It is important to correlate what is observed on a peripheral blood smear with the RBC parameters obtained on the automated blood cell count. The red cell indices and red cell distribution width (RDW) are considered "descriptors" that may assist the practitioner with diagnosis of red blood cell disorders, particularly anemias. Visually observing the RBCs on a smear is often done to confirm the parameters reported by the hematology instrument. The following procedures are recommended when evaluating the RBC population:
Mean Cell Hemoglobin
- Examine the hemoglobin and hematocrit for agreement-- If the red cells are normochromic and normocytic, the hemoglobin X 3 will approximately equal the hematocrit
- Following evaluation of hemoglobin and hematocrit, the next parameter to be evaluated is the MCV. This value represents the average volume of the RBCs present in the sample. The MCV will provide a clue to the size of RBCs you can expect to observe on the blood smear. General guidelines for the correlation includes:
- MCV within normal limits -- normal red blood cell size (normocytic)
- MCV below normal limits -- small red blood cell size (microcytic)
- MCV greater than normal limits -- large red blood cell size (macrocytic)
- Note that this general categorization may be deceptive, as a mixture of small and large cells may give an MCV that will be within normal limits.
- Evaluate the MCHC to determine the amount of hemoglobin contained within the red blood cells. If the MCHC is within the normal limits, the amount of hemoglobin in the red blood cells is sufficient with an appropriate central pallor. These cells are described as normochromic, as shown in the top image on the right.
- If the MCHC is below the normal limits, the expectation is that the amount of hemoglobin within the red blood cells will be reduced with an increased central pallor. These cells are described as hypochromic and are illustrated in the bottom image on the right.
- An MCHC that is above the normal limits occurs infrequently. Spherocytes are the only true hyperchromic cells (although not reported as "hyperchromic"). These cells have a decreased surface area that create more concentrated content. Note that interference from lipemia, hemolysis, and red cell agglutination may falsely elevate the hemoglobin and consequently impact the calculation of the MCHC.
- The RDW is examined by observing the width of the red blood cell distribution curve. This value is reflected statistically as the coefficient of variation (CV) or standard deviation (SD).
- The greater the CV or SD, the more variable the size of the cells and hence the wider the curve.
- If the cells are all similar size, the RDW will be within normal limits and the curve will be narrow.
- When the RDW is outside the normal range, a red cell population of variable sizes will be observed on the peripheral blood smear. A red cell population that is variable in size is termed "anisocytosis."
- A red cell distribution curve that is shifted either left or right of the "normal" peak is labeled as having a "left shift" or "right shift" respectively. The left shift is indicative of a population of cells that is smaller than normal (<6 µm) while a right shift is indicative of a population of cells that is larger than normal (>8 µm).
Remember that the MCH provides a measurement of the hemoglobin in the average red blood cell, where MCHC provides the ratio of the hemoglobin to the volume
of the average cell. MCV and MCHC are the indices that are most often evaluated when anemia is being considered and the amount of hemoglobin produced parallels changes in the MCV. However, when evaluating conditions with a decreased red cell volume, such as thalassemia or iron deficiency, or increased absolute erythrocyte hemoglobin, as happens with hemochromatosis, changes in the volume makes the MCHC less useful. The MCH does not often change. Therefore, when it decreases, it may signal a condition such as thalassemia, iron deficiency, or hemochromatosis.