Blood Cells and the CBC

Ed Uthman, MD

Diplomate, American Board of Pathology

This is a document in a five-part series
on blood cells and anemia:
1. Blood cells and the CBC
2. Anemia: Pathophysiologic Consequences,
Classification, and Clinical Investigation
3. Nutritional Anemias and
Anemia of Chronic Disease
4. Hemolytic Anemias
5. Hemoglobinopathies and Thalassemias

Introduction

Hematopathology is not only the study of disease of the blood and bone marrow, but also of the organs and tissues which employ blood cells as principal effectors of their physiologic functions. Such would include the lymph nodes, spleen, thymus, and the many foci of lymphoid tissue found along the aerodigestive tract. Generally two types of medical subspecialists intensively practice in this area, the hematologist and the hematopathologist. The hematologist usually is a Board-certified internist who has completed additional years of training in hematology, usually as part of a combined fellowship in hematology and oncology. The thrust of this individual's work is toward the diagnosis and medical management of patients with hematologic disease, especially neoplasms, and medical management of other nonhematologic cancer. The hematopathologist, on the other hand, is usually Board-certified in anatomic and clinical pathology and has taken additional years of training in hematopathology. His or her principal activity is the morphologic diagnosis of conditions of the hematopoietic and lymphocyte-rich tissues and in the performance of laboratory testing that assists such diagnosis.

Hematopathology is somewhat unique in its approach to the patient and the disease, in that 1) many diseases are understood at the molecular level, 2) the patient's tissue is easily obtainable in large quantities (in the case of peripheral blood, at least) and easily kept viable for special studies, and 3) the function of the blood (or at least the erythroid component) is relatively simple when compared to that of other organ systems. Because it is a scientifically integrated discipline hematology/hematopathology is an area which is intellectually gratifying to the eclectic individual who is well-rounded in various biomedical endeavors, including biochemistry, physiology, pharmacology, microanatomy, morphologic diagnosis, and patient care.

The Blood

A few nights working in a trauma center would tend to convince one that the body is just a huge bag of blood. In fact, an "average" 70 liter human body contains only about 5 liters of blood, or 7% by volume. In the normal state, blood has no business anywhere except in the confines of the heart and blood vessels and in the sinusoids of the marrow, liver, and spleen. Of the average 5 L of blood, only 2.25 L, or 45%, consists of cells. The rest is plasma, which itself consists of 93% water (by weight) and 7% solids (mostly proteins, the greatest proportion of which is albumin). Of the 2.25 L of cells, only 0.037 L (1.6%) are leukocytes. The entire circulating leukocyte population, if purified, would fit in a bartender's jigger. The total circulating platelet volume is even less -- about 0.0065 L -- or a little over one teaspoonful.

Erythrocytes

Image of red cells Structurally the simplest cell in the body, volumes have been written about the lowly red blood cell. The basic function of the rbc is the creation and maintenance of an environment salutary to the physical integrity and functionality of hemoglobin. In the normal state, erythrocytes are produced only in the skeleton (in adults only in the axial skeleton), but in pathologic states (especially myelofibrosis, which will be covered subsequently) almost any organ can become the site of erythropoiesis. Numerous substances are necessary for creation of erythrocytes, including metals (iron, cobalt, manganese), vitamins (B12, B6, C, E, folate, riboflavin, pantothenic acid, thiamin), and amino acids. Regulatory substances necessary for normal erythropoiesis include erythropoietin, thyroid hormones, and androgens. Erythrocytes progress from blast precursors in the marrow over a period of five days. Then they are released into the blood as reticulocytes, distinguishable from regular erythrocytes only with special supravital stains. The reticulocyte changes to an erythrocyte in one day and circulates for 120 days before being destroyed in the reticuloendothelial system.

Clinical laboratories measure several important parameters that reflect rbc structure and function. These measurements are used to 1) evaluate the adequacy of oxygen delivery to the tissues, at least as is related to hematologic (as opposed to cardiopulmonary) factors, and 2) detect abnormalities in rbc size and shape that may provide clues to the diagnosis of a variety of hematologic conditions. Most of these tests are performed using automated equipment to analyze a simple venipuncture sample collected in a universal lavender- (or purple-) top tube containing EDTA as an anticoagulant. Let us consider each of these tests.

Further reading on red cell disease

Anemia: Pathophysiologic Consequences, Classification, and Clinical Investigation is an introduction to anemia

Nutritional Anemias and Anemia of Chronic Disease deals with anemias caused by iron, folate, and vitamin B12 deficiencies.

Hemolytic Anemias is concerned with anemias caused by red cells being destroyed faster than a healthy marrow can replace them.

Hemoglobinopathies and Thalassemias covers sickle cell disease, hemoglobins C and E, and alpha- and beta-thalassemias.

Understanding Anemia, my first book, is now available in hardback and paper. The publisher has kindly allowed me to post the full text of Chapter 1 online. You can access it through the book outline at this link. There is also a link to buy the book from online bookstores at a substantial discount. This book is aimed at general readers and presumes a knowledge of biology at the high school level, then builds from there.

Leukocytes and the leukocyte differential count

To consider the leukocytes together as a group is something of a granfalloon, because each type of leukocyte has its own function and ontogeny semi-independent of the others. To measure the total leukocyte count and allow this term to mean anything to the doctor is a travesty, yet the "wbc" count has traditionally been considered a cardinal measurement in a routine laboratory workup for just about any condition. I cannot emphasize too much that to evaluate critically the hematologic status of a patient, one must consider the individual absolute counts of each of the leukocyte types rather than the total wbc count. For such a critical evaluation, the first step is to order a wbc count with differential. In many labs, the result will be reported as a relative differential, something like this:

WBC 6000/µL
segmented neutrophils 60%
band neutrophils 2%
lymphocytes 25%
monocytes 8%
eosinophils 3%
basophils 2%

Your first task is to multiply the wbc count by each of the percentages given for the cell types; this gives you an absolute differential. Now you're in business to get some idea as to the pathophysiologic status of the patient's blood and marrow. Thus, the illustration above becomes:

WBC 6000/µL
segmented neutrophils 3600/µL
band neutrophils 120/µL
lymphocytes 1500/µL
monocytes 480/µL
eosinophils 180/µL
basophils 120/µL

The total wbc count is invariably done using an automated method. Routinely, the differential count is done "by hand" (i.e., through the microscope) in smaller labs, and by automated methods in larger facilities. The automated methods are amazingly accurate, considering the fine distinctions that must often be made in discerning one type of leukocyte from the other. One manufacturer's machine can quite reliably pick out one leukemic blast cell in eight hundred or more leukocytes. Now we shall consider each of the leukocyte types individually.

Platelets

Image of platelets The main thing to remember about platelets is to look for them first! A typical tyro maneuver is to study a blood smear for an hour looking for some profound hematological abnormality, never to realize there is nary a platelet in sight. It is therefore necessary to discipline yourself to first check for a normal number of platelets when sitting down with a slide, before being seduced by the midnight beauty of the basophil's alluring granules or the monocyte's monolithic sovereignty. The normal platelet count is 133 - 333 x 103/µL.

Platelets are counted by machine in most hospital labs and by direct phase microscopy in smaller facilities. Since platelets are easily mistaken for garbage (and vice versa) by both techniques, the platelet count is probably the most inaccurate of all the routinely measured hematologic parameters. Actually, you can estimate the platelet count fairly accurately (up to an absolute value of about 500 x 103/µL) by multiplying the average number of platelets per oil immersion field by a factor of 20,000. For instance, an average of ten platelets per oil immersion field (derived from the counting of ten fields) would translate to 200,000/µL (10 x 20,000). Abnormal bleeding generally does not occur unless the platelet count is less than 30,000/µL, if the platelets are functioning properly. Screening for proper platelet function is accomplished by use of the bleeding time test.

Other cells in peripheral blood

Plasma cells sometimes appear in the peripheral blood in states characterized by reactivity of lymphocytes. Old time hematologists often maintain that the cells that look exactly like plasma cells on the smear are really "plasmacytoid lymphs," and it is usually nonproductive to argue this point with them. Endothelial cells occasionally get scooped up into the phlebotomy needle during blood collection and show up on the slide. They are huge and tend to be present in groups. Histiocytes, complete with pseudopodia and phagocytic vacuoles, may appear in states of extreme reactivity, especially in septic neonates. Nucleated red cells may also be seen in small numbers in the peripheral blood of newborns; however, in adults, even a single nucleated rbc on the slide is abnormal, indicating some sort of serious marrow stress, from hemolytic anemia to metastatic cancer. Myeloblasts are always abnormal and usually indicate leukemia or an allied neoplastic disease. Rarely they may be seen in non-neoplastic conditions, such as recovery from marrow shutdown (aplasia). Later stages of myeloid development (promyelocyte, myelocyte, metamyelocyte) may be represented in the peripheral blood in both reactive states and leukemias.

Bone marrow examination

This is one of the most common biopsy procedures performed on both outpatients and the hospitalized. Two types of specimens are generally obtained, the aspirate and the core biopsy. The site of biopsy is usually the posterior iliac crest (via the posterior superior iliac spine) in adults and the anterior tibia in children, although other sites are available. After local anesthesia is applied to the periosteum and overlying skin, a small needle (usually the "University of Illinois needle") is introduced (or crunched actually) into the medullary space through a small skin incision. About 0.5 mL of marrow material is aspirated and smeared onto several glass slides and stained with a stain identical or similar to the Wright stain used on peripheral blood. Some material usually remains in the syringe where it is allowed to clot. It is then fished out of the syringe, processed like all other biopsy tissue, embedded in paraffin, sectioned, and stained with hematoxylin/eosin and other selected stains. The core biopsy, generally performed after the aspirate is done, is taken with a larger, tapered needle, typically the "Jamshidi needle." This yields a core of bone (similar to a geologic core sample) which is fixed, decalcified, processed, and sectioned. The H&E-stained core biopsy and aspirate clot sections are best for assessment of marrow cellularity and the presence of metastatic neoplasms or granulomas. The Wright-stained aspirate smears are best for studying the detailed cytology of hematopoietic cells.

The bone marrow biopsy procedure produces some pain for the patient, since it is impossible to anesthetize the inside of bone. The level of pain ranges from mild discomfort to agony, depending on the individual's pain threshold and level of apprehension. Some physicians elect to precede the biopsy with a benzodiazepine or other minor tranquilizer. Generally the aspiration action produces much more pain than the core biopsy.

For a procedure that involves invasion of bone, the marrow biopsy is remarkably free of complications. Bleeding and infection may occur but are rare, even in severely thrombocytopenic and immunosuppressed patients. It is highly recommended that med students learn how to perform this useful procedure during the clinical years of their training.

Copyright © 1997-2000, Edward O. Uthman. Free for non-commercial use only.

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