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488 Part VI: The Erythrocyte Chapter 32: Erythropoiesis 489

extensively examined existing data and concluded that the most repro- Generally, the ratio of total-body Hct as estimated by direct mea-
ducible expressions of red cell mass are related to body surface area esti- surements of red cell volume and plasma volume to the large-vessel Hct
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mated from height and weight : ranges from 0.89 to 0.92. Consequently, when using the determined
plasma volume to calculate RCM and total blood volume, a correction
RCM Males = (1486 × S) − 285 RCM Females = (822 × S) + (1.06 × Age) factor is necessary, and a value of 0.90 is generally used:
where RCM = red cell mass, S = body surface area in square meters,
×
×
and Age = age in years. The calculated values, ±25 percent, included 98 Corrected redcellmass= Hctplasmavolume0.90
percent of the measured male values and 99 percent of the measured 100 − Hct
female values. 13 Recommended procedures for determination and evaluation of
Despite the ICSH recommendation, the most common method is blood volume are outlined by the ICSH. 155
to report red cell mass values in terms of milliliters per kilogram. How-
ever, this method of expression gives erroneously low values in obese
individuals because fat is hypovascular. A better method might be to MEASUREMENTS OF RED CELL
express the red cell mass in terms of lean weight. In general, lean weight
is 20 percent less than actual weight in normal males and 25 percent less PRODUCTION
in normal females. However, estimation of lean weight in obese indi-
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viduals is inaccurate. From a practical point of view, RCM probably is Under normal circumstances, most human red cells produced in the
best reported in terms of actual weight, with mental adjustments made marrow live, or have the potential to live, a normal life span. Under
based on body configuration. In general, the RCM of normal females certain conditions, however, a fraction of red cell production is ineffec-
ranges from 23 to 29 mL/kg of body weight and of normal males ranges tive; with destruction of nonviable red cells either within the marrow or
from 26 to 32 mL/kg of body weight. 151 shortly after the cells reach the blood. 38

PLASMA LABELS EFFECTIVE RED CELL PRODUCTION
RCM also can be estimated from plasma volume. Radioactive iodine Effective erythropoiesis is most simply estimated by determining the
( I) is used to label albumin and measure its distribution volume. reticulocyte count. Most modern automated counters measure reticu-
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Other radioactive isotopes of iodine other than 99m Tc have been locytes by nucleic acid binding dyes such as thiazole orange using flow
used, but I has virtually supplanted all other plasma labels. Albu- cytometry. These are fast reliable assays. This count usually is expressed
125
min labeled with radioactive iodine is commercially available, and as the percentage of red cells that are reticulocytes, but it also can be
a known amount is injected intravenously. Several blood samples expressed as the total number of circulating reticulocytes per unit of
are obtained within the first 15 minutes and centrifuged. CPM per blood (absolute reticulocyte count and corrected reticulocyte counts;
milliliter of plasma is measured, plotted on semilogarithmic paper, equation 1 below).
and extrapolated to zero time. This procedure is necessary because, Equation 1
in contradistinction to labeled red cells, labeled albumin is removed %reticulocytes × redcellcount
gradually, beginning immediately after injection. Plasma volume is Absolutereticulocytecount=
calculated according to the equation: 100

CPMoflabeled albumininjected A simple clinical method to estimate effective erythropoiesis uses
Plasma volume(mL) = the reticulocyte count to calculate the reticulocyte index (see equation
CPMmLplasmaat0hour 2 below). This measurement depends on several assumptions: (1) the
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The continuous exchange of intravascular with extravascular albu- human red cell life span is approximately 100 days (actually approxi-
min is the major problem encountered when plasma volume is mea- mately 115); (2) the life span is finite and, thus, the oldest 1 of 100 or
sured with labeled albumin. Even with extrapolation to 0 hour, plasma 1 percent of red cells is removed (and replaced) each day; (3) the retic-
volume is somewhat larger than that measured with a strictly intravas- ulocyte is identifiable as such in the blood for 1 day using supravital
cular protein such as fibrinogen. Consequently, if measurement of the stain; and (4) the reticulocyte count of 1 percent in a person with a nor-
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plasma volume is used to calculate the size of the total red cell mass, it mal Hct represents normal red cell production and thus “1” is the basal
is a less-reliable measure than determining red cell mass directly with reticulocyte index.
tagged red cells. This inaccuracy is further augmented by the fact that In anemic patients, two calculations are needed to measure the
the venous Hct used to calculate red cell mass from measured plasma reticulocyte index and compare it to the normal of 1 in the basal state.
volume does not reflect accurately the distribution of plasma and red To correct the reticulocyte percentage for the lower red cell count in
cells in the body. However, from a practical point of view, the results anemic subjects, the reticulocyte percent is multiplied by the ratio of
of estimating RCM from plasma volume are surprisingly accurate and the patient’s Hct over the normal mean Hct, providing a corrected retic-
have been advocated based on simplicity and low cost. 152 ulocyte percent.
Conversion of the reticulocyte count (see equation 1 above) to the
reticulocyte index (equation 2 below) is achieved by taking into account
TOTAL-BODY HEMATOCRIT the estimated life span of reticulocytes. The life span of reticulocytes in
blood in a normal individual is approximately 1 day. However, when red
When total RCM is measured with labeled red cells, the value is approx- cell production is increased under conditions of erythropoietic stress,
imately 10 percent lower than that calculated from plasma volume and for example, in severe anemia, reticulocytes are released prematurely
the Hct of blood. In fact, the mean Hct of blood in all of the vessels and circulate as reticulocytes for 2 to 4 days, except in situations with
(total-body Hct) clearly is somewhat lower than the Hct measured from low EPO levels, as in renal insufficiency. These appear as polychro-
blood obtained from large vessels; these differences are a result of vary- matophilic erythrocytes on Wright-stained blood film and are called
ing proportions of plasma in different size vessels. stress or shift reticulocytes.

Kaushansky_chapter 32_p0479-0494.indd 489 9/17/15 6:11 PM