A complete blood count (CBC) measures three main types of blood cells: red blood cells, white blood cells, and platelets. Your lab report will list each measurement alongside a reference range, and any result flagged with an “H” (high) or “L” (low) falls outside that range. Understanding what each line means can help you make sense of your results before your next conversation with your doctor.
How to Read the Lab Report Format
CBC reports follow a standard layout. Each row lists a test name (often abbreviated), your result, a flag column, the unit of measurement, and a reference range. The reference range represents the values seen in most healthy people of your age and sex. If your result falls outside that range, the report flags it: “H” for higher than normal, “L” for lower. Some labs use “WNL” to indicate a value is within normal limits. A result marked with an asterisk (*) is a critical value, meaning it’s far enough outside the range that the lab contacted your doctor immediately.
One flagged result does not necessarily mean something is wrong. Mild deviations are common and can reflect dehydration, a recent illness, medications, or even the time of day your blood was drawn. Patterns across multiple values tell a much more useful story than any single number.
Red Blood Cells, Hemoglobin, and Hematocrit
These three measurements all describe your blood’s ability to carry oxygen from your lungs to the rest of your body. They tend to move together: if one is low, the others usually are too.
Hemoglobin is the iron-rich protein inside red blood cells that actually binds oxygen. Normal hemoglobin for adult men is 13.5 to 17.5 g/dL, and for adult women it’s 12.0 to 16.0 g/dL. Low hemoglobin is the most common way anemia shows up on a CBC. It can leave you feeling tired, short of breath, or lightheaded.
Hematocrit tells you what percentage of your blood is made up of red blood cells. Normal ranges for adult men are 41% to 53%, and for adult women 36% to 46%. A low hematocrit mirrors low hemoglobin and points toward anemia. A high hematocrit can signal dehydration (because less plasma makes the red cell percentage look artificially high) or, less commonly, conditions where the body produces too many red blood cells.
If your hemoglobin or hematocrit is low, the next thing to look at on your report is the red blood cell indices, which help narrow down the cause.
Red Blood Cell Indices: MCV, MCH, MCHC, and RDW
MCV (mean corpuscular volume) is the average size of your red blood cells. This is one of the most useful numbers on the entire report because the size of your red blood cells points toward different types of anemia. A low MCV means your red blood cells are smaller than normal, which is the hallmark of iron deficiency. A high MCV means they’re larger than normal, which often results from a deficiency in vitamin B12 or folate. A normal MCV with low hemoglobin suggests other causes, like chronic disease or recent blood loss.
MCH and MCHC measure how much hemoglobin is packed into each red blood cell. MCH is the average weight of hemoglobin per cell, and MCHC is the average concentration. In practice, they reinforce what MCV already tells you. Low MCH and MCHC typically accompany small, pale red blood cells seen in iron deficiency.
RDW (red cell distribution width) measures how much your red blood cells vary in size. A normal RDW means your red blood cells are fairly uniform. A high RDW means there’s a wider range of sizes, which can be an early signal of nutritional deficiencies, chronic liver disease, heart disease, or diabetes. A low RDW simply means your cells are very similar in size and is not a concern.
White Blood Cells and the Differential
White blood cells are your immune system’s workforce. The normal adult range is 3.5 to 10.8 K/mm³. A total white blood cell count above the reference range often reflects infection, inflammation, or stress. A count below the range can indicate that your immune system is suppressed, whether from a medication, a viral infection, or a bone marrow problem.
If your CBC includes a differential (sometimes listed as “CBC with diff”), you’ll see the count broken into five types, each with a different job:
- Neutrophils are the most abundant type and the first responders to bacterial infections. An elevated neutrophil count is one of the clearest signs of a bacterial infection on a CBC.
- Lymphocytes primarily target viral infections. A high lymphocyte count often shows up during a cold, flu, or other viral illness.
- Monocytes clean up damaged cells and help coordinate the immune response. Mild elevations can accompany chronic inflammation.
- Eosinophils respond to parasitic infections and allergic reactions. If your eosinophil count is high, allergies or asthma are common explanations.
- Basophils are the rarest white blood cells and play a role in allergic and inflammatory responses. They’re rarely elevated on their own.
The differential is reported both as a percentage of total white blood cells and as an absolute count. The absolute count is more reliable because percentages can shift when one cell type rises or falls. For example, if neutrophils spike during a bacterial infection, lymphocytes may look low as a percentage even though their actual number hasn’t changed.
Platelets and MPV
Platelets are small cell fragments that clump together to form clots and stop bleeding. The normal platelet count for adults is 150 to 400 K/mm³. A count below 150 is called thrombocytopenia, and it can cause easy bruising or prolonged bleeding from minor cuts. A count above 400 is called thrombocytosis and may increase the risk of abnormal clotting.
Many CBC reports also include MPV (mean platelet volume), which measures the average size of your platelets. Larger platelets are typically younger, freshly released from the bone marrow. A high MPV often means your body is destroying platelets faster than usual and compensating by producing new ones quickly. This pattern shows up in conditions like immune-related platelet destruction, preeclampsia during pregnancy, and some blood cancers. A low MPV suggests the bone marrow may not be producing new platelets at a normal rate, which can occur with certain infections, autoimmune conditions, or as a side effect of medications.
How Pregnancy Changes Normal Ranges
If you’re pregnant, many of your CBC numbers will look different from standard adult ranges, and that’s expected. Blood volume increases significantly during pregnancy, but plasma (the liquid part) increases faster than red blood cells. This dilution effect causes hemoglobin and hematocrit to drop naturally. Hemoglobin levels above 11.1 g/dL in the first trimester, above 10.6 g/dL in the second, and above 10.5 g/dL in the third trimester are considered normal for pregnant women taking iron supplements. Hematocrit normally decreases during pregnancy but should stay above 30%.
White blood cell counts also tend to run higher during pregnancy, particularly neutrophils, so a mildly elevated WBC in pregnancy does not automatically signal infection.
What a CBC Can and Cannot Tell You
A CBC is a screening tool, not a diagnosis. It can reveal that something is off, like low hemoglobin or a high white blood cell count, but it usually can’t tell you exactly why. If your results are abnormal, your doctor may order follow-up tests to pinpoint the cause. For example, low hemoglobin with a low MCV might prompt a check of your iron levels and iron stores. A high white blood cell count might lead to a blood smear, where a lab technician examines your cells under a microscope to look at their shape and maturity.
Some medications can also shift CBC results. Chemotherapy drugs lower white blood cells and platelets. Anti-inflammatory medications can affect white cell counts. Blood thinners don’t change the CBC directly, but recent blood loss from any cause will lower red blood cell numbers. If your results seem unexpected, it’s worth considering what medications or supplements you were taking at the time of the draw.
Do You Need to Fast Before a CBC?
No fasting is required for a standard CBC. You can eat and drink normally before the test. If your blood draw also includes other panels like a metabolic panel or cholesterol test, those may require fasting, but the CBC portion itself is unaffected by food. The draw takes less than a minute, and results are typically available within a day.