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 guidelines for assessing the degree 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 the most immature and more reflective of& current bone marrow production).

Degree of Regeneration
Canine Reticulocytes
(thou/µL)
Feline Aggregate Reticulocytes (thou/µL)
Inadequate or no regeneration
< 95
< 60
Mild
95-150
60-100
Moderate
150-300
100-200
Marked
> 300
> 200

 

For example, a dog with 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 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 RBC count has a reticulocyte count of 8%, which is also above our reference interval. The absolute reticulocyte count is 120 thou/µL (8/100 x 1.5 x 1000 or 8 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.

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.

Hematocrit
Reticulocyte lifespan
45
1.0
35
1.5
25
2.0
15
2.5

 

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. 

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