CBC blood test: what a complete blood count measures

What is in a CBC

A CBC reports three groups of cells, plus measurements of their size and contents:

Red blood cells: hemoglobin, hematocrit and the indices

Hemoglobin, hematocrit and the red cell count show whether you have enough oxygen-carrying capacity. Low values mean anemia; high values can come from dehydration, smoking, living at altitude, sleep apnea, lung or heart disease, testosterone therapy or, rarely, a bone marrow condition.

The red cell indices (MCV, MCH, MCHC and RDW) help explain why. Small, pale cells usually point toward low iron or thalassemia trait. Large cells point toward low vitamin B12 or folate, alcohol, thyroid or liver causes, or certain medications. A high RDW means cell sizes are mixed, which often happens with low iron, B12 or folate.

Hemoglobin falls late in iron deficiency, so a normal hemoglobin does not rule out low iron. If you are tired or have heavy periods, ask about ferritin.

White blood cells and the differential

The total white cell count rises with infection, inflammation, stress, steroids, smoking and some allergic reactions. It can fall with some medications, chemotherapy, autoimmune conditions, bone marrow problems and some infections such as HIV.

The differential breaks the total into five types:

• Neutrophils. The most common type and the first responders to bacterial infection.
• Lymphocytes. B and T cells that fight viruses and remember past infections.
• Monocytes. Clear germs and dead cells and support the immune response.
• Eosinophils. Fight parasites and rise with allergies and asthma.
• Basophils. Release histamine in allergic reactions.

Platelets and MPV

Platelets plug small breaks in blood vessels so bleeding stops. A low count can come from viral infections, autoimmune conditions, liver scarring (cirrhosis), low vitamin B12 or bone marrow problems. A high count can follow infection, inflammatory conditions such as inflammatory bowel disease or rheumatoid arthritis, anemia or heavy blood loss. MPV is the average platelet size and is read alongside the count.

What to do with an abnormal CBC

1. Look at the pattern, not one flag. A single value just outside the range, with everything else normal, is often a normal variation.
2. Note anything that can shift results: recent illness, a hard workout, dehydration, your period, smoking, altitude and new medications.
3. Ask about the follow-up tests that fit. Low hemoglobin or small cells usually lead to ferritin and an iron panel. Large cells lead to vitamin B12, folate, thyroid and liver tests. Unusual white cell or platelet counts may lead to a repeat CBC and a blood smear.
4. Recheck after a change. Red cells live about 120 days, so red cell measures move over weeks to months. White cells and platelets can change in days.

See a clinician promptly if an abnormal CBC comes with shortness of breath, chest pain, fainting, black or bloody stools, unusual bruising or bleeding, fevers or night sweats that won't go away, or unexplained weight loss.