Absolute retic count

To determine the adequacy of the regenerative response in dogs and cats, absolute reticulocyte counts can be calculated. This is the method used at Cornell University for determining the adequacy of a regenerative response as it takes into account the severity of the anemia but does not make assumptions about a “normal” hematocrit or correction factors for reticulocyte release. The reticulocyte percentage alone is not as useful as it does not account for the severity of the anemia. The absolute reticulocyte count is calculated automatically in our laboratory from the reticulocyte % in dogs and cats (whenever a reticulocyte count is automatically added to a hemogram or a reticulocyte count is requested for a sample that also has a red blood cell [RBC] count). To calculate an absolute reticulocyte count, we use the following formula:

Absolute reticulocyte count (thou/µL) = reticulocyte % (#/100) x RBC count (mill/µL) x 1000

or (even easier)

Absolute reticulocyte count (thou/µL) = reticulocyte % x RBC count (mill/µL) x 10

Outlined below are approximate guidelines for assessing the degree or adequacy of regeneration in anemic dogs and cats, along with examples of using the results. Remember that cats have two different types of reticulocytes, aggregate and punctate. Only the aggregate reticulocytes are included in a reticulocyte count (as they are more reflective of current bone marrow production due to their short lifespan of <24 hours). 

We can use the above table as a guideline for assessing the adequacy of the regenerative response, while taking into account the degree of anemia. For example, a dog with a severe anemia and a hematocrit (HCT) of 15% and a RBC count of 1.5 mill/µL has a reticulocyte count of 3%, which is above our reference interval of 0-1.5%. Based on the reticulocyte percentage alone, you would consider the severe anemia is regenerative, however the absolute reticulocyte count is 45 thou/µL (3/100 x 1.5 x 1000 or 3 x 1.5 x 10), which indicates inadequate or no regeneration based on the severity of anemia (using the above table). This indicates that the bone marrow is not responding appropriately to the severe anemia (as long as the bone marrow has had sufficient time to respond, i.e. 3-5 days) and that decreased production of erythrocytes is contributing to the anemia (i.e. the anemia is multifactorial) or is the main cause of the severe anemia. In contrast, a different dog with the same low HCT of 15% and a reticulocyte count of 12%, which is also above our reference interval. The absolute reticulocyte count is 180 thou/µL (12/100 x 1.5 x 1000 or 12 x 1.5 x 10), which indicates a mild regenerative response and you would consider causes of blood loss or hemolysis for the severe anemia or a multifactorial anemia, with inflammation suppressing the regenerative response. The stimulus for erythropoiesis should also be considered when determining adequacy of regeneration. A mild anemia is not expected to stimulate as strong an erythropoietic drive as a severe anemia.

An alternative explanation for a high absolute reticulocyte count (and % reticulocyte count) in an anemic or non-anemic animal is a drive for erythropoiesis, such as lower oxygen saturation of hemoglobin (e.g. cardiopulmonary disease), localized renal hypoxia (could be present in many diseases) stimulating erythropoietin release, or stress erythropoiesis (splenic erythropoiesis in inflammatory or other “stress” states; for more on the latter, refer to the erythropoiesis page). In a study of 5922 dogs and 2142 cats, a reticulocytosis was seen in 4.4 and 3.1% of non-anemic dogs (>110,000/uL) and cats (>50,000/uL), respectively (animals with erythrocytosis were excluded). Less than 2% of the animals were clinically healthy, which is below the expected percentage of normal animals having results for an analyte above the reference interval (2.5%). In sick animals, various diseases were associated with a reticulocytosis without anemia, including cardiovascular and respiratory disease, blood loss, inflammation and cancer (Fuchs et al 2018). The reticulocytosis was marked in a few cases (up to 700,000/uL in dogs and 300,000/ul in cats, but a compensated hemolytic anemia was not ruled out). Small breed dogs with congestive heart failure due to mitral valve disease can also have a reticulocytosis without anemia (up to 262,000/uL, n=18), which decreased in most of the dogs in the study with successful treatment of the cardiac disease (Choi et al 2022). In a small study of 11 brachycephalic dogs, higher packed cell volumes (average, 48 vs 44%) and lower average partial pressure of oxygen (paO₂, 86 vs 100 mmHg) was identified versus dogs with medium muzzle lengths (Hoareau et al 2012). However, an additional study of 18 brachycephalic dogs found no significant difference in paO₂, although the mean paO₂ values were trending lower (79 versus 84 mmHg) versus dogs with medium or long muzzles (Aralpulgasam et al 2018). The data suggests that some brachycephalic dogs could have systemic hypoxia and a reticulocytosis, but this theory has not been tested directly and brachycephalic breeds were not over-represented in a retrospective study of 1035 dogs with reticulocytosis without anemia (the study did include dogs with erythrocytosis and defined a reticulocytosis as >2.5% reticulocytes) (Pattullo et al 2015).

A moderate or marked reticulocytosis in a non-anemic animal should prompt a hunt for causes of inappropriate erythropoietin release (renal or liver disease) or a compensated hemolytic anemia. 

Please see the reticulocyte percentage page for elaboration on interpretation of high reticulocyte counts.

Corrected reticulocyte percentage

This also takes into account the degree of anemia and is calculated as follows:

Corrected reticulocyte percentage = reticulocyte % x (patient’s hematocrit or packed cell volume÷normal hematocrit or packed cell volume)

Guidelines for interpretation:

• Dog: An anemia is considered regenerative if the corrected reticulocyte percentage is > 1%
• Cat: An anemia is considered regenerative if the corrected reticulocyte percentage is > 0.4% in the cat.

This technique assumes that the normal hematocrit or packed cell volume is 45% for a dog and 35% for a cat and does not take into account the reticulocyte maturation time (or how long it takes a released immature anucleated RBC to mature in the circulation). If the individual dog or cat hematocrit/packed cell volume was known from a previous hemogram (always to get a baseline result in a healthy adult animal), this is a reasonable technique for assessing regeneration. It can be readily performed in most private practices (only a microhematocrit centrifuge to measure the packed cell volume and assessment of a new methylene blue-stained smear to obtain the reticulocyte percentage are needed) versus the absolute reticulocyte count (which requires an accurate RBC count from an automated analyzer). Because our hematology analyzer provides us with the RBC, we prefer the absolute reticulocyte count to the corrected reticulocyte percentage for assessing regeneration.

Reticulocyte production index

This takes into account the degree of anemia (using the corrected reticulocyte percentage) and how long reticulocytes persist in the circulation. Thus it takes into account the release of more immature reticulocytes in severe anemias that are thought to persist in the circulation for longer. It is calculated using the following formula:

Reticulocyte production index = Corrected reticulocyte percentage ÷ reticulocyte lifespan

The reticulocyte lifespan (or correction factor) is shown in the table below.

These correction factors were derived for human beings and have not been validated for animals. They do appear to work in dogs.

Guideline for interpretation: Values greater than 1.0 indicate increased production in response to anemia.