Lecture #6: Erythrocyte Indices

The red cell indices are used to determine the morphologic type of anemia, which is helpful in outlining a diagnostic approach to the patient with anemia. These are useful only, if they are reasonably accurate, and this depends upon the accuracy of the measurement from which they are calculated. Three accurately determined values are needed: red cell count, hemoglobin and hematocrit.

1. Mean Corpuscular Volume (MCV) – this is the average volume of the individual red cell in cubic microns or femtoliter. It is increased in macrocytic anemia and decreased in microcytic anemia.

1 femtoliter = 10^–15 liter = 1 cubic micron

Formula:    Hct% x 10                                =          MCV in fl.

                  RBC count (millions/ul

Normal values:     90 ± 8 fl. Per cell

2. Mean Corpuscular Hemoglobin (MCH) – this is the content (weight) of hemoglobin of the average red cell in micrograms or pictograms. It is increased in macrocytic anemia and lowered in hypochromic anemia.

1 picogram = 10^–12 grams = 1 micromicrogram

Formula:    Hemoglobin (g/liter)               =          MCH in pg

                  RBC count (millions / ul)

Normal values:     30 ± 3 pg

3. Mean Corpuscular Hemoglobin Concentration (MCHC) – this is the average concentration of hemoglobin in a given volume of packed red cells. It is elevated only in hereditary spherocytosis but not in the macrocytic anemias. Decreased values are found in hypochromic anemias.

Formula:

Hemoglobin (g/liter)              x          100      =          MCHC in %

RBC count (millions/ul)

Normal values:     34 ± 2%

4. Color Index (C.I.) – this is the average volume of an erythrocyte as compared with the average volume of a normal erythrocyte. This is increased in macrocytic anemia and decreased in microcytic anemia and hypochromic anemia.

Formula:    Hemoglobin (g/dl) x 6.9                      =          C.I.

                  RBC (millions) x 20

Normal values:     0.9 to 1.1  

5. Volume Index (V.I.) – this is the average volume of erythrocytes as compared with the average volume of normal erythrocyte.

Formula:    Hematocrit % x 2.3                 =          V.I. 

                  RBC (millions) x 20

6. Saturation Index (S.I.) – this is the average amount of hemoglobin in an erythrocyte per volume of cell in relation to normal.

Formula:    Color Index                  =          S.I.

                  Volume Index

Normal values:     0.9 to 1.1

7. Mean Corpuscular Average Thickness (MCAT) – assuming that the red cell is in the form of a short cylinder, the thickness can be calculated as follows:

               MCV                         =     MCAT in micrometers           

      Pi (3.1416) (MCD)²

                              2

Normal values:     1.7 to 2.1 micrometers

8. Mean Corpuscular Diameter (MCD) – this is measured in microns or micrometers by the following special methods:

a.      Price Jones method

b.      Diffraction or Halometer method

Normal values:     6.8 to 7.2 micrometers

The rule of three:

            3 x RBC                                    =          Hemoglobin

            3 x Hemoglobin                       =          Hematocrit ± 3%