Reticulocyte Index

Reticulocyte production index explained

The RPI is the corrected reticulocyte count which accounts for all reticulocytes in all development stages. This is particularly useful in conditions such as anemia where there are RBCs in circulation at different stages of maturation.

The reticulocyte count provided in a normal complete blood count test is provided as a percent, defining their fraction in the red blood cells but does not account for different stages.

By associating reticulocytes with hematocrit or hemoglobin determinations, an accurate view in the bone marrow response can be obtained.

There are two different methods, that can be found in the two tabs of the calculator.

RPI method 1

The first method uses the following variables:

■ Percentage of reticulocytes (from RBCs);

■ Hematocrit (as measured);

■ Normal haematocrit (most commonly accounted as 45%).

There are two steps in this method: firstly, the reticulocyte index is calculated then it needs to be adjusted by a maturation correction to reflect the longer life span of premature reticulocytes.

Reticulocyte Index = Reticulocyte count x Hematocrit / Normal Hematocrit

RPI = Reticulocyte index/ Maturation correction

The correction rates used can be found in the table below:

RPI method 2

Uses the following variables:

■ Percentage of reticulocytes;

■ Hemoglobin (as measured);

■ Normal haemoglobin (depending on patient age and gender).

RPI = Reticulocyte count x Hemoglobin / Normal Hemoglobin x 0.5

RPI interpretation

The reticulocyte production index is most often used in the diagnosis of anemias. This is because in these conditions the raw RBC count may be misleading (appear higher than it actually is) because the cells are depleted.

In anemia, the reticulocyte production should increase as response to the red blood cells loss and the RPI verifies if the bone marrow response is appropriate.

In cases RPI does not adapt, there are two main causes:

■ The anemia is due to an acute cause;

■ There is a bone marrow defect such as insufficient erythropoietin.

Normal RPI varies between 0.5 and 2.5%. However, in case anemia is present and RPIs are lower than 2%, this is still consistent with RBC loss and decreased production of reticulocytes.

Values lower than 0.5 indicate a very decreased reticulocyte production.

Values greater than 3, when accompanied by anemia, indicate RBC loss but an increased production of reticulocytes.

Reference

Poorana P, Subhashree AR. Role of Absolute Reticulocyte Count in Evaluation of Pancytopenia-A Hospital Based Study. J Clin Diagn Res. 2014; 8(8): FC01–FC03.