• Reticulocytes are newly-produced red blood cells. They are slightly larger than totally mature red blood cells, and have some residual ribosomal RNA. The presence of RNA allows a visible blue stain to bind or, in the case of fluorescent dye, result in a different brightness. This allows them to be detected and counted as a distinct population.
• The Reticulocyte production index (RPI, also called a corrected reticulocyte count) is a calculated value used in the diagnosis of anemia.
• This calculation is necessary because the raw reticulocyte count is misleading in anemic patients.
• The problem arises because the reticulocyte count is not really a count but rather a percentage: it reports the number of reticulocytes as a percentage of the number of red blood cells.
• In anemia, the patient’s red blood cells are depleted, creating an erroneously elevated reticulocyte count.
• The idea of the RPI is to assess whether the bone marrow is producing an appropriate response to an anemic state.
• Reticulocyte production should increase in response to any loss of red blood cells.
• It should increase within 2-3 days of a major acute hemorrhage, for instance, and reach its peak in 6-10 days.
• If reticulocyte production is not raised in response to anemia, then the anemia may be due to an acute cause with insufficient time to compensate, or there is a defect with red blood cell production in the bone marrow.
• Marrow defects include nutritional deficiencies (i.e. iron, folate, or B12) or insufficient erythropoietin, the stimulus for red blood cell production.

• Reticulocyte Production Index is calculated as follows:
• The degree of anemia (done by normalizing hematocrit of 45%)
• Reticulocyte maturation time (RMT)
• The Reticulocyte maturation time is 1.0 days for a hematocrit of 45 % to 2.5 days for a hematocrit of 15 %:
• Reticulocyte production index = Reticulocytes % x (HCT / 45) x (1 / RMT)
• The reticulocyte index (RI) should be between 1.0 and 2.0 for a healthy individual.
• RI < 2 with anemia indicates decreased production of reticulocytes and therefore red blood cells.
• RI > 2 with anemia indicates loss of red blood cells (destruction, bleeding, etc) leading to increased compensatory production of reticulocytes to replace the lost red blood cells.** Value of 45 is usually used as a normal hematocrit.
  • So,in a person whose reticulocyte count is 5%, hemoglobin 7.5 g/dL, hematocrit 25%, the RPI would be:

5 x [corrected retic count]/[maturation correction] = 5 x (25/45) /2 = 1.4

• ^ Adamson JW, Longo DL. Anemia and polycythemia. in: Braunwald E, et al. Harrison’s Principles of Internal Medicine. (15th Edition). McGraw Hill (New York), 2001.
• ^ Hoffbrand AV, et al. Essential Haematology. (Fourth Edition) Blackwell Science (Oxford), 2001.

• Medical Calculator Online RPI calculator, “for educational purposes only”.

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