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.

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12 Responses to Absolute vs. corrected reticulocyte count

  1. SP says:

    But, can’t we just look at the absolute reticulocyte number instead?

  2. Kristine says:

    Sure – you can do that. But if you want to know if the reticulocytosis is appropriate for the degree of anemia present, you have to either do the calculations involved in the corrected retic count (or preferably, the reticulocyte production index), or just guess at them in your head…

  3. SP says:

    I can see why one has to calculate the RPI, when he uses a percentage as the measure of reticulocyte production; you explained it beautifully above. What I meant to say is that I think the absolute reticulocyte number would be enough for the investigation of an anemia. It will increase, when the anemia is due to blood loss or hemolysis and be depressed when the anemia is hypoproliferative. And if it does not increase, when it is supposed to, then that is a clue pointing towards marrow failure.

  4. Kristine says:

    Oh I see what you mean – you’re absolutely (haha) right! If the absolute retic count does not increase, that points towards marrow failure. Yes – you can definitely use the absolute count as a diagnostic tool to help point you towards hemolysis vs. marrow faiure. Good point.

  5. Raffi says:

    Thanks for the post. By chance, is the “shift reticulocyte” the same as polychromasia? I heard that if you see that polychromasia is present, you devide the corrected reticulocyte count by two. I’m assuming that’s a short hand for the reticulocyte production index, and using the reticulocyte maturation time is more specific. I’m pretty sure we have to know how to calculate corrected reticulocyte count for USMLE, but do you have to know how to do the reticulocyte production index too?

  6. Kristine says:

    Thanks, Raffi. No – the concept of shift reticulocytes is not the same as polychromasia. Polychromatophilic cells are just reticulocytes (technically you should call them polychromatophilic cells on Wright-Giemsa stain, and reticulocytes if you’re using a retic stain). Polychromasia just means that you see polychromatophilic cells in the blood – which you do normally. So if you had increased polychromasia, that would mean that you had more polychromatophilic cells than usual (more reticulocytes than usual). It’s just a relative term. Shift reticulocytes are reticulocytes (or polychromatophilic cells) that leave the bone marrow early (a few days before they normally would) due to the extreme demand for red cells in the blood. Since they hang around longer (they mature in the blood instead of in the marrow), technically you should do a calculation to account for them in the blood (otherwise, you’d overestimate the retic count!). As for whether you need to remember how to calculate the RPI for the boards, I don’t know! I haven’t heard one way or the other from my students. My guess is that’s a little more detail than you’d need to know…but I’m just not sure.

  7. Baraniko Eromanga says:

    Thanks for discussing this, it’s confusing me for long time, now I understand the differences.

  8. Sandeep Jain says:

    As always, fantastic explanation! The delay in maturation time with decreasing Hgb is good to know!

  9. Fatima says:

    As the hemoglobin drops, you need to make more reticulocytes to get up to the normal range of 0.5 – 2.0%
    Really don’t understand if hb drops so RBC mass drops too so why we need more retics to get to 0.5 to 2%
    By the way your explanation is more than amazing

  10. Raffi says:

    Thank you for your detailed response!

  11. Manjula says:

    Please can you use polychromatic cells from a wrights stain film to estimate reticulocyte %.

  12. Kristine says:

    Yes – you can use a regular Wright-Giemsa-stained blood smear to estimate the reticulocyte count. Polychromatophilic red cells are a little bigger and a little more purplish in color than mature red cells, and you can easily pick them out on a regular blood smear.

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