In case of anemia, corrected reticulocyte count for Red blood cells or absolute reticulocyte count must be used instead of simple reticulocyte percentage because reticulocyte percentage is simply the ratio of reticulocytes to red blood cells, which may give a falsely high reading of reticulocyte percentage when the number of RBCs is low.
Let’s have an example;
If you have 100 apples and 3 apples are not ripped, then it means 3 percent of the apples are not ripped.
But if you have 70 apples and 3 apples are not ripped then you will have 4.2% unripped apples.
Similar is the case with RBCs, if you have a low RBC count you may get a falsely high reading of reticulocyte count, so you must correct the Reticulocyte count for RBC count.
To do this the formula is;
Corrected Retic count for RBCs or Absolute Reticulocyte count = RBC count x Retic precentage x 10000 / mm3
Iterpretation:
If the corrected retic count for RBCs is greater than 40,000/mm3 it means bone marrow response is adequate if it is less than 40,000 it means the bone marrow response is inadequate.
For example, if we have the following CBC report;
WBC 9000/mm3
RBC 1.2 x 10^9 / mm3
Hb 5.0 g/dl
Hct: 16.2 %
Retic count: 3%
Then the absolute reticulocyte count or corrected retic for RBCs will be
= 1.2 x 3 x 10000 = 36000 / mm3
and this value is less than 40000, it means the reticulocyte count of 3% was giving us a wrong impression of reticulocytosis when in reality the patient has a low reticulocyte count and impaired bone marrow response as compared to the degree of anemia.
We may use hemoglobin or hematocrit instead of RBC count in the above formula, but I found the above formula the simplest and easiest.
