Polychromasia: Definition, Characteristics, Clinical Significance and Cell Description

The term suggests that red blood cells stain many colors.

In practice, it means that some of the red blood cells stain shades of bluish-gray: these are the reticulocytes. The cells that stain shades of blue, ‘polychromasia blue’, are unusually young reticulocytes.

Blue polychromasia is most commonly seen when there is a strong erythropoietic impulse or when there is extramedullary erythropoiesis, as, for example, in primary myelofibrosis or carcinomatosis.

It should be noted that, in certain circumstances, the absence of polychromasia is significant; in a severely anemic patient indicates that the bone marrow response is inadequate (eg, in aplastic anemia and pure red cell aplasia).

Polychromasia is a feature of immature enucleated erythrocytes (which are also aggregated reticulocytes) in the blood.

Immature red blood cells are blue because they contain moderate to large amounts of RNA (ribosomes, polyribosomes) that compensate for the red in hemoglobin, imparting a purple color to the cells.

In many species, once the cell reaches the reticulocyte stage, it remains in the bone marrow for about 2 days, then is released to complete its maturation by losing its RNA and part of its surface membrane while circulating. This is usually accomplished in the spleen.

Consequently, a low number of polychromatophils is observed in healthy dogs (<1.5% reticulocytes). Immature RNA red blood cells are not released from the marrow in normal horses and ruminants. In all species except the horse, it is regenerative if sufficient polychromatophils are present.

The horse (and other equines) is an exception, as they generally do not release polychromatophils in response to anemia (they release cells that are larger than normal, called macrocytes).

In dogs and cats, the number of reticulocytes can be quantified as a percentage or an absolute count.

There is often confusion as to the meaning of the terms, reticulocytes and polychromatophils (polychromatophilic erythrocytes).

Polychromasia is used to describe red blood cells in PBF that have their cytoplasm stained shades of blue-gray or purple (rather than their normal pink / red color). These are previously mature red blood cells released from the bone marrow during blood formation.

Polychromasia is usually an indication of stress on the bone marrow, as well as immature red blood cells. These young premature erythrocytes are conveniently called reticulocytes, however, not all reticulocytes are polychromatophilic.

The dark blue-gray / purple staining seen in polychromatic red blood cells is due to ribosomes remaining in immature blood cells, which are not found in mature red blood cells.

The typical lifespan of an erythrocyte is approximately 120 days and the duration of a reticulocyte found in the bloodstream is one day.

Clinical significance

The percentage of reticulocytes calculated to be in the blood at any given time indicates the rapidity of red blood cell turnover in a healthy patient. However, the number of reticulocytes reflects the amount of erythropoiesis that has occurred on a given day.

The absolute number of reticulocytes is called the reticulocyte index and is calculated by adjusting the percentage of reticulocytes by the ratio of the observed hematocrit to the expected hematocrit to obtain the ‘corrected’ reticulocyte count.

Cell description

The polychromatic red blood cells, as mentioned above, appear to be more densely stained and have shades of purple / grayish blue. Sometimes it can appear noticeably larger than normal red blood cells.


Distinction between immature red blood cells in a standard blood sample (Wright’s stain) and reticulocytes (new methylene blue stain):

Reticulocytes are RNA-containing immature enucleated erythrocytes that stain blue with non-methylene blue (NMB) staining or fluoresce with RNA-binding dyes (eg, oxazine).

When they contain moderate or large amounts of RNA, they are called aggregated reticulocytes and stain purple in a Wright or Diff-quick stain, but if they contain only a little RNA, they are called stippled reticulocytes, they will not stain purple and will be red .

Therefore, there are always more reticulocytes than polychromatophils (polychromatophils are just aggregated reticulocytes and are not stippled reticulocytes).

This distinction is important in cats, where only aggregated reticulocytes (or polychromatophils) are counted as part of the regenerative response.


Polychromatophils are reticulocytes that contain enough RNA to stain blue-purple with Wright’s stain. They consist of the most immature reticulocytes (i.e. aggregated reticulocytes), as they contain the highest amount of RNA.

All polychromatophils are reticulocytes, however, not all reticulocytes are polychromatophils in a Wright stained blood smear, as mentioned above).

Reticulocyte evaluation helps determine if an anemic patient has a healthy marrow response to anemia, as indicated by an increased number of young red blood cells produced to replace lost red blood cells.

The interpretation of the reticulocyte count differs with species.

Phantom red blood cells

Red blood cell ghosts represent cells that have broken down in the circulation, losing their hemoglobin. The remaining red blood cell membranes look like “ghosts.”

Phantom red blood cells represent lysis of red blood cells (hemolysis). This can be a true in vivo finding or an in vitro artifact.