Complete blood count (CBC), or complete blood count test, is a blood test that counts the three types of blood cells: red cells (which carry oxygen), white cells (which help fight infection), and platelets (which prevent bleeding). One tube is ordered most often in the US to check for anemia, infection, inflammation, and bleeding disorders all in one. No fasting is needed. The vast majority of results are normal, and the very few marked results have a normal explanation.

What a CBC counts

A CBC test measures the number and size of the cells in a sample of blood. An automated analyzer counts millions of cells in seconds and reports a dozen or so values, which fall into three groups.

Red blood cells carry oxygen. The report lists the count, the hemoglobin (the oxygen-carrying protein inside the cells), and the hematocrit (the share of your blood that is red cells). It also lists the average cell size, MCV, which is one of the most useful numbers on the page.

White blood cells are your immune system in circulation. A plain CBC gives the total count. A CBC with differential breaks that total into five types: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Each rises for different reasons.

Platelets are the fragments that clump together to plug a cut. The report gives the count and the average platelet size.

Who should get a CBC blood test

The CBC is ordered with many other tests and is a part of the majority of annual physicals. Fatigue, weakness, shortness of breath on the stairs and pallor all suggest a possibility of anemia. When the fever keeps returning, or the glands are swollen or the infection won't go away, the question of white cells arises. If a person bleeds easily, has nosebleeds, heavy periods or bleeding gums, platelets may be of concern.

This is also a monitoring test. People on chemotherapy, some rheumatology medications and antibiotics have regular counts as these medications can make the bone marrow inactive. The number of kidneys present affects the count in a predictable manner, as does inflammatory bowel disease and heavy alcohol consumption. A CBC is always done prior to surgery, as if the patient has a low platelet count or anemia, the surgery plan changes.

How to prepare

There is nothing to do. A CBC test does not require fasting, and MedlinePlus notes that no preparation is needed. Drink water, since dehydration thickens the blood and nudges hemoglobin and hematocrit up.

Two things are worth mentioning to whoever reviews your result. Hard exercise the day before raises white cells for a while, and a recent cold can push the count up or down for a week or two. Neither is a reason to delay the test, but both can explain a flag.

Reading your result

Each lab prints its own reference range, and that is the one to use. Ranges for hemoglobin and hematocrit differ by sex, and some values shift with age and altitude. The Merck Manual publishes a typical table, but your report's numbers take precedence. Our guide to the CBC normal range walks through each line.

Low hemoglobin means anemia. The next question is what kind, and MCV usually answers it. Small red cells point to iron deficiency, by far the most common cause, especially in people who menstruate. Large red cells point to a shortage of vitamin B12 or folate, or to alcohol, thyroid disease, or certain medications. Normal-sized cells with anemia often mean a chronic condition, kidney disease, or recent blood loss. A ferritin test confirms iron status, and our guide to low ferritin covers what comes next.

High hemoglobin is usually dehydration or smoking. It is also seen at altitude, with sleep apnea, and on testosterone therapy. Rarely, it reflects a bone marrow condition that overproduces red cells.

A high white count most often means infection or inflammation, though steroids, smoking, stress, and hard exercise raise it too. The differential helps: high neutrophils lean bacterial, high lymphocytes lean viral, high eosinophils suggest allergy or parasites. A very high count, or one that stays high with no explanation, needs a closer look. A low count can follow a viral illness, certain medications, or a marrow problem.

Low platelets raise bleeding risk and have many causes, from a recent virus to medications to immune conditions. High platelets usually reflect inflammation or iron deficiency and settle when the cause does.

The pattern matters more than any single number. One mildly low value on an otherwise normal report is common. Three related values moving together is a signal.

What a CBC blood test cannot tell you

A complete blood count is an enumeration of cells. It doesn't measure cholesterol, blood sugar, thyroid hormone, vitamins or kidney or liver function. These are found on other panels and typically a CBC is ordered along with a metabolic panel and often a lipid panel.

It additionally does not identify a contagious illness. The high count of white blood cells is indicative of immune activity, not the immune response. Some infections, such as HIV and hepatitis require separate testing and a normal CBC does not exclude these. A hint may be given in the differential and the red cell size on a CBC lab test report, but that is about as close as you get to a cause.

How to get tested

You can order one yourself. August's lab tests include a CBC with differential on its own and as part of routine, energy, and fitness panels, at one price that covers the draw. A CBC lab test needs a venous sample, so a phlebotomist comes to your home or you visit a nearby lab; there is no finger-prick kit that can run a true complete blood count. Results post the next day in most cases, and a clinician reviews every one before you see it, so a flagged value comes with context. Critical values are escalated the same day. August Care members get up to 50% off next-day lab tests and a first consultation included.

If you are unsure what your symptoms point to, you can describe them to August, a free AI health assistant, to help decide what to test. It is a starting point, not a diagnosis.

Frequently Asked Questions

It shows the counts and sizes of red blood cells, white blood cells, and platelets, plus hemoglobin and hematocrit. Together these screen for anemia, infection, inflammation, and bleeding or clotting problems, and the red cell size gives a clue to the cause of any anemia found.

No. Food has no effect on blood cell counts, so you can eat and drink normally beforehand. Fasting only matters if the CBC is drawn alongside tests that need it, such as fasting glucose or a lipid panel, in which case follow the instructions for those.

It depends on the lab, your sex, and your age. Hemoglobin ranges, for example, run higher for men than for women. Your report prints the range that applies to each value, and that range, not a figure from a website, is the one to compare against.

MCV is the average size of your red blood cells. A high value means the cells are larger than normal, which most often points to low vitamin B12 or folate. Alcohol use, an underactive thyroid, liver disease, and some medications can also raise it.

Usually infection, inflammation, or a recent stressor such as hard exercise, steroids, or smoking. The differential narrows it down by cell type. A count that is very high, or that stays high with no obvious cause, should be reviewed by a clinician promptly.

Not directly. Some blood cancers, such as leukemia, produce abnormal white cell counts that prompt further testing, and anemia can be the first sign of a bleeding tumor. But a CBC is a screen that raises questions, not a cancer test, and a normal result does not rule cancer out.

No. A CBC counts cells only. HIV needs a specific antibody or antigen test, and vitamin D needs a 25-hydroxyvitamin D test. If your routine blood work was a CBC and a metabolic panel, neither of those was checked unless it was ordered separately.

Yes. August sends a phlebotomist to draw the sample at home, and a certified lab runs the same analysis a clinic would. Results post the next day in most cases with a clinician's review. Finger-prick kits cannot run a complete blood count, so a venous draw is required.