Immature Reticulocyte Fraction

The orthochromatic erythroblast expels its nucleus to form a reticulocyte. This cell’s name is derived from the reticular (web-like) ribosomal RNA seen within the cell when stained with methylene blue. The presence of ribosomal RNA is why reticulocytes appear slightly blue when stained with with Wright Giemsa. Reticulocytes are larger than mature red blood cells (MCV~160) and normally comprise about 1% of the red blood cell count.

The reticulocyte count is an important indicator of effective erythropoiesis. In healthy individuals, reticulocytes circulate in the peripheral blood for 1-2 days after being released from the bone marrow. Then, they lose sufficient ribosomal RNA to become red blood cells. 

During periods of increased erythropoietic demand, reticulocyte lifespan in peripheral blood increases to 3 or more days, as a result of premature release of immature or “stress” reticulocytes from the bone marrow. 

The introduction of automated reticulocyte counts has permitted more precise counting and determination of reticulocyte maturity based on their RNA content. It is now possible to precisely quantitate the proportion of all reticulocytes that is immature. This is termed the “immature reticulocyte fraction”, or IRF. It is calculated as the ratio of immature reticulocytes to the total number of reticulocytes. The ratio provides a very early and sensitive index of marrow erythropoietic activity.

Serial determination of IRF after bone marrow transplantation (BMT) can be used to demonstrate successful engraftment. In most cases, a rise in the IRF has been shown to occur earlier than any other available laboratory parameter, including absolute neutrophil count. An increase in the IRF of more than 20% suggests successful erythroid engraftment. IRF is also a sensitive measure of early hematopoietic recovery following intensive chemotherapy.

The IRF can be used as an early and reliable indicator of adequacy of response to erythropoietin (EPO) therapy in patients with anemia associated with chronic renal failure, or other diseases such as AIDS and malignancy. The IRF could potentially be used to monitor response to EPO in a blood conservation program. It can also be used to monitor response to other treatments for anemia such as iron, folate, and vitamin B12.

Evaluation of the IRF together with the absolute reticulocyte count can provide additional useful information, as shown in the following table.

IRF result is reported together with the reticulocyte count The reference range is 0.11 – 0.38. 

Specimen requirement is one 5ml EDTA (lavender-top) tube.  IRF can be measured on the same tube submitted for a reticulocyte count.

References

Kim NH, Assessment of Immature Reticulocyte Fraction as an Early Predictor of Marrow Engraftment After Hematopoiectic Stem Cell Transplantation, Transplant Cell Therapy, 2011;17(2),S235.

Piva E et al, Clinical Utility of Reticulocyte Parameters, Clinical Lab Med.2015;35(1):133-63.