Definition of red cell indices?
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Here is a comprehensive definition and breakdown of red cell indices based on authoritative medical textbooks:
Red Cell Indices
Red cell indices are a set of calculated parameters that describe the size and hemoglobin content of red blood cells (RBCs). They are routinely reported as part of a complete blood count (CBC) and are derived by most electronic cell counters.
"Red cells can also be described by parameters called red cell indices, which are routinely calculated by most electronic cell counters."
- Goldman-Cecil Medicine
The Classic Red Cell Indices
| Index | Full Name | Formula | Normal Range | What it Reflects |
|---|---|---|---|---|
| MCV | Mean Corpuscular Volume | Hct / RBC count × 10 | 85-95 fL | Average size of a red cell |
| MCH | Mean Corpuscular Hemoglobin | Hgb / RBC count × 10 | 28.5-32.3 pg | Average weight of Hgb per cell |
| MCHC | Mean Corpuscular Hemoglobin Concentration | (Hgb / Hct) × 100 | 33.8-34.2 g/dL | Average concentration of Hgb per cell |
| RDW | Red Cell Distribution Width | CV of red cell volume | ~11.5-14.5% | Degree of variation in cell size (anisocytosis) |
- Harrison's Principles of Internal Medicine, 22E, Table 66-1
- Quick Compendium of Clinical Pathology, 5th ed., §4.4.1
Individual Index Details
1. MCV (Mean Corpuscular Volume)
- The most clinically used index.
- Directly measured by electronic counters (by sensing cell volume) or calculated as: Hct (%) × 10 / RBC count (×10¹²/L).
- Classifies anemia into microcytic (<80 fL), normocytic (80-100 fL), or macrocytic (>100 fL).
- Note: pronounced reticulocytosis can falsely raise the MCV.
2. MCH (Mean Corpuscular Hemoglobin)
- Varies linearly with MCV and therefore adds limited independent information.
- Low in hypochromic states (iron deficiency, thalassemia).
3. MCHC (Mean Corpuscular Hemoglobin Concentration)
- Changes very little in most anemias and is therefore of limited value for classification.
- High MCHC is a key clue to: hereditary spherocytosis, cold agglutinin disease, or lipemic specimens.
4. RDW (Red Cell Distribution Width)
- Reflects the coefficient of variation (CV) of red cell volume, i.e., how uniformly sized the cells are.
- High RDW = anisocytosis (mixed cell sizes), as seen in iron deficiency anemia (IDA).
- Clinical use: Helps distinguish IDA (high RDW) from thalassemia trait (normal or mildly elevated RDW) - both are microcytic.
Additional / Modern Indices (from automated counters)
- Reticulocyte Hemoglobin Concentration (CHr / Ret-He): reflects availability of iron in the marrow; a marker of functional iron deficiency.
- Immature Reticulocyte Fraction (IRF): indicates bone marrow erythropoietic activity; higher fraction = more immature reticulocytes.
- Hematocrit (Hct): also derivable as MCV × RBC / 10.
Clinical Significance
The combination of MCV, RDW, and the Reticulocyte Production Index (RPI) examined together allows significant narrowing of the differential diagnosis of anemia, even if no single index is pathognomonic.
"Although these red cell indices are not pathognomonic of the cause of a particular type of anemia, the combination of MCV, RDW, and RPI examined together will often significantly narrow the differential."
- Henry's Clinical Diagnosis and Management by Laboratory Methods
Sources:
- Harrison's Principles of Internal Medicine, 22E
- Goldman-Cecil Medicine
- Henry's Clinical Diagnosis and Management by Laboratory Methods
- Quick Compendium of Clinical Pathology, 5th ed.
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