Interpretation of Reticulocyte lab values

Reticulocytes and their significance

This is a really confusing topic because of misnomers.  Wikipedia and link above (which wikipedia cited) helped a lot in breaking this down.


Reticulocyte Count is not really a "count".  It might be called a "count" but it's usually a percentage.  If the retic count is 5, that means that 5% of the RBCs are reticulocytes.
This becomes an issue in anemia, where the total number of RBCs decrease, and therefore the "retic count" will be spuriously high.  Example - if x = number of reticulocytes and T = total number of RBCs, then the ratio of X/T ("the retic count") will increase as T decreases.

The next part is intuitive.  The "retic count" is corrected for the degree of anemia by multiplying by the ratio of the patient's Hct/normal Hct.  The normal Hct in calculations is generally 45.
So if the patient's "retic count" is 5% or 5, and the patient's Hct is 25.  Then the "corrected retic count" is 5 x (25/45) = 25/9 .  In calculations, 5 (and not 0.05) is used for 5%.

To make things more confusing, sometimes the "corrected retic count" is also known as "reticulocyte index."  (This is confusing because another term we'll address known as the "reticulocyte production index" is also sometimes called the "reticulocyte index."  WTF)



Okay, so we've somewhat corrected the "retic count" for the degree of anemia.  The next part is to take into account that reticulocytes take about 4 days to mature --- In normal conditions this means 3 days in the bone marrow and 1 day in the peripheral blood.  In anemia, however, the bone marrow will "prematurely release" reticulocytes.  So this means that you'll have more circulating reticulocytes, but this doesn't necessarily mean that the BM has amped up its erythropoiesis in response to the anemia.  It could just mean that the BM just prematurely released the reticulocytes.

My interpretation of this calculation is that you're not supposed to consider "premature reticulocytes" as reticulocytes in your final reticulocyte value.
So to correct for the premature release of reticulocytes, we use something called the "maturation factor."  It makes sense intuitively ---- If we're going to divide by this "maturation factor,"as the degree of anemia worsens (Hct decreases), the maturation factor increases.  (interestingly the maturation factor = number of days in the peripheral blood spent by the reticulocyte to mature)

Maturation factor for degree of anemia:
                   
                     Maturation Factor
Hct 36 - 45         1
Hct 26 - 35         1.5
Hct 16 - 25         2
Hct < 15             2.5


Reticulocyte Production Index =  (Corrected Reticulocyte Count) / Maturation Factor

Once again:
-Reticulocyte Count is usually reported as a percentage
- In calculations using the "retic count", if it's 5%, use "5" and not "0.05"
-Corrected Reticulocyte Count can also be known as the Reticulocyte Index
-Reticulocyte Production Index can also be known as Reticulocyte Index

From Wiki:

• The reticulocyte index (RI) should be between 1.0% and 2.0% for a healthy individual.
• RI < 2% with anemia indicates loss of red blood cells, but decreased production of reticulocytes (ie, and inadequate response to correct the anemia) and therefore red blood cells.^[2]
• RI > 3% with anemia indicates loss of red blood cells (from causes such as destruction, bleeding, etc.), with an increased compensatory production of reticulocytes to replace the lost red blood cells.^[2]