CBC and Anemia: Understanding the Complete Blood Count
Order the Lab Tests in This Guide
Book the panels reviewed in this BloodWorkAppointment guide through our partner, where a referral and an insurance policy are both unnecessary.
FSA and HSA eligible where applicable.We may earn a commission on partner links.
Quick Answer
A complete blood count (CBC) is one of the most commonly ordered laboratory tests. Because it measures the number, size, and hemoglobin content of red blood cells, it is often the first test used when anemia is suspected. This guide explains what the CBC measures, how its red blood cell indices are interpreted, and why follow-up testing may be needed.
Key takeaways
- A CBC measures red blood cells, white blood cells, and platelets, along with related indices such as hemoglobin and hematocrit.
- Anemia is generally defined by hemoglobin or hematocrit below the reference range for a person's age, sex, and condition.
- MCV describes average red blood cell size and helps classify anemia as microcytic, normocytic, or macrocytic.
- RDW describes variation in red blood cell size and can support or refine the interpretation of an abnormal MCV.
- The CBC alone often cannot identify the cause of anemia; additional tests such as iron studies, vitamin levels, or a blood smear may be needed.
- Reference ranges vary by laboratory, so results should be interpreted in the context of the reporting lab and clinical history.
Ready to get tested? Examine the included markers, read the collection notes, and reserve the panel with the BloodWorkAppointment partner today.
Order TestsWhat a Complete Blood Count Measures
A complete blood count is a panel of measurements performed on a blood sample, usually collected in a tube containing an anticoagulant. The core components include the red blood cell count, hemoglobin, hematocrit, white blood cell count with differential, and platelet count. Automated analyzers also calculate red blood cell indices such as mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC), and red cell distribution width (RDW).
Hemoglobin is the iron-containing protein in red blood cells that carries oxygen, and hematocrit is the proportion of blood volume occupied by red blood cells. These two values are central to detecting anemia. The white blood cell and platelet portions of the CBC provide additional information about the bone marrow and can help identify conditions that affect multiple cell lines at once.
How Anemia Appears on a CBC
Anemia is generally identified when hemoglobin or hematocrit falls below the reference interval established for a person's age, sex, and physiologic state. Because reference ranges differ among laboratories and populations, a result slightly outside the range is not automatically diagnostic. Symptoms such as fatigue, pallor, shortness of breath, or rapid heartbeat may accompany low values, but mild anemia can also be found incidentally on routine testing.
Once anemia is identified, the CBC provides a starting framework for classification. The MCV indicates whether red blood cells are smaller than normal (microcytic), normal in size (normocytic), or larger than normal (macrocytic). The RDW indicates how much red blood cell sizes vary within the sample. Together, these indices narrow the list of possible causes and guide the selection of follow-up tests.
Interpreting MCV and RDW
Mean corpuscular volume is calculated by dividing hematocrit by red blood cell count and is expressed in femtoliters. A low MCV suggests microcytic anemia, which may be associated with iron deficiency or certain inherited hemoglobin disorders. A high MCV suggests macrocytic anemia, which may be associated with vitamin B12 or folate deficiency, among other causes. A normal MCV indicates normocytic anemia, which can occur with acute blood loss, chronic disease, or kidney-related causes.
Red cell distribution width reflects the range of red blood cell sizes. A high RDW means the cells vary more in size than expected, which can occur early in nutritional deficiencies or when two different processes coexist. A normal RDW with a low MCV may point toward a more uniform population of small cells. Clinicians often interpret MCV and RDW together rather than in isolation, because the combination provides more information than either value alone.
Limits of the CBC and Common Follow-Up Tests
The CBC is a screening and classification tool, not a definitive diagnostic test for the cause of anemia. For example, a low MCV is consistent with iron deficiency but does not confirm it, and a high MCV does not by itself establish a vitamin deficiency. Additional laboratory testing is usually required to identify the underlying mechanism, and the choice of tests depends on the CBC pattern and the person's history.
Common follow-up tests include iron studies such as ferritin, serum iron, transferrin saturation, and total iron-binding capacity; vitamin B12 and folate levels; reticulocyte count to assess bone marrow response; and a peripheral blood smear to examine cell shape and appearance. In some cases, evaluation of kidney function, markers of inflammation, or hemoglobin electrophoresis may be appropriate. Results are best interpreted by a qualified clinician who can integrate laboratory data with symptoms, physical findings, and medical history.
How It Works
Three steps from ordering to secure results.Place the Order
Choose your panel and complete payment securely through our partner.Visit a Collection Site
Attend an accredited laboratory at a time that works for your schedule.Receive Results
View your results securely, usually within 1-3 business days.Why Order Laboratory Panels Through Our Partner
- Order through our accredited laboratory partner
- FSA and HSA eligible — pre-tax payment where applicable
- No referral, physician note, or insurance required
- Testing performed at CLIA-certified laboratories nationwide
- Secure results, usually within 1-3 business days
Frequently Asked Questions
what is a rdw lab test
RDW stands for red cell distribution width. It is a calculated value reported as part of a complete blood count that describes how much red blood cells vary in size. A higher RDW means greater variation in cell size, which can occur with nutritional deficiencies, mixed anemia causes, and other conditions. RDW is usually interpreted alongside MCV and other red blood cell indices.
what does lab test mcv mean
MCV stands for mean corpuscular volume. It is a calculated red blood cell index that reports the average size of red blood cells in femtoliters. A low MCV indicates smaller-than-normal red blood cells, a high MCV indicates larger-than-normal cells, and a normal MCV indicates average size. MCV helps classify anemia and guide further testing.
what is a cbc lab test
A CBC, or complete blood count, is a common blood test that measures red blood cells, white blood cells, and platelets. It includes hemoglobin, hematocrit, and calculated red blood cell indices such as MCV, MCH, MCHC, and RDW. The CBC is used to screen for and monitor conditions including anemia, infection, and disorders affecting blood cell production.
Can a CBC tell what is causing anemia?
A CBC can identify that anemia is present and help classify it based on red blood cell size and variability, but it usually cannot determine the specific cause on its own. Additional tests, such as iron studies, vitamin B12 and folate levels, reticulocyte count, or a blood smear, are often needed. A clinician interprets the CBC together with symptoms, history, and other laboratory results.
Sources
Reviewed by the BloodWorkAppointment Editorial Team
The BloodWorkAppointment editorial team reviews laboratory panels against published reference material before publication.
Ready to Order Your Lab Tests?
Book the panels highlighted in this BloodWorkAppointment guide through our partner, then stop by an approved collection location for collection.
Order Complete Blood Count Test (CBC Test) with DifferentialInformation in this BloodWorkAppointment guide is educational and does not replace professional guidance. Review your results with a qualified clinician.