Reticulocyte production index

Physiology

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 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. Reticulocytopenia, or "aplastic crisis", is the medical term for an abnormal decrease of reticulocytes in the body

The reticulocyte production index may find new use as a more reliable detector of erythropoietin-doping in athletes. The use of this method is referred to as "biological passport."

Calculation of RPI

Reticulocyte Production Index is calculated as follows:

1. C o r r e c t e d R e t i c P e r c e n t a g e = R e t i c P e r c e n t a g e ∗ H e m a t o c r i t N o r m a l H e m a t o c r i t

A value of 45 is usually used as a normal hematocrit.

2.The next step is to correct for the longer life span of prematurely released reticulocytes in the blood—a phenomenon of increased red blood cell production. This relies on a table:

So, in a person whose reticulocyte percentage is 5%, hemoglobin 7.5 g/dL, hematocrit 25%, the RPI would be:

R P I = C o r r e c t e d R e t i c P e r c e n t a g e M a t u r a t i o n C o r r e c t i o n → RPI = 5 ∗ 25 45 2 = 1.4

Alternatively some books provide the following formula:

R P I = R e t i c P e r c e n t a g e ∗ H e m o g l o b i n ( o b s e r v e d ) N o r m a l H e m o g l o b i n ∗ 0.5

Interpretation

Interpretation of these values are not standard and vary based on specific laboratory values and clinical context.