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A CBC is performed using an automated laboratory instrument which measures the numbers of RBC's, WBC's, and platelets per unit volume. In addition, the RBC indices are generated. Some instruments can also generate an automated WBC differential count.
A standard complete blood count is performed on an automated laboratory instrument that quantitates the amount of Hgb as well as the size, shape, and number of RBC's. A variety of calculations are performed to produce indices that provide information about RBC disorders. The standard indices are:
The number of RBC's per unit volume is measured directly and given in millions per microliter.
The MCV is measured directly; the unit is a femtoliter. The MCV measures the size of RBC's and is the most important index for classification of anemias into "macrocytic" with higher than normal MCV and "microcytic" with low MCV.
The Hgb content is measured directly and given in grams per deciliter. This value, along with Hct, provides the most useful measure of the oxygen carrying capacity of the blood.
The Hct is a calculated value and provides a measure of the amount of oxygen carrying capacity of the blood.
The MCH is calculated and gives the average mass of Hgb in an individual RBC; the unit is a picogram.
The MCHC is calculated and provides a measure of the concentration of Hgb in the cells.
The RDW is calculated to provide a measure of the anisocytosis, or variation in size of the RBC's.
An example of the data generated by an automated instrument for performing a CBC is shown below. The graph at the lower right demonstrates the red cell distribution width (RDW) by relative numbers of sizes of RBC's as measured in femtoliters (fl).
In addition to these indices obtained from an automated instrument, a reticulocyte count (RC) can be calculated. The "retic" count is generated manually, because a peripheral smear must be stained with a supravital dye so that the number of reticulocytes can be enumerated. Most retic counts are given as a "corrected reticulocyte count" which compensates for a falsely elevated retic count with anemia: RC (corrected) = RC x (patient Hct ÷ 45).
The following terms are used in describing the morphology of RBC's, as seen on a standard peripheral blood smear:
| Acanthocyte | Cell with irregular, long, asymmetrical projections; sporadically seen with severe liver disease or post-splenectomy; seen with rare disorder of abetalipoproteinemia |
| Anisocytosis | Variation in size of RBC's |
| Babesiosis | Similar to ring forms of malaria |
| Basophilic stippling | Small aggregates of RNA are seen as small blue dots in the RBC; fine stippling may be a feature of reticulocytes; coarse stippling can appear with toxic marrow damage, myelodysplasia, and thalassemias |
| Elliptocyte | Elongated, elliptical cell; non-specific when occasionally seen; rare disorder of hereditary elliptocytosis |
| Heinz body | Precipitated Hgb seen as a perimembranous blue dot only after supravital staining; seen with some hemoglobinopathies |
| Howell-Jolly body | Small, round deeply basophilic nuclear remnant; seen when spleen is absent |
| Hypochromia | Cells with decreased MCH, typical of iron deficiency |
| Macrocytosis | Cells with increased MCV, typical of megaloblastic anemias |
| Malaria | Plasmodium infection seen as "ring forms", stippling, and gametocytes, depending upon the species |
| Microcytosis | Cells with decreased MCV, typical of iron deficiency anemia and thalassemias |
| Pappenheimer body | Multiple, tiny iron containing granular blue dots; seen when spleen is absent and with iron overload |
| Poikilocytosis | Variation in shape of RBC's |
| Polychromatophilia | The bluish tint to young RBC's with high RNA content |
| Reticulocyte | Young RBC's with increased RNA content that can be precipitated by supravital staining for identification and enumeration |
| Rouleaux | Linear aggregation of RBC's that resembles a stack of coins; seen when surface charge is reduced with increased serum protein, particularly increased fibrinogen or globulin |
| Sideroblast | Non-nucleated RBC with stainable iron |
| Spherocyte | Small, round dense cell without central pallor; suggests extravascular (splenic) hemolysis in previously normal persons; with hereditary spherocytosis there is increased osmotic fragility |
| Schistocyte | Fragmented, irregularly shaped seen with intravascular hemolysis such as microangiopathic hemolytic anemias (DIC, TTP). A variant called a "helmet cell" appears cut in half |
| Sickle cell | Curved, banana-shaped cell with pointed ends found in sickle cell disease from aggregation of Hgb S |
| Stomatocyte | Cell with slit-like central pallor; occasional stomatocytes are non-specific or an artefact; many are seen with rare hereditary stomatocytosis |
| Target cell | Cell with central and peripheral staining with intervening pallor due to increased redundancy of RBC membrane; seen with liver disease, in some thalassemias, and with Hgb C |
| Tear drop cell | Cell pinched at one end, prominent in myelofibrosis and myelophthisic conditions |
Morphologic changes seen in RBC's on a peripheral blood smear include the findings pictured below. Moving the mouse over each image will reveal the name in the status bar at the bottom.
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The following images illustrate findings with RBC's in peripheral blood smears: