Complete blood count interpretation – cells and numbers (Proceedings)
A complete blood count (CBC) is a useful and very often-used method of screening and diagnosing patients who present for a wide variety of conditions. It may include both quantitative information about cell numbers, sizes, variability, etc. as well as descriptive information based on evaluation of a blood smear and description of any morphologic abnormalities or infectious agents present. A CBC is probably the most useful when both types of information are reviewed.
Classification of anemiasIf a patient has a significant anemia, an important step is to figure out if the anemia is regenerative or non-regenerative. Being able to classify an anemia in this way is often key to determining the underlying cause, and thus, the most appropriate treatment and prognosis. Regenerative anemias are most often due to blood loss or hemolysis, while non-regenerative anemias are usually referable to a problem with effective bone marrow production of erythroid cells.
Sometimes it's relatively easy to tell whether an anemia is regenerative but other times it is not and it's very useful to have several different tools that can be used to evaluate these cases.
Classification by cell appearance
Reviewing a blood smear from an anemic patient can provide some very good subjective information, as well as occasionally revealing an underlying etiology, such as Mycoplasma spp. Regenerative anemias will (except in horses) show polychromasia and anisocytosis. Polychromatophilic (bluish) cells are immature erythrocytes released prematurely from the bone marrow, and these are usually also macrocytic cells. There are, however, some causes of larger-than-normal, or macrocytic cells that are not part of a regenerative response. These include FeLV infection in cats, bone marrow disorders (myelodysplasia), folate deficiency, and poodle macrocytosis.
Nucleated red blood cells, also known as metarubricytes, can also be seen as part of a regenerative response as they are erythrocytes that are even less mature than polychromatophilic cells. These cells may also be seen in other conditions, however, including lead toxicity, hemangiosarcoma or other splenic disorder, corticosteroid influence, and bone marrow disorders so they are not specific indicators of regeneration.
Similarly, basophilic stippling may be seen occasionally in a very regenerative anemia, especially in ruminants and sometimes cats and dogs, but is does not confirm regeneration, as it also may be seen in acute lead poisoning.
Although reviewing a blood smear may give a very good subjective view of the regenerative response (or lack thereof), in some cases, a more quantitative and accurate way of assessing regeneration is to perform a reticulocyte count. This may be easily done manually by mixing equal parts of fresh blood and new methylene blue stain. The blood and stain mixture is allowed to incubate for 10-15 minutes and then a blood smear made in the usual manner and air-dried. Some of the newer in-house hematology analyzers will provide a reticulocyte count as part of a CBC.
Usually, the percentage of reticulocytes out of 1000 red blood cells is counted. In dogs, this is relatively straightforward, but in cats, the distinction is made between aggregate and punctuate reticulocytes. Aggregate reticulocytes are those that have clumped or "stringy" dark blue reticulum, while punctuate reticulocytes have dots of reticulum. The punctuate reticulates are older as they can circulate for up to two weeks. Thus, they don't really reflect the current regenerative ability of the bone marrow and are not typically counted in a reticulocyte count.
The percentage of reticulocytes can be misleading as it doesn't allow and correct for the degree of anemia. There are a couple of ways that this can be handled. One is by calculating an absolute number, rather than percentage of reticulocytes. This can only be done if the RBC number is known. As normal, non-anemic animals can have up to 60,000 reticulocytes/ml, an animal with a regenerative anemia should have > 60,000 reticulocytes/ml with markedly regenerative anemias associated with > 200,000 reticulocytes/ml. If the absolute RBC count is not done, the correction for degree of anemia can be done by using the patient's PCV divided by the "normal" PCV for the species (e.g. 45% for dogs) as a correction factor. This factor is multiplied by the reticulocyte percentage obtained to come up with a corrected reticulocyte percentage.
In reviewing reticulocyte counts or percentages to determine whether regeneration is occurring, it's important to keep in mind that, if there is a sudden demand for erythrocytes (i.e. acute onset of anemia), it will take 2-3 days before any reticulocytes will be seen in the peripheral blood and they will not peak until 4-7 days in dogs. Some people use the term "pre-regenerative" to describe the situation when reticulocytes and other indicators of regeneration are not seen in the blood, but there has not been sufficient time for them to appear.