What are RBC Indices?
Red Blood Cell (RBC) indices are calculated values derived from the Complete Blood Count (CBC) that describe the size, hemoglobin content, and hemoglobin concentration of red blood cells. They are essential tools for classifying anemias and directing the diagnostic workup toward the underlying cause.
The three primary RBC indices are:
- MCV (Mean Corpuscular Volume): The average volume (size) of a single red blood cell
- MCH (Mean Corpuscular Hemoglobin): The average mass of hemoglobin per red blood cell
- MCHC (Mean Corpuscular Hemoglobin Concentration): The average concentration of hemoglobin in a given volume of packed red blood cells
RBC Indices Formulas
Note: Hematocrit is expressed as a percentage (e.g., 42%). In some formulas, it is expressed as a decimal (0.42). The formulas above use the percentage form.
Normal Ranges
| Index | Normal Range | Unit | What It Measures |
|---|---|---|---|
| MCV | 80 – 100 | fL (femtoliters) | Average RBC volume (size) |
| MCH | 27 – 33 | pg (picograms) | Average hemoglobin mass per RBC |
| MCHC | 32 – 36 | g/dL | Average hemoglobin concentration in RBCs |
RBC Morphology Diagram
Anemia Classification by MCV
| MCV Category | MCV Range | Common Causes | Key Lab Findings |
|---|---|---|---|
| Microcytic (<80 fL) | < 80 fL | Iron deficiency, thalassemia, chronic disease, sideroblastic anemia, lead poisoning | Low ferritin, low iron, high TIBC (iron deficiency); low MCV with normal/high iron (thalassemia) |
| Normocytic (80–100 fL) | 80–100 fL | Chronic disease, acute blood loss, hemolysis, renal failure, aplastic anemia | Variable; reticulocyte count helps distinguish |
| Macrocytic (>100 fL) | > 100 fL | B12 deficiency, folate deficiency, alcoholism, liver disease, hypothyroidism, medications (methotrexate, AZT) | Hypersegmented neutrophils (megaloblastic); round macrocytes (non-megaloblastic) |
MCHC Interpretation
| MCHC Status | Range (g/dL) | Term | Associated Conditions |
|---|---|---|---|
| Low MCHC | < 32 | Hypochromic | Iron deficiency anemia, thalassemia, sideroblastic anemia |
| Normal MCHC | 32 – 36 | Normochromic | Normal; or normochromic anemias |
| High MCHC | > 36 | Hyperchromic | Hereditary spherocytosis, severe burns, specimen error (lipemia, cold agglutinins) |
Note: An MCHC above 36 g/dL is relatively uncommon and should prompt consideration of spherocytosis or specimen artifact. True hyperchromia is limited by the maximum hemoglobin concentration a red blood cell can contain.
Iron Studies & Further Workup
When RBC indices suggest microcytic anemia, iron studies are typically the next step:
| Test | Iron Deficiency | Chronic Disease | Thalassemia |
|---|---|---|---|
| Serum Iron | Low | Low | Normal/High |
| TIBC | High | Low/Normal | Normal |
| Ferritin | Low (<12) | Normal/High | Normal/High |
| Transferrin Sat. | Low (<16%) | Low/Normal | Normal/High |
For macrocytic anemias, check serum B12 and folate levels, reticulocyte count, and review the peripheral blood smear for megaloblastic changes (hypersegmented neutrophils, oval macrocytes).
Worked Example
A patient with Hemoglobin 10.5 g/dL, Hematocrit 32%, and RBC count 3.8 ×10&sup6;/µL:
This pattern of normocytic, normochromic anemia suggests chronic disease, acute blood loss, or hemolysis as likely causes. Further workup should include reticulocyte count, peripheral smear, and evaluation for underlying chronic conditions.
Frequently Asked Questions
What does a low MCV mean?
A low MCV (below 80 fL) indicates microcytic red blood cells that are smaller than normal. The most common cause worldwide is iron deficiency anemia, followed by thalassemia trait. A useful discriminator is the Mentzer index (MCV/RBC): values greater than 13 favor iron deficiency, while values less than 13 favor thalassemia trait.
Can MCV be normal in iron deficiency?
Yes, especially in early iron deficiency. MCV may remain normal until iron stores are significantly depleted. Ferritin is a more sensitive early marker of iron deficiency than MCV. Combined iron deficiency and B12/folate deficiency can also result in a "normal" MCV as the opposing effects cancel out.
What causes elevated MCHC?
The most clinically significant cause of elevated MCHC is hereditary spherocytosis, where red blood cells lose membrane surface area and become spherical, concentrating their hemoglobin. However, spurious MCHC elevation due to specimen errors (lipemia, cold agglutinins, hemolysis) is more common than true hyperchromia.
How do RBC indices relate to the peripheral blood smear?
RBC indices provide numerical data that corresponds to visual findings on the peripheral smear. Low MCV corresponds to microcytes, high MCV to macrocytes, low MCHC to hypochromic cells (increased central pallor), and high MCHC to spherocytes (no central pallor). The smear provides additional information about RBC shape, inclusions, and other cell lines that indices cannot capture.