Lab results explained
Complete Blood Count (CBC) Explained: Red Cells, Iron and B12
A complete blood count (CBC) is one of the most common blood tests. It counts your red cells, white cells and platelets and describes the size and color of your red cells. When something looks off, ferritin and vitamin B12 are often the next tests. This page explains how they connect. Each marker has its own detailed guide.
Tests in this panel
- RDW (red cell distribution width)How much your red cells vary in size. Often rises early in iron or B12 deficiency.Read the guide
- MCH (mean corpuscular hemoglobin)How much hemoglobin each red cell carries. Low with iron deficiency, high with low B12 or folate.Read the guide
- Ferritin (iron stores)Your body's iron stores. The most useful single test for iron deficiency.Read the guide
- Vitamin B12A vitamin needed to make healthy red cells and nerves. Low levels can enlarge red cells.Read the guide
What a CBC measures
A CBC reports hemoglobin and the red cell count, the white cell count (often with a breakdown of the different types) and platelets. It also lists red cell indices: MCV (average size), MCH and MCHC (hemoglobin per cell) and RDW (variation in size). These indices help explain why hemoglobin is low or high.
Small cells, large cells
Clinicians often sort anemia by red cell size. Small, pale cells (low MCV and MCH) most often point to iron deficiency, which ferritin can confirm. Large cells (high MCV and MCH) can come from low vitamin B12 or folate, heavy alcohol use, liver disease, an underactive thyroid or some medicines. A high RDW adds that the cells are a mix of sizes, which can show up early, before hemoglobin falls.
Why ferritin and B12 are checked next
Ferritin shows how much iron your body has stored, and a low ferritin confirms iron deficiency even when hemoglobin is still normal. Vitamin B12 is checked when red cells are large or there are symptoms such as tingling, numbness or memory problems. Finding the cause matters: in adults, iron deficiency often leads to a search for blood loss.
What else can change a CBC
Recent infection, pregnancy, heavy periods, endurance training, smoking, living at high altitude and dehydration can all shift CBC values. Inflammation can raise ferritin and hide low iron stores, so ferritin is often read together with CRP.
White cells and platelets
A CBC also checks white cells, which fight infection, and platelets, which help blood clot. Values just outside the range are common and often settle on a repeat test. Values far outside the range, or several abnormal values together, need prompt follow-up with your clinician.
Frequently asked questions
Do I need to fast for a CBC?
No. Food does not affect a CBC. If ferritin, iron or other tests are drawn at the same time, your clinician may ask you to fast or come in the morning.
Can I have iron deficiency with a normal hemoglobin?
Yes. Iron stores run down before hemoglobin falls, so ferritin can be low while the CBC still looks normal. Tiredness, hair shedding or restless legs can appear at this stage.
What is the difference between MCV and MCH?
MCV is the average size of your red cells; MCH is the average amount of hemoglobin in each one. They usually move together: both are low in iron deficiency and both are high when B12 or folate is low.
Should I take iron or B12 if my CBC is off?
Not before the cause is known. Iron taken when you do not need it can build up in the body, and starting iron or B12 can blur later tests. Talk to your clinician about which test comes next.
Keep your blood counts in context
Add your results to VHT to see your CBC, ferritin and B12 together over time, and ECHNA, your AI health assistant, explains what changed in plain language.
