MCV, MCH, and RDW: the red cell indices decoded

In shortMCV, MCH, and RDW are the red blood cell indices: the average size of red cells, the hemoglobin amount per cell, and the variation in cell size. They exist to classify anemia — the pattern of the three points toward its cause.

What each index measures

The red blood cell indices are part of the complete blood count and are mostly used to help diagnose the cause of anemia. MCV — mean corpuscular volume — is the average size of a red blood cell. MCH is the average amount of hemoglobin per cell, and its close relative MCHC is the hemoglobin concentration inside the cells. RDW describes how much the cell sizes vary.

Together they turn “hemoglobin is low” into a description: whether the red cells are small or large, pale or normally colored, uniform or mixed. That description is what separates one kind of anemia from another.

Why cell size classifies anemia

Different causes shrink or swell red cells in characteristic ways. Iron deficiency produces small cells; B12 and folate deficiency produce large ones. A low MCV therefore points the workup toward iron studies, while a high MCV points toward the B vitamins — which is why the indices decide the next test rather than just decorating the report.

RDW adds the texture: a high RDW means the cell population is uneven — a mix of small and large cells — which happens when a deficiency is developing and young and old cells differ in size.

Reading the indices together

No index is read alone. MCV without hemoglobin is a size without a story; hemoglobin without MCV is a shortage without a shape. The report places them side by side precisely so that the pattern — size, content, variation — can be matched against the classic anemia profiles.

And the same rule applies here as everywhere on the report: the reference ranges printed by the laboratory that ran the sample are the only valid yardstick, because methods and therefore ranges differ between laboratories.

The four indices in one glance

MCV — mean corpuscular volume: the average size of a red blood cell
MCH — mean corpuscular hemoglobin: the hemoglobin amount per cell
MCHC — the hemoglobin concentration inside the average cell
RDW — red cell distribution width: how much cell sizes vary

Markers covered in this article

Hemoglobin Red blood cells (RBC)Hematocrit (Hct)Ferritin Vitamin B12

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Frequently asked questions

What does MCV tell the provider?

The average size of the red blood cells. Small cells (low MCV) point toward iron deficiency; large cells (high MCV) toward B12 or folate deficiency. It is the main line for classifying an anemia once hemoglobin is found low.

Is RDW a separate disease marker?

No. RDW describes how varied the red cell sizes are. A high value means a mixed population of small and large cells, typical of a developing deficiency. It refines the picture the other indices draw; it does not diagnose anything alone.

Why are the indices part of a routine CBC?

Because they cost nothing extra on an automated counter and turn a low hemoglobin into a classified finding: size and hemoglobin content per cell narrow the list of causes before any further test is ordered.

Sources

RBC Indices: MedlinePlus Medical Encyclopedia — MedlinePlus, National Library of Medicine. Accessed: August 29, 2026.
Complete Blood Count (CBC): MedlinePlus Medical Test — MedlinePlus, National Library of Medicine. Accessed: August 29, 2026.