Reticulocyte Count

Reticulocytes are non-nucleated immature red cells in peripheral blood, containing residual RNA. After erythroid precursors lose their nuclei, another 4 days is required for the resulting reticulocytes to mature and lose their RNA. Normally the first 3 days are spent in the marrow, and the last day in peripheral blood.  Early reticulocytes continue to synthesize hemoglobin; approximately 25% of total red cell hemoglobin content is produced during this stage of development.

The reticulocyte count is useful as an index of effective red cell production. It is usually expressed as a percentage of total red cells and as an absolute count (number of reticulocytes per uL). The percentage value is falsely elevated in patients with anemia. This bias is overcome by correcting reticulocyte percentage according to the patient’s red cell count using the following formula.

Corrected Reticulocyte Count = Reticulocyte % X Patient’s red cell count/5.00 million per uL

The percentage value reported by Saint Luke’s Regional Laboratories has already been corrected in this way. Absolute reticulocyte count does not require correction.

Clinical laboratory techniques of reticulocyte enumeration are based on detection of cytoplasmic RNA. Originally, reticulocyte count was performed by microscopic examination of peripheral blood smears stained with a supravital dye. Today, reticulocyte counts have been included in automated hematology analyzers. Automated methods have the advantage of enumerating large numbers of cells, thereby greatly improving precision, accuracy and efficiency. Additional reticulocyte parameters such as reticulocyte maturation index and immature reticulocyte fraction can also be determined.

Automated reticulocyte counting is based on flow cytometry technology that includes light scatter and immunofluorescence of a RNA specific dye, such as auramine-O used by Sysmex hematology analyzers. Red blood cells are discriminated from platelets and white blood cells by forward versus side scatter analysis. Reticulocytes are then differentiated from mature red cells from reticulocytes by the level of green fluorescence intensity. The reticulocyte population is automatically divided into low, intermediate and high fluorescence populations and the percentage of cells in each fraction is calculated.

Indications for a reticulocyte count include:

  • Investigation of anemia
  • Monitoring the effect of hematinic or recombinant erythropoietin therapy
  • Monitoring bone marrow regenerative capacity after chemotherapy or bone marrow transplantation

The reticulocyte count is one of the major parameters used in the initial classification of anemia, allowing one to distinguish hypoproliferative from hyperproliferative causes. Reticulocytes are increased in anemic patients with functional bone marrows. This includes patients with blood loss or hemolytic anemias and patients who have been successfully treated for other types of anemia. In contrast, patients with bone marrow aplasia, bone marrow infiltration or nutritional deficiencies have a very low corrected reticulocyte count (less than 2%).





Hemolytic anemia

Acute blood loss

Response to replacement therapy


Aplastic anemia

Marrow suppression by drug, toxin, or virus

Pure red cell aplasia

Bone marrow infiltraion (leukemia, lymphoma, carcinoma)


Iron deficiency anemia

Anemia of chronic disease

Chronic renal failure

Megaloblastic anemia



Automated reticulocyte counting methods have led to the ability to measure stages of reticulocyte maturity, based on RNA content. The immature reticulocyte fraction (IRF) is the sum of reticulocyte fractions with medium and high fluorescence.  A rise in IRF is one of the earliest indicators of bone marrow engraftment or recovery from intensive chemotherapy. The IRF can also be used as an early indicator of response to erythropoietin therapy in patients with chronic renal failure and other diseases.

Reference range for reticulocyte count is shown in the following table.






0.6 – 1.6

24 – 80


90.6 – 1.8

26 - 105


Reference range for the IRF is 0.0-0.1. Specimen requirement is one 5mL lavender top tube of blood and can be the same tube used for CBC.


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