What is a complete blood count (CBC)?
A complete blood count, or CBC, measures the number of red blood cells, white blood cells, and platelets in your blood. It also measures hemoglobin (the protein that carries oxygen) and other details about your red blood cells.
It is one blood draw, usually a single small tube, and it needs no fasting unless another test ordered at the same time requires it.
The three lines on the report
Think of a CBC as three separate reports printed on one page. Each one answers a different question.
Red blood cells carry oxygen. The CBC gives the red cell count, the hemoglobin level, the hematocrit (the share of your blood made up of red cells), and the average size of your red cells, reported as MCV. Low values mean anemia. That is what makes you short of breath climbing stairs, or tired in a way sleep does not fix.
White blood cells fight infection. The count alone tells you how many. A CBC "with differential" breaks that total into the five types of white cell, which is where most of the useful detail lives.
Platelets stop bleeding. They plug small breaks in blood vessels. When they run low, the condition is called thrombocytopenia, and you may bruise easily or see tiny red or purple dots on the skin.
The number to learn if you are on chemotherapy
Within the white cell differential, the one that matters most for infection is the neutrophil. Multiply the total white count by the percentage of neutrophils and you get the absolute neutrophil count, or ANC.
Doctors use fixed cutoffs:
- Below 1,500 cells per microliter is neutropenia
- Below 500 is severe neutropenia
- Below 100 is profound neutropenia
Your ANC is often what decides whether treatment goes ahead on a given day. It is also why fever is treated so urgently during chemotherapy. NCI advises calling your team for a temperature of 100.4°F (38°C) or higher.
Platelets matter for a different set of warnings. NCI says to call about bleeding that does not stop after a few minutes, bleeding from the mouth or nose, blood when you vomit, pink or red urine, black or bloody stools, unusually heavy periods, or vaginal bleeding outside a period. Severe headache, vision changes, confusion, or unusual sleepiness also belong on that list.
What a CBC can and cannot tell you
A CBC can point toward a diagnosis. NCI notes it helps diagnose some cancers, especially leukemias, where abnormal cells crowd the marrow and show up directly in the count.
For most solid tumors it does not diagnose anything. Abnormal numbers there are a clue, not an answer, and NCI is explicit that lab tests alone cannot diagnose cancer. Imaging and a biopsy do that work.
What the CBC does well is track. Run before every cycle, it shows how your marrow is holding up and whether the dose or the schedule needs to change.
Reading your own results
Results come with a reference range, a low and high limit drawn from testing large numbers of people. Ranges differ slightly between labs, so compare a result to the range printed on that same report, not to one from another hospital.
A single flagged value is usually less telling than the direction of travel across several draws. If you can see your results online, look at the trend before you worry about one number.
Sources
Want the full picture? Read our complete explanation: Blood and Lab Tests Used in Cancer
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