A complete blood count (CBC) measures three things in your blood: red blood cells that carry oxygen, white blood cells that fight infection, and platelets that help your blood clot. Your results will include a dozen or more line items with numbers, reference ranges, and abbreviations. Once you know what each value represents, the report becomes surprisingly straightforward to read.
Most CBC reports follow the same format: a measured value next to a reference range. If your number falls within that range, it’s considered normal. Values flagged as high (H) or low (L) fall outside the expected range and may point to an underlying issue, though a single abnormal value doesn’t always mean something is wrong.
Red Blood Cell Count (RBC)
This line tells you how many red blood cells are in a tiny sample of your blood, measured in millions of cells per microliter. The normal range for adult men is 4.7 to 6.1 million cells per microliter. For adult women, it’s 4.2 to 5.4 million. A low count can signal anemia, blood loss, or nutritional deficiencies. A high count can result from dehydration, smoking, or living at high altitude, all of which push your body to produce more oxygen-carrying cells.
Hemoglobin (Hb) and Hematocrit (Hct)
Hemoglobin measures the actual oxygen-carrying protein inside your red blood cells, reported in grams per deciliter (g/dL). The World Health Organization considers normal hemoglobin to be 13 to 18 g/dL for adult men and 12 to 16 g/dL for adult women. During pregnancy, the lower limit drops to about 10 g/dL because the body naturally expands its blood volume. Low hemoglobin is the textbook definition of anemia, while a high level is called erythrocytosis.
Hematocrit is closely related. It measures what percentage of your blood is made up of red blood cells. Normal ranges are 40% to 54% for men and 36% to 48% for women. Because hemoglobin and hematocrit move together, they usually tell the same story. If one is low, the other almost always is too.
The most common reason for low values in both is iron deficiency, but blood loss, chronic disease, and vitamin deficiencies (particularly B12 and folate) can also be responsible. High values can come from dehydration, lung disease, or smoking.
Red Cell Indices: MCV, MCH, MCHC, and RDW
These four values describe the size, shape, and hemoglobin content of your red blood cells. They’re especially useful for figuring out what type of anemia someone has.
MCV (Mean Corpuscular Volume) tells you the average size of your red blood cells. Normal is about 80 to 94 femtoliters. A low MCV means your cells are smaller than normal (microcytic), which is the hallmark of iron deficiency anemia. A high MCV means they’re larger than normal (macrocytic), often caused by a deficiency in B12 or folate.
MCH (Mean Corpuscular Hemoglobin) measures the average amount of hemoglobin in each red blood cell. Normal is around 27 to 31 picograms per cell. It tends to track with MCV: small cells carry less hemoglobin, large cells carry more.
MCHC (Mean Corpuscular Hemoglobin Concentration) looks at hemoglobin relative to the size of the cell. Normal is about 32 to 36 g/dL. A low MCHC means red blood cells are paler than usual, which can happen in iron deficiency. This value stays normal in many other types of anemia, so it helps narrow the diagnosis.
RDW (Red Cell Distribution Width) measures how much variation there is in the size of your red blood cells. Normal is roughly 11.5% to 14.5%. A high RDW means your red blood cells vary widely in size. In iron deficiency anemia, RDW often rises before any other value changes, making it one of the earliest markers. In contrast, people with certain inherited blood conditions like thalassemia trait tend to have uniformly small cells and a normal RDW, which helps doctors tell the two apart.
White Blood Cell Count (WBC)
Your total white blood cell count reflects how active your immune system is. It’s usually reported in thousands per microliter. A typical range is about 4,500 to 11,000. A high count (leukocytosis) most commonly means your body is fighting an infection, but stress, inflammation, allergies, and certain medications like corticosteroids can push it up too. A low count (leukopenia) can result from viral infections, autoimmune conditions, or bone marrow problems.
The WBC Differential
If your CBC includes a differential, you’ll see a breakdown of the five types of white blood cells, each with a different job. These are reported as percentages of the total white cell count, and some labs also give an absolute number for each type.
- Neutrophils (40% to 60%): Your first responders against bacterial infections. A high percentage usually points to a bacterial infection or inflammation. A low count makes you more vulnerable to infections.
- Lymphocytes (20% to 40%): These include B cells and T cells, the core of your adaptive immune system. They rise during viral infections and drop in certain immune disorders.
- Monocytes (2% to 8%): These clean up damaged cells and help fight chronic infections. Elevated monocytes can show up with chronic inflammatory conditions.
- Eosinophils (1% to 4%): These respond to parasitic infections and allergic reactions. A high eosinophil count is common with asthma, allergies, or parasites.
- Basophils (0.5% to 1%): The rarest white blood cell. They play a role in allergic responses. Elevated basophils are uncommon and can sometimes indicate a blood disorder.
The pattern matters more than any single number. For example, a high total WBC with elevated neutrophils suggests a bacterial infection, while a high WBC driven mostly by lymphocytes points more toward a viral one.
Platelet Count and MPV
Platelets are the tiny cell fragments that help your blood clot when you’re injured. A normal platelet count ranges from 150,000 to 450,000 per microliter. Counts below 150,000 (thrombocytopenia) can increase your risk of bruising and bleeding. Counts above 450,000 (thrombocytosis) can raise the risk of abnormal clotting, though mild elevations are often a temporary response to infection or inflammation.
Many CBC panels also include MPV, or mean platelet volume, which measures the average size of your platelets. Newly made platelets are larger than older ones, so a high MPV suggests your body is rapidly producing new platelets to replace ones being destroyed or used up. A low MPV may mean the bone marrow isn’t keeping up with demand. MPV is most useful when read alongside the platelet count itself: a low platelet count with a high MPV tells a different story than a low count with a low MPV, because it suggests the problem is destruction rather than production.
How to Read Your Report
When you get your results, start by looking at which values are flagged outside the reference range. Keep in mind that reference ranges can vary slightly between labs, so always compare your number to the range printed on your specific report rather than a generic chart online.
A single mildly abnormal value, with everything else in range, is often meaningless. Dehydration, a recent workout, your menstrual cycle, or even the time of day your blood was drawn can shift numbers slightly. Patterns matter more than isolated readings. If your hemoglobin, hematocrit, and RBC count are all low, that consistently points toward anemia. If your WBC count is high with elevated neutrophils, that cluster suggests infection or inflammation.
Trends over time are also valuable. If you’ve had CBCs done before, compare your current results to previous ones. A hemoglobin that’s been slowly dropping over several months tells a very different story than a single reading that’s slightly below range. Many patient portals now display this kind of trending data automatically, which makes tracking changes easier than ever.
Fasting is generally not required before a CBC, since the test measures blood cells rather than substances affected by food. However, if your blood draw is also being used for other tests like cholesterol or blood sugar, your provider may ask you to fast for those.