Wintrobe's Clinical Hematology 14e SC

Chapter 1: Examination of the Blood and Bone Marrow 5

that captures digital images of cells in a stained smear and classifies them

larger platelet volumes (secondary to new platelet production) seen in

thrombocytopenic patients in whom platelets are decreased because of peripheral destruction (as in immune thrombocytopenia). 76-78 MPVmay

to provide a differential that includes mature and immature WBCs and other cells, such as variant lymphocytes and plasma cells. 59 The images are reviewed by trained technologists to further refine the classifications if needed. RBC and platelet counts and morphology can also be analyzed. 64

also be increased in myeloproliferative disorders. However, it should be

noted that platelets tend to swell during the first 2 hours in EDTAantico- agulant, shrinking again with longer storage. 79 Decreased MPV has been associated with megakaryocytic hypoplasia and cytotoxic drug therapy. 80

The systems have the capacity to store images and are useful in training

technologists as well as providing an easily accessible means, whereby

smears obtained at different times from a single patient may be compared morphologically. 65 These systems perform well in normal blood specimens

Other platelet parameters may also be reported, depending on the

analyzer. The immature platelet fraction, or reticulated platelets, rep-

but have limitations in their ability to identify morphologically abnormal

resents newly released platelets that retain residual RNA, analogous to red cell reticulocytes. 64 Reticulated platelet counts are determined

cells, so specimens with dysplastic changes, unusual morphologic vari-

ants, or significant artifacts may not be evaluable or may provide false data. 11,59,66-69 Often, these systems will designate a certain percentage of

using RNA staining dyes, give an estimate of thrombopoiesis, and

may be useful in distinguishing platelet destruction syndromes from hypoplastic platelet production in bone marrow failure conditions. 64,81

cells as unclassifiable, requiring review by a technologist for definitive

identification of the cell type and completion of the differential.

Normal values vary between 3% and 20%, and 2.5- to 4.5-fold increases

in reticulated platelet counts are seen in the clinical setting of immune thrombocytopenia. 82 Increased reticulated platelets may herald the return of platelet production after chemotherapy. 83

PLATELET ANALYSIS

Platelets are anucleate cytoplasmic fragments that are 2 to 4 µm in

ADVANTAGES AND SOURCES OF ERROR WITH AUTOMATED HEMATOLOGY

diameter. As with the other blood components, they may be counted

by either manual or automated methods. Manual methods involve

dilution of blood samples and enumeration in a counting chamber or

hemocytometer using phase-contrast microscopy. Sources of error are

similar to other manual counting techniques and include dilution errors

Clearly, the use of automated hematology analyzers has reduced labo-

and low numbers of events counted. The CV of manual methods, es-

ratory costs and turnaround time while also improving the accuracy and

> 15%. 70

pecially in patients with thrombocytopenia, may be

Platelets

reproducibility of blood counts. Thorough verification of hematology

are counted in automated hematology analyzers after removal of red

analyzers prior to clinical use and adequate technical and quality control procedures are essential. 8,25,84 Despite the high level of accuracy and

cells by sedimentation or centrifugation, or using whole blood. Platelets

are identified by light scatter, impedance characteristics, and/or platelet antigen or platelet-specific cytoplasmic staining. 24,64 These give reliable

precision, automated hematology analyzers may generate a warning flag

in 10% to 25% of samples, requiring manual examination of the blood smear. 15-17,24,85 Blood smear examination still plays an important role

platelet counts with a CV of approximately 3% in the normal range.

in characterizing these samples. In addition, some cell types are only

However, achieving accurate counts in patients with thrombocytopenia

remains a challenge, and CVs in thrombocytopenic samples are closer to 5%. 25 Falsely low platelet counts may be caused by the presence of large platelets, platelet clumps/agglutinins, 52 or adsorption of platelets to leukocytes. 71 Fragments of RBCs or WBCs may falsely elevate the automated platelet count, but this usually gives rise to an abnormal histogram that identifies the spurious result. 72,73 Automated hematology analyzers also determine mean platelet vol- ume (MPV), which has been correlated with several disease states. 74-76

identified morphologically, such as Sézary cells, and red cell morphology is best analyzed by direct smear examination. 36

Laboratory Hematology

Certain disease states are associated with spuriously high or low results

from analyzers, although some of these are specific to a particular type of

instrumentation (summarized in Table 1.1 ). Therefore, values obtained

from the automated hematology analyzer must be interpreted in the context

of clinical findings. As previously mentioned, careful examination of the

stained blood film often imparts additional information that may not be

reflected in the average values reported by the automated CBC.

In general, MPV has an inverse relationship with platelet count, with

Table 1.1 Disorders and Conditions That May Reduce the Accuracy of Blood Cell Counting

Component Disorder/Condition

Effect on Cell Count

Rationale

Red cells

Microcytosis or schistocytes

May underestimate RBC

Lower threshold of RBC counting

window is greater than microcyte size

Howell–Jolly bodies

May spuriously elevate platelet count

Howell–Jolly bodies are similar in size

(in whole blood platelet counters only)

to platelets

Polycythemia

May underestimate RBC

Increased coincidence counting

White cells

Leukocytosis

Overestimate RBC

Increased coincidence counting

Acute leukemia and chronic lymphocytic

May spuriously lower WBC

Increased fragility of leukocytes,

leukemia, viral infections

including immature forms

Chemotherapy of acute leukemia

May artifactually increase platelet count

Leukemic cell nuclear or cytoplasmic

fragments identified as platelets

Platelets

Platelet agglutinins

May underestimate platelet count, sometimes

Platelet clumping

with spurious increase in WBC

Aggregates may be identified as

leukocytes

Copyright © 2019 Wolters Kluwer, Inc. Unauthorized reproduction of the content is prohibited. Plasma Cold agglutinins May underestimate RBC with spurious macrocytosis Red cell doublets, triplets, and so forth have increased volume

Cryoglobulins, cryofibrinogens

Variation in platelet count

Protein precipitates may be identified as

platelets

Some of these examples affect counts only when certain instruments are used. The effects depend on methodology, dilution, solutions used, and specimen temperatures.

Abbreviations: RBC, red blood cell count; WBC, white blood cell count.

Adapted from Koepke JA. Laboratory Hematology . New York, NY: Churchill Livingstone; 1984.

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