to the American Cancer Society (ACS), most people with AML have too many immature white cells in the blood and not enough red cells or platelets; myeloblasts, which are not normally in the blood, may be seen. In CLL the key sign is too many lymphocytes (lymphocytosis): more than 5,000 lymphocytes per mm³ of blood strongly suggests CLL, but other tests — above all flow cytometry — are needed to confirm it. In acute leukemia the white count can also be normal or even low, because leukemia cells sometimes stay in the marrow without spilling into the blood.

The absolute neutrophil count (ANC) is calculated by multiplying the total white count by the fraction of neutrophils (segmented and band forms). The NCI describes it as a measure of infection risk. During chemotherapy the ANC is the most closely watched number: when it is very low, a fever is treated as an emergency and needs immediate contact with your care team.

Watch the units: in Azerbaijan and Russia, white cells are usually reported as ×10⁹/L, red cells as ×10¹²/L and hemoglobin in g/L; US sources use “thousand/µL” and g/dL. 4.5 thousand/µL = 4.5 ×10⁹/L, and 13 g/dL = 130 g/L. Approximate adult ranges given by Testing.com (US): white cells 4.5–11.0 ×10⁹/L, platelets 150–400 ×10⁹/L, hemoglobin 13–18 g/dL in men and 12–16 g/dL in women. Each laboratory has its own ranges — always check the range printed on your report.

04What happens in the body?

When and why is it ordered?

A CBC is done at routine check-ups and when someone has unexplained tiredness, frequent infections, bruising or fever. Many leukemias are found in exactly this way — through an unexpected result on an “ordinary” test. Such a result leads the doctor to further tests: a blood smear and a bone marrow examination.

Three cell families at a glance

Red cells, white cells and platelets are all made in the same place — the bone marrow. As leukemia cells fill the marrow, fewer healthy cells are produced. That is why several values changing together, for example anemia alongside low platelets, tells the doctor more than any single number.

Three groups of softly glowing cells on a dark navy background: red cells on the left, pale granular white cells with nuclei in the centre and small golden platelets on the right. — **Figure 2.**A CBC counts three cell families: red cells, white cells and platelets.

During treatment: the ups and downs

A temporary drop in blood counts after chemotherapy is expected — this low point is called the “nadir”. During this time transfusions and protection from infection matter most. Then, as the marrow recovers, neutrophils and platelets gradually rise, and that is an encouraging sign.

A stream of blood cells on a dark navy background that becomes denser and brighter from left to right, with a warm, hopeful light on the right. — **Figure 3.**After treatment, blood counts gradually rise as the bone marrow recovers.

Which other blood tests are done alongside it?

Together with the CBC, doctors often order blood chemistry tests (liver and kidney values, electrolytes, LDH, uric acid) and clotting tests. The ACS notes that these tests do not diagnose leukemia, but they help uncover liver or kidney problems, abnormal mineral levels and problems with blood clotting. This information is important for planning treatment safely.

How to take in a result

It is natural to feel anxious when a result pops up on your phone. Rather than drawing conclusions from one number, look at the trend — how it compares with earlier tests — and write down your questions. Ask your hematologist which values matter most for you and when you should call.

05The complete blood count step by step

Four steps — from the blood draw to your doctor’s decision.

1Blood sampleA few millilitres of blood are taken from a vein.

2Automated countAn analyser counts and measures the cells.

3Smear (if needed)Unusual cells are checked under the microscope.

4Doctor’s reviewResults are compared with symptoms and earlier tests.

AnemiaLow hemoglobin — tiredness, paleness.

Infection riskLow neutrophils — fever is dangerous.

Bleeding riskLow platelets — bruising, bleeding.

06Key values and possible changes in leukemia

Value	What it shows	What may happen in leukemia	What to watch
White cells (WBC)	Total number of white blood cells	High, normal or low	Does not make a diagnosis on its own
Neutrophils / ANC	Main defence against bacteria	Often low	Low ANC + fever = urgent
Lymphocytes	Immune “memory” cells	Can be very high in CLL	Clarified by flow cytometry
Blasts	Immature cells	May appear in the blood in acute leukemia	A bone marrow test is needed
Hemoglobin / red cells	Oxygen transport	Anemia	Need for transfusion
Platelets (PLT)	Blood clotting	Often low	Signs of bleeding
MCV	Average red cell size	May change	Helps work out the type of anemia

* This table is a general guide. Ranges differ by laboratory, age and sex; your hematologist will explain what each number means for you.

07Key terms

CBC: Complete blood count — a test that measures the number and size of blood cells.
Differential: A test showing the percentage or number of the five types of white cells.
Hemoglobin: The iron-containing protein in red cells that carries oxygen.
Hematocrit: The share of blood made up of red cells (as a percentage).
MCV: Mean corpuscular volume — the average size of red cells.
ANC: Absolute neutrophil count — the key measure of infection risk.
Neutropenia: A lower-than-normal number of neutrophils.
Thrombocytopenia: A lower-than-normal number of platelets.
Blast: An immature cell that is not normally found in the blood.
Reference range: The range of values a laboratory expects in healthy people.

08Key points

A CBC counts red cells, white cells and platelets.
The differential shows the five types of white cells separately.
In leukemia the white count can be high, normal or low.
Anemia and low platelets are common.
Blasts in the blood are a reason for a bone marrow test.
A diagnosis is usually not made from a blood test alone.
During treatment, the ANC shows the risk of infection.
Ranges vary by laboratory — check the range on your report.

09Frequently asked questions

My blood count is normal. Does that mean I do not have leukemia?

A normal CBC is usually reassuring, but sometimes, especially early on, the changes can be small. If your symptoms continue, talk openly with your doctor — further tests will be ordered if needed.

My white count is very high. Is it definitely leukemia?

No. White cells most often rise because of infection, inflammation, stress, smoking and some medicines. Leukemia is usually suspected when several values change together, or when blasts or unusual cells are seen in the blood.

Do I need to fast before the test?

Usually not. A CBC needs no special preparation. But if other tests are being run on the same sample, you may need to fast — ask for instructions.

Why are blood tests taken so often during treatment?

Chemotherapy lowers blood counts for a while. Frequent tests let the team spot the need for transfusion, the risk of infection and the recovery of the marrow in good time.

My neutrophils are low. What should I watch for?

The key rule: if you get a fever, chills or suddenly feel unwell, call your care team straight away. We have separate topics on protecting yourself from infection during neutropenia; your hematologist will give you personal advice.

Why are the results from another laboratory different?

Laboratories use different machines, units and reference ranges. To follow the trend, try to use the same laboratory where possible and pay attention to the units (×10⁹/L, g/L).

10Questions for your doctor

Take these questions to your appointment. The ones you save are collected on the “My questions” page and stay on this device only.

Which values in my complete blood count are outside the normal range?
Are these changes due to the leukemia or to treatment?
How often do I need a blood count?
Can I have a copy of my results with the reference ranges?

My questions

12Sources

MedlinePlus (U.S. National Library of Medicine). Complete Blood Count (CBC)
MedlinePlus (U.S. National Library of Medicine). White Blood Count (WBC)
MedlinePlus (U.S. National Library of Medicine). Blood Differential
MedlinePlus (U.S. National Library of Medicine). Hemoglobin Test
MedlinePlus (U.S. National Library of Medicine). Platelet Tests
National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: complete blood count
National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: absolute neutrophil count
National Cancer Institute (NCI). Adult Acute Myeloid Leukemia Treatment (PDQ®) — Patient Version
National Heart, Lung, and Blood Institute (NHLBI). Blood Tests
American Cancer Society. Tests for Acute Myeloid Leukemia (AML)
American Cancer Society. Tests for CLL
American Cancer Society. Blood Tests
Testing.com. CBC With Differential Test: Order, Results & What They Mean

Medical disclaimerThis information is for educational purposes only and does not replace medical advice.

Last updated: 7 October 2026← Back to the list

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