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The page below is a sample from the LabCE course Describing a Red Blood Cell Population Using RBC Indices and Red Cell Distribution Width. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Red Cell Distribution Width (RDW) Correlated with Peripheral Smear

As discussed earlier, the RDW indicates the amount of variation in the size of RBCs in the blood sample. A normal RDW indicates a homogenous population of RBCs, which means the cells are of similar sizes. The RDW does not necessarily indicate that the cells are normocytic but rather that they are homogenous. When the peripheral smear is examined, the RBCs within the population will be of similar size, as seen in the top image on the right.
An increase in RDW indicates that the red cell population is not homogenous. When the peripheral smear is evaluated, the observations should include variation in RBC sizes (heterogenous population), as seen in the bottom image on the right.
The RDW can also be correlated with the histogram that is produced by the automated blood cell counter. A narrow distribution curve indicates a homogenous population of red cells, as seen in Figure 6
Figure 6

The wider the distribution curve, the more heterogenous the population of red cells, as shown in Figure 7.

Figure 7