The Reticulocyte production index is a calculation used by physicians in the diagnosis of anemia. Reticulocytes are newly produced red blood cells. They are slightly larger than totally mature red blood cells, and have some residual ribosomal RNA; the RNA makes them stain a different color (or, in the case of fluorescent dye, a different brightness), allowing them to be detected 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. If reticulocyte production is not raised in response to anemia, this may either be because the anemia is due to an acute cause (so that the bone marrow has not yet had time to respond), or because the bone marrow is in some way impaired.

Reticulocyte production should rise within 2-3 days of a major acute hemorrhage, for instance, and reach its peak in 6-10 days.

The first step in producing the RPI is to come up with a reticulocye count that is corrected for the level of anemia. This equation is:

(observed retic count) x (patient hematocrit)/(normal hematocrit) = corrected retic count.

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:

Hematocrit (%)......Retic survival (days)/maturation correction

• 36-45........................1.0
• 26-35........................1.5
• 16-25........................2.0
• 15 and below.............2.5

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

5 x retic count/correction = 5 x (25/45)/2 = 1.4

In general, an RPI of less than 2 suggests either inadequate time to respond to the anemia or a production problem in the marrow.

References

• 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.

External links

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