Q. I have one question, and you are my last hope. What is the difference between the absolute and corrected reticulocyte count? I thought they were the same thing, but I read on a forum that they are not.A. That’s a great question! There are actually three different ways of addressing the reticulocyte count in a patient: the absolute reticulocyte count, the corrected reticulocyte count, and the reticulocyte production index.

Absolute reticulocyte count

First, there’s the absolute reticulocyte count. This is really a percentage, not a count, but we won’t get into that. It’s just the straight-up percentage of red cells that are reticulocytes (normal is about 0.5 – 2.0%).

That’s useful as it is. However, it doesn’t reflect that fact that as the hemoglobin drops, you should have more reticulocytes, percentage-wise (in other words, at a hemoglobin of 5, you should have a much higher percentage of reticulocytes than you should at a hemoglobin of 10). To take this into account, you can do a corrected retic count.

Corrected reticulocyte count

The corrected reticulocyte count = reticulocyte % x (Hgb/15).

This formula “corrects” for hemoglobin – meaning that it will show you if the patient is making enough reticulocytes for the degree of anemia present. As the hemoglobin drops, you need to make more reticulocytes to get up to the normal range of 0.5 – 2.0%.

There’s one more factor that you need to be aware of regarding reticulocytosis. If the patient is severely anemic, the bone marrow will release reticulocytes prematurely into the blood. These prematurely-released reticulocytes are called “shift reticulocytes,” and they will circulate in the peripheral blood for longer than normally-released reticulocytes (because shift reticulocytes are younger than normal retics!).

For example, at a hemoglobin of 15, it takes reticulocytes 1 day to mature in the peripheral blood. But at a hemoglobin of 12, it takes reticulocytes 1.5 days to mature in the blood (they take longer to mature into normal red cells because they were released prematurely into the blood!).

So if you want to be accurate about your retic count, you wouldn’t want to count all those shift reticulocytes (if you did, you’d get an artifactually high count, since you’d be counting all the shift reticulocytes, which hang around longer in the blood!). To correct for these shift reticulocytes, you can do a reticulocyte production index.

Reticulocyte production index

The reticulocyte production index = (reticulocyte %/reticulocyte maturation time) x (Hgb/15).

To figure out the reticulocyte maturation time, use the following guidelines (if your hemoglobin is in between the numbers mentioned, just estimate the maturation time):

  • At a hemoglobin of 15, the maturation time = 1 day
  • At a hemoglobin of 12, the maturation time = 1.5 days
  • At a hemoglobin of 8, the maturation time = 2 days
  • At a hemoglobin of 5, the maturation time = 2.5 days

With this formula, you can see that the lower the hemoglobin is, the more retics you need to make to balance out the “shift” reticulocytes. Most guidelines say that an RPI less than 2 indicates that the patient’s bone marrow is not making enough reticulocytes, and  an RPI over 3 indicates that the marrow is responding appropriately to the anemia and making enough retics.