Page 40 - Williams Hematology ( PDFDrive )

16 Part I: Clinical Evaluation of the Patient Chapter 2: Examination of Blood Cells 17

on the reticulocyte count (Chap. 32). The correlation between manual These parameters have the advantage of ready access in the context
and automated methods of reticulocyte enumeration is good, but refer- of an automated blood count, but the availability of differently derived
ence ranges differ slightly among the methods, given the different dyes and calculated parameters from various instrument makers is a chal-
and conditions used and the continuous nature of the variables separat- lenge to remember and compare across laboratories.
ing reticulocytes from mature red cells.
Many hematology analyzers now report some quantitative mea- Other Red Cell Findings
sure of reticulocyte RNA content. Increase in the immature (highest Nucleated Red Cells Nucleated red cells are present in newborns,
RNA content) reticulocyte fraction is an early sign of marrow recovery particularly if physiologically stressed, and in a variety of disorders,
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from cytotoxic therapy or treatment for nutritional anemias, usually including hypoxic states (congestive heart failure), severe hemolytic
preceding the rise in total reticulocyte count. A limitation at present anemia, primary myelofibrosis, and infiltrative disease of the marrow
is that the methods lack standardization and reference ranges for these (Chap. 45). Most modern automated hematology analyzers are capable
parameters are instrument dependent. 44 of detecting and quantitating nucleated red blood cells, which were a
source of spuriously elevated leukocyte counts in earlier instruments, at
Additional Red Cell and Reticulocyte Indices a level of 1 to 2 nucleated red cells per 100 leukocytes.
Current high-end automated cell counters measure unique properties of Malarial Parasites Malarial parasites can also be detected by some
mature red cells and reticulocytes on a cell-by-cell basis, not just as pop- current analyzers, based on detecting parasite infected red cells or neu-
ulation averages. The result is a plethora of new indices that are in many trophils containing ingested hemozoin in regions of the multiparam-
cases specific to an instrument manufacturer, presenting new diagnostic eter display that are not characteristically populated in normal blood
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opportunities but also a confusing nomenclature and a potential lack (sometimes causing spurious eosinophilia ). Some reports indicate
of comparability. Some examples of parameters that have been studied high sensitivity and specificity with certain instrumentation, a useful
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include %HypoHe, %MicroR, RET-He (available on Sysmex instru- consideration in endemic areas where access to technologists with mor-
ments), CHr, HYPO% (Siemens), RSf, LHD% (Beckman-Coulter), and phologic expertise may not be consistent. Careful attention to instru-
FRC (fragmented red cells; Sysmex and Siemens). ment characteristics and limitations as well as the relative prevalence
New formulas for distinguishing causes of microcytosis based on of disorders causing instrument flags in the laboratory’s patient pop-
45
several novel red cell indices function about as well or somewhat bet- ulation is essential in fine tuning instrument review criteria to provide
ter than traditional formulas for differentiating iron deficiency from reasonable sensitivity and specificity.
46
thalassemia trait. More sophisticated mathematical modeling of individ- Other Abnormalities Not Detected by Automation Some disor-
ual cell-based volume and hemoglobin content data available in current ders, such as immune and hereditary spherocytosis (Chaps. 46 and 54),
analyzers has been used in a systems biology approach to demonstrate hemoglobin C disease (Chap. 49), elliptocytosis (Chap. 46), inherited
latent iron deficiency and to distinguish causes of microcytosis. 47,48 The granule abnormalities (Chap. 66), and malaria and other parasitic dis-
ability of new automated analyzers to measure parameters specifically eases (Chap. 53), may not be reliably detected by the various flagging
in reticulocytes on a cell-by-cell basis also opens up the possibility of strategies on automated analyzers, and morphologic findings such as
reticulocyte-specific indices. The theoretical advantage is that acute basophilic stippling (Chap. 31), toxic granulation (Chap. 60), sidero-
changes in red cell function would be detected more rapidly and reliably cytes (Chap. 31), and pathologic rouleaux (Chap. 109) are only detect-
in the reticulocyte fraction as opposed to the total red cell population. able by microscopic examination of the blood film.
Estimates of reticulocyte-specific hemoglobin content (CHr and
RET-He, which are comparable) by light-scatter measurements of AUTOMATED ANALYSIS OF LEUKOCYTES
reticulocytes are closely related to adequacy of iron availability to ery-
throid precursors during the preceding 24 to 48 hours, and have been Leukocyte Count
described as diagnostically useful in detecting functional iron deficiency Leukocyte counts are performed by automated cell counters on blood
49
in complex clinical settings, such as chronic inflammation and chronic samples appropriately diluted with a solution that lyses the erythrocytes
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renal disease. The increase in serum ferritin as an acute phase reactant (e.g., an acid or a detergent), but preserves leukocyte integrity. Man-
combined with the physiologic variation of serum iron and iron-bind- ual counting of leukocytes is used only when the instrument reports a
ing capacity limits the value of conventional parameters in these set- potential interference or the count is beyond instrument linearity lim-
tings. The CHr may be a better predictor of depleted marrow iron stores its. Manual counts are subject to much greater technical variation than
than traditional serum iron parameters in nonmacrocytic patients, automated counts because of technical and statistical factors, and with
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and is a more sensitive predictor of iron deficiency than hemoglobin modern instrumentation, need to be done infrequently. Instruments
for screening infants and adolescents for iron deficiency. Estimates that perform an automated 5-part differential can measure absolute
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of percentages of red cells falling below a cutoff for hemoglobin con- neutrophil counts accurately down to 100/μL. Automated leukocyte
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centration (HYPO%) or hemoglobin content (%HypoHe) may provide counts may be falsely elevated as a result of cryoglobulins or cryofibrin-
greater sensitivity than the corresponding mean values averaged over all ogen, clumped platelets or fibrin from an inadequately anticoagulated
red cells, for instance with respect to iron deficiency in renal disease. or mixed sample, ethylenediaminetetraacetic acid (EDTA)–induced
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Four of the newer parameters (HYPO%, %HypoHe, CHr, RetHe) sim- platelet aggregation, nucleated red blood cells, or nonlysed red cells,
ilarly outperformed transferrin saturation and ferritin in hemodialysis and falsely decreased because of EDTA-induced neutrophil aggregation.
patients for diagnosis of iron deficiency. However, both the CHr and This potential interference is instrument dependent, and current analyz-
54
RET-He are less effective than the MCH in screening elderly patients ers use a variety of algorithms to minimize their effect and flag those rare
for iron-deficiency anemia. The RSf (square root of MCV times MRV samples on which accurate automated analysis cannot be performed.
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[mean reticulocyte volume]) and LHD% (a mathematical transforma-
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tion of the MCHC) have similar diagnostic utility as RET-He. Frag- Leukocyte Differential
mented red cell (FRC) counts by automated analyzers, based on better Leukocytes in the blood serve different functions and arise from dif-
methods of separating small red cells from platelets, appear to lack spec- ferent hematopoietic lineages, so it is important to evaluate each of the
ificity and their clinical role is not yet defined. major leukocyte types separately. Modern automated instruments use

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