What Is a CBC Blood Test and What Does It Show?

A Complete Blood Count — or CBC — is one of the most frequently ordered blood tests in medicine. It gives a detailed picture of the cells circulating in your blood: red cells, white cells, and platelets. Each component tells a different story about your health.

Whether you are getting a routine annual physical or investigating symptoms like fatigue, frequent infections, or unexplained bruising, a CBC is often the first test ordered.

A standard CBC with differential measures the following:

Red Blood Cell Count (RBC) The total number of red blood cells per volume of blood. Red cells carry oxygen from the lungs to the rest of the body. Low RBC is a hallmark of anemia.

Hemoglobin (Hgb) The protein inside red cells that actually binds and carries oxygen. Low hemoglobin is the most direct measure of anemia severity.

Hematocrit (Hct) The percentage of blood volume made up of red cells. Low hematocrit confirms anemia; high hematocrit can indicate dehydration or polycythemia.

MCV (Mean Corpuscular Volume) The average size of red blood cells. This is one of the most useful values for determining the type of anemia:

• Low MCV (microcytic anemia) → often iron deficiency or thalassemia
• High MCV (macrocytic anemia) → often B12 or folate deficiency
• Normal MCV with low hemoglobin → often chronic disease or acute blood loss

MCH (Mean Corpuscular Hemoglobin) The average amount of hemoglobin per red cell. Usually mirrors MCV.

MCHC (Mean Corpuscular Hemoglobin Concentration) The concentration of hemoglobin in red cells. Low MCHC suggests iron deficiency; high MCHC can indicate hereditary spherocytosis.

RDW (Red Cell Distribution Width) Measures variation in red cell size. High RDW means your red cells are uneven in size — a sign that something is disrupting normal red cell production.

Total White Blood Cell Count (WBC) White cells are your immune system's soldiers. The total count tells you whether your immune system is ramped up (high WBC) or suppressed (low WBC).

Differential (the "with differential" part) A CBC with differential breaks the WBC count into five types of white cells:

• Neutrophils — the first responders to bacterial infection. High neutrophils often indicate bacterial infection or inflammation. Low neutrophils (neutropenia) increase infection risk.
• Lymphocytes — key players in viral immunity and adaptive immune response. High lymphocytes can indicate viral infection or certain leukemias. Low lymphocytes can indicate immune suppression.
• Monocytes — clean up debris and help coordinate immune response. Elevated monocytes can indicate chronic infection or inflammatory conditions.
• Eosinophils — elevated in allergic reactions, asthma, and parasitic infections.
• Basophils — rare; elevated in certain allergic and inflammatory conditions.

Platelet Count (PLT) Platelets are tiny cell fragments that form blood clots. Low platelets (thrombocytopenia) can cause excessive bleeding. High platelets (thrombocytosis) can increase clotting risk.

MPV (Mean Platelet Volume) The average size of platelets. Large platelets are more active; high MPV alongside low platelet count can suggest the bone marrow is trying to compensate.

A CBC can flag early signs of:

• Anemia — low hemoglobin, hematocrit, or RBC count
• Iron deficiency — low MCV, low MCH, high RDW
• B12 or folate deficiency — high MCV
• Bacterial infection — elevated WBC with high neutrophils
• Viral infection — elevated lymphocytes
• Leukemia — abnormal WBC count or differential, often with abnormal cell morphology
• Immune suppression — low WBC or neutropenia
• Bleeding disorders — low platelet count
• Dehydration — elevated hematocrit and hemoglobin
• Bone marrow problems — abnormalities across multiple cell lines

No. A CBC does not require fasting. You can eat and drink normally before the draw. However, if your CBC is part of a larger panel that includes a CMP or lipid panel, fasting may be required for those components.

A CBC is appropriate for:

• Annual wellness screening — it is one of the most informative routine tests available
• Anyone experiencing unexplained fatigue — anemia is one of the most common and treatable causes
• People with frequent infections — may indicate immune dysfunction
• Anyone with unexplained bruising or bleeding — may indicate low platelets or clotting disorders
• Women of reproductive age — iron-deficiency anemia is common due to menstrual blood loss
• Vegetarians and vegans — at higher risk for B12 deficiency anemia
• People on certain medications — chemotherapy, immunosuppressants, and some antibiotics can affect blood cell counts
• Anyone with a family history of blood disorders — sickle cell disease, thalassemia, and hereditary anemias have genetic components

Your lab report will show each value alongside a reference range. A result outside the range is flagged with an "H" (high) or "L" (low). A few things to keep in mind:

• Reference ranges are based on population averages. Mild deviations are common and often not clinically significant.
• The pattern matters more than any single value. Anemia, for example, is diagnosed by looking at hemoglobin, hematocrit, MCV, and RDW together — not just one marker.
• A single abnormal CBC should be repeated before drawing conclusions. Illness, dehydration, and even vigorous exercise can temporarily shift values.

If multiple values are abnormal or a value is significantly outside the reference range, follow up with a physician.

You do not need a physician's order to get a CBC in most states. At Reliable Exams, a CBC with Differential can be ordered online and drawn at any of 4,500+ LabCorp or Quest Diagnostics locations. Results are available in 1–2 business days.

For a more complete annual screening, our Female Wellness Panel and Male Wellness Panel both include a CBC with Differential alongside a full metabolic panel, lipid panel, thyroid panel, and more.