An FBC (full blood count) is a routine blood test that measures the number, size, and health of the three main types of cells in your blood: red blood cells, white blood cells, and platelets. In many countries, including the United States, it’s called a CBC (complete blood count). It’s one of the most commonly ordered blood tests, used both as a general health screen and as a tool for diagnosing conditions like anemia, infections, and blood cancers.
What an FBC Measures
The test returns a panel of results, typically around 13 different values, all from a single blood sample. These fall into three main groups based on the type of blood cell being measured.
Red Blood Cells
Red blood cells carry oxygen from your lungs to the rest of your body. The FBC measures several things about them:
- Red blood cell count (RBC): The total number of red cells in your blood. Normal ranges are 4.0 to 5.4 million cells per microlitre for women and 4.5 to 6.1 million for men.
- Hemoglobin (Hb): The oxygen-carrying protein inside red blood cells. Normal levels are 11.5 to 15.5 g/dL for women and 13 to 17 g/dL for men.
- Hematocrit: The percentage of your blood made up of red cells. Normal is 36 to 48% for women and 40 to 54% for men.
- MCV (mean corpuscular volume): The average size of your red blood cells. Normal is around 87 femtolitres. This value is particularly useful for identifying what type of anemia you might have.
- MCH and MCHC: These measure how much hemoglobin each red cell contains and its concentration. They help distinguish between different causes of anemia.
White Blood Cells
White blood cells are your immune system’s front line. A normal total white cell count falls between 4,000 and 10,000 cells per microlitre. The FBC may also include a “differential,” which breaks down the five types of white blood cells individually:
- Neutrophils: The most abundant type, responsible for fighting bacterial and viral infections.
- Lymphocytes: Include B cells, which produce antibodies against invaders, and T cells, which can destroy infected or cancerous cells directly.
- Monocytes: Kill bacteria and viruses while also clearing away dead cells.
- Eosinophils: Defend against parasites and play a role in allergic reactions and inflammation.
- Basophils: Release enzymes during allergic reactions and asthma attacks.
Platelets
Platelets are tiny cells that clump together to form blood clots when you’re injured. A normal platelet count ranges from 150,000 to 400,000 cells per microlitre. The test may also report mean platelet volume (MPV), which measures the average size of your platelets. Larger-than-normal platelets can suggest your bone marrow is working overtime to replace platelets that are being destroyed too quickly, while smaller platelets may indicate the bone marrow isn’t producing enough new ones.
Why Doctors Order It
An FBC is often part of a routine checkup, even when nothing is obviously wrong. It gives a broad snapshot of your overall health. Beyond routine screening, doctors order it to investigate specific symptoms like unexplained fatigue, bruising, fever, or weight loss. It’s also used to monitor existing conditions that affect blood cells, such as autoimmune diseases, chronic kidney disease, or blood cancers, and to track the effects of treatments like chemotherapy.
What Abnormal Results Can Tell You
A low hemoglobin or red cell count points to anemia, but the FBC goes further by helping identify the cause. If your red cells are smaller than normal (low MCV), the most common culprit is iron deficiency. If they’re larger than normal (high MCV), it often signals a deficiency in vitamin B12 or folate. Other causes of low hemoglobin include chronic kidney disease, inflammatory bowel disease, heavy menstrual bleeding, and conditions that destroy red cells faster than your body can replace them, like sickle cell anemia or thalassemia.
A high white cell count can signal infection, inflammation, allergic reactions, or in some cases blood cancers like leukemia. A low white cell count may result from bone marrow disorders, chemotherapy, certain medications, or HIV. The differential breakdown is what makes this especially useful: a spike in eosinophils points toward allergies or parasites, while elevated neutrophils suggest a bacterial infection.
Platelet counts outside the normal range raise their own set of questions. Low platelets (thrombocytopenia) can cause easy bruising and prolonged bleeding, and may be linked to autoimmune conditions, certain infections, or medications. High platelets (thrombocytosis) can occur with infections, inflammation, iron deficiency, or more rarely with blood cancers that cause the bone marrow to overproduce cells.
One important thing to understand: a single abnormal value doesn’t automatically mean something serious. Results fluctuate with hydration, recent exercise, stress, and even the time of day. Your doctor will interpret your FBC in context, considering your symptoms, medical history, and trends over multiple tests.
What an FBC Cannot Detect
Despite its usefulness, an FBC has clear limits. It tells you how many blood cells you have and what they look like, but it doesn’t explain why numbers are abnormal. A low hemoglobin flags anemia but won’t tell you whether it’s caused by iron deficiency, chronic disease, or blood loss. Follow-up tests are almost always needed. These might include iron studies, vitamin B12 and folate levels, kidney function tests, or a blood smear where a lab technician examines your cells under a microscope.
An FBC also doesn’t measure blood sugar, cholesterol, liver enzymes, or kidney markers. Those require separate tests. If your doctor wants a broader picture, they’ll typically order an FBC alongside a metabolic panel or other targeted tests.
How the Test Works
The blood draw itself takes a few minutes. A healthcare professional draws a small sample from a vein in your arm, usually from the inside of your elbow or the back of your hand. The sample goes into a tube containing an anticoagulant to prevent clotting, then gets sent to a lab where an automated analyzer counts and categorizes the cells.
Fasting is not required for an FBC. You can eat and drink normally beforehand. Fasting is only necessary for specific tests like blood glucose or cholesterol panels, so if your FBC is being drawn alongside those, your doctor will let you know. It’s worth mentioning any medications or supplements you’re taking, as some can affect blood cell counts.
How Long Results Take
An FBC is one of the faster lab tests to process. Once your sample reaches the lab, the actual analysis typically takes under 90 minutes. In practice, most results are available within a few hours to one business day, depending on how the sample is transported and how busy the lab is. If your doctor marks the test as urgent, results can be ready in as little as three hours. In a routine outpatient setting, you’ll usually hear back within 24 hours, and many clinics now post results to an online patient portal as soon as they’re authorized.