At steady state there is equilibrium between red cell production and destruction through red cell senescence. Red cell production and destruction can be estimated from the reticulocyte parameters.
blood volume in mL =
= (weight in kilograms) * (blood volume per kilogram based on gender)
where:
• blood volume per kilogram in males is 69 mL/kg; women 64 mL/kg
RBC volume in mL =
= (blood volume in mL) * (hematocrit as a fraction)
reticulocyte volume in mL =
= (RBC volume) * (reticulocyte percentage as fraction)
steady state daily red cell dynamics in mL/day=
= (reticulocyte volume) / (lifespan of reticulocytes in days)
where:
• The lifespan for reticulocytes depends on concurrent anemia, reflecting early release of reticulocytes into the circulation.
|
hematocrit
|
lifespan
|
|
40-45%
|
1.0
|
|
35-40%
|
1.5
|
|
25-35%
|
2.0
|
|
< 25%
|
2.5
|
Limitations:
• If erythropoiesis is suppressed, reticulocyte lifespan is variable, so that the estimated lifespan for reticulocytes are not reliable.
• If the patient is actively bleeding or hemolyzing, then the patient is not in steady state.
