Wintrobe's Clinical Hematology 14e SC

Chapter 1: Examination of the Blood and Bone Marrow 3

Mean Corpuscular Hemoglobin MCH is a measure of the average Hb content per RBC. It may be calcu- lated manually or by automated methods using the following formula 19 :

trapping. Manual Hct methods have a coefficient of variation (CV) of approximately 2%. 20

Automated analyzers do not depend on centrifugation techniques to

determine Hct, but instead calculate Hct using direct measurements of red × mean red cell volume. Alternatively, some analyzers measure Hct directly by cell number and red cell volume as follows: Hct = red cell number

= hemoglobin (g/L)/red cell count (10 12 /L)

MCH

MCH is expressed in picograms (pg, or 10 −12

g). In anemias sec-

comparing the sum of all RBC size measurements to the volume of the

ondary to impaired Hb synthesis, such as iron deficiency anemia, Hb

specimen. Automated Hct values closely parallel manually obtained

mass per red cell decreases, resulting in a lower MCH value. MCH measurements may be falsely elevated by hyperlipidemia 30 because

measurements, and the manual Hct is used as the reference method for

hematology analyzers (with correction for the error induced by plasma

increased plasma turbidity will erroneously elevate Hb measurement.

trapping). Errors of automated Hct calculation are more common in patients with polycythemia 22 or abnormal plasma osmotic pressures. 23

The CV for automated analysis of MCH is 1% to 2% in most modern analyzers, compared with approximately 10% for manual methods. 20,25 Mean Corpuscular Hemoglobin Concentration

Manual methods of Hct determination may be preferable in these cases.

< 1.5%. 24,25

The CV of most automated Hcts is

Hemoglobin Concentration

The average concentration of Hb in a given red cell volume, or MCHC, may be calculated by the following formula 19 :

Hb is an intensely colored protein, allowing its measurement by spectro-

= hemoglobin (g/dL)/Hct (L/L)

photometric techniques. Hb is found in the blood in a variety of forms,

MCHC

including oxyhemoglobin, carboxyhemoglobin, methemoglobin, and other

The MCHC is expressed in grams of Hb per deciliter of packed

minor components. These may be converted to a single stable compound,

RBCs, representing the ratio of Hb mass and the volume of red cells.

cyanmethemoglobin, by mixing blood with Drabkin solution (contains potassium ferricyanide and potassium cyanide). 26,27 Sulfhemoglobin is not

With the exception of hereditary spherocytosis and some cases of

homozygous sickle cell or hemoglobin C disease, MCHC values will

converted, but is rarely present in significant amounts. The absorbance of

not exceed 37 g/dL. This level is close to the solubility value for Hb,

the cyanmethemoglobin is measured in a spectrophotometer at 540 nm

and further increases in Hb may lead to crystallization. The accuracy

to determine Hb. This technique is used both in manual determinations

of the MCHC determination is affected by factors that have an impact

and in most automated hematology analyzers, although cyanide-free

on measurement of either Hct (plasma trapping or the presence of

methods are used by some. Hb is reported in grams per deciliter (g/dL) of

abnormal red cells) or Hb (hyperlipidemia and leukocytosis), which is methodology specific. 28 The CV for MCHC for automated methods ranges between 1.0% and 1.5%. 24

whole blood. The main errors in measurement arise from dilution errors

or increased sample turbidity caused by improperly lysed red cells, leu- kocytosis, or increased levels of lipid or protein in the plasma. 28-30 Older

As noted earlier, the MCV, MCH, and MCHC reflect average values

analyzers reported spurious increases in Hb levels when white cell counts

× 10 9

and may not adequately describe blood samples when mixed popula-

exceeded 30

/L because of increased turbidity, but this is decreased

tions of red cells are present. For example, in sideroblastic anemias,

with newer flow systems so that Hb levels remain extremely accurate in the

× 10 9

/L. 24

a dimorphic red cell population of both microcytic hypochromic and

face of WBC counts as high as 100

With automated methods,

< 1% (CV). 24,25

normocytic normochromic cells may be present, yet the indices may

the precision for Hb determination is

Laboratory Hematology

be normochromic and normocytic. It is important to examine the blood

Red Cell Count

smear as well as instrument red cell histograms to detect such dimorphic populations. 15 The MCV is an extremely useful value in classification of anemias, 24,32,36 but the MCH and MCHC often do not add significant,

Manual methods for counting red cells have proven to be very inaccurate,

and automated counters provide a much more accurate reflection of red cell numbers. 31 Both erythrocytes and leukocytes are counted after whole blood dilution in an isotonic solution. Because the number of red cells (expressed as 10 12 cells/L) greatly exceeds the number of white cells (by

clinically relevant information.

Red Cell Distribution Width

The red cell distribution width (RDW) is a red cell measurement that

a factor of 500 or more), the error introduced by counting both cell types

quantitates cellular volume heterogeneity reflecting the range of red cell sizes within a sample. 37,38 RDW has been proposed to be useful in early

is negligible. However, when marked leukocytosis is present, red cell

counts and volume determinations may be erroneous unless corrected

classification of anemia because it becomes abnormal earlier in nutritional

for white cells. The observed precision for RBC counts using automated hematology analyzers is approximately 1% (CV) 24,25 compared with a minimum estimated value of 11% with manual methods. 31 Mean Corpuscular Volume

deficiency anemias than other red cell parameters, especially in cases of iron deficiency anemia. 39 RDW is particularly useful in characterizing

microcytic anemia, allowing discrimination between uncomplicated iron

deficiency anemia (high RDWand normal-to-lowMCV) and uncomplicated heterozygous thalassemia (normal RDWand lowMCV), 39-41 although other tests are usually required to confirm the diagnosis. 42 RDW is also useful in

The average volume of the RBC is a useful parameter that is used to

classify anemias and may provide insights into the pathophysiology of red cell disorders. 32 The MCV is measured in femtoliters (fL or 10 −15 L)

identifying red cell fragmentation, agglutination, or dimorphic cell popu-

lations (including patients who have had transfusions, have sideroblastic anemias, or have been recently treated for a nutritional deficiency). 39 Reticulocyte Counts

and is usually measured directly in automated analyzers by dividing the

sum of the individual RBC volumes by the RBC count but may also be calculated from the RBC count and the Hct using the following formula 19 :

= Hct (L/L)

× 1000/red cell count (10 12

MCV

/L)

Determination of the numbers of reticulocytes or immature, non-nucleated

The CV in most automated systems is approximately 1%, 24,25 com- pared to 10% for manual methods. 20 Agglutination of cells, as with cold

RBCs that still contain RNA provides useful information about the ca-

pacity of the bone marrow to synthesize and release red cells in response

Copyright © 2019 Wolters Kluwer, Inc. Unauthorized reproduction of the content is prohibited. agglutinin disease or paraproteinemia, may result in a falsely elevated MCV. 33 Most automated analyzers gate out MCV values above 360 fL, thereby excluding most red cell clumps, although this may falsely to anemia and helps to distinguish between decreased RBC production and enhanced peripheral destruction. Corrected reticulocyte counts or the reticulocyte production index (RPI) can be used to compare the

magnitude of reticulocytosis with the magnitude of anemia to determine

lower calculated Hct determinations. In addition, severe hyperglycemia > 600 mg/dL) may cause osmotic swelling of the red cells, leading to a falsely elevated MCV, which could also lead to a falsely high Hct and falsely decreased MCHC. 23,34,35 Leukocytosis may also spuriously elevate MCV values. 28 (glucose

whether the bone marrow response is adequate. In the past, reticulocyte

counts were performed manually using supravital staining with methylene

blue that stains precipitated RNA as a dark blue meshwork or granules (at least two per cell). 43 Normal values for reticulocytes in adults are

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