11.1 Hematology Terminology, Blood Functions, and Cell Types

CMLA Work Area III (Examination / Analytical Considerations) is 83 items, or 41.5% of the 200-question exam. Hematology is competency 6. This section is III.6.A: know terminology related to hematology, know functions of the blood, and recognize the functional differences among platelets, erythrocytes, and leukocytes. American Medical Technologists (AMT) lists the words you must define: hematology, hemoglobin, cells, blood, reticulocyte, plasma, serum, mononucleosis, erythrocyte, anemia, hematocrit, complete blood count, thrombocyte (platelet), erythrocyte sedimentation rate, and leukocyte (neutrophil, lymphocyte, monocyte, eosinophil, basophil). Instrument quality control, coagulation assays, and waived devices come later in this chapter. A Certified Medical Laboratory Assistant (CMLA) names the cells and the liquid they float in so a complete blood count (CBC) order, a lavender-top tube, and a waived hemoglobin result make sense—without independently performing a high-complexity manual differential.

Quick Answer: Hematology is the study of blood and blood-forming tissues. Erythrocytes carry oxygen on hemoglobin. Leukocytes defend against infection. Thrombocytes (platelets) plug injured vessels. Hematocrit is the packed-red-cell fraction of whole blood. A CBC reports those formed elements on mixed whole blood; a CMLA does not independently sign out a high-complexity differential.

Hematology, blood, cells, plasma, and serum

Hematology is the laboratory discipline that studies blood, the cells that circulate in it, and the organs that produce those cells (bone marrow, spleen, lymph nodes). Blood is a liquid plasma in which formed elements are suspended. Adult circulating volume is often taught as about 5 liters; CMLA does not require a body-weight formula, but you must treat blood as both a transport medium and a cellular suspension.

Plasma is the liquid portion of anticoagulated blood. It still contains fibrinogen and other clotting proteins. Serum is the liquid left after blood has clotted—fibrinogen has been consumed in the fibrin clot. Chapter 6 taught this pair for tubes. The hematology point is mechanical: cell counts, hemoglobin, and hematocrit are whole-blood assays. You do not centrifuge a lavender-top ethylenediaminetetraacetic acid (EDTA) tube and test the plasma as a CBC. Visual color does not identify plasma versus serum; the additive and whether a clot formed do.

Erythrocytes, hemoglobin, hematocrit, reticulocytes, and anemia

An erythrocyte is a red blood cell (RBC). Mature circulating human erythrocytes lack a nucleus. They are biconcave discs packed with hemoglobin (Hgb or Hb), the iron-containing protein that binds oxygen in the lungs and releases it in tissues. Hemoglobin also carries a portion of carbon dioxide. A hemoglobin result is a concentration (commonly g/dL). It is not a cell count and it is not a hematocrit.

Hematocrit (Hct) is the percentage (or fraction) of whole blood occupied by packed red cells after centrifugation, or the equivalent value an analyzer calculates from red-cell count and mean cell volume. Teaching shorthand is that hematocrit in percent is often about three times the hemoglobin in g/dL when red cells are normal (for example, hemoglobin 15 g/dL with hematocrit about 45%). That rule of three is a check, not an AMT-published conversion you report instead of a measured result. It fails in microcytosis, macrocytosis, and some analyzer flags.

A reticulocyte is a young erythrocyte newly released from marrow; it still contains residual ribonucleic acid (RNA). The reticulocyte count estimates marrow response to anemia. It is typically a nonwaived hematology assay performed by qualified testing personnel. Recognize the word: more reticulocytes often mean the marrow is making red cells (blood loss, hemolysis, recovery); inappropriately low reticulocytes in anemia suggest production failure. You do not independently perform a high-complexity reticulocyte count or smear review.

Anemia is a decrease in hemoglobin, hematocrit, and/or red-cell mass sufficient to reduce oxygen-carrying capacity. The CMLA does not diagnose the cause. You collect the EDTA specimen, keep it from clotting, and escalate a critically low hemoglobin per the laboratory's critical-value standard operating procedure (SOP) (Chapter 17). Too many red cells (polycythemia) can raise hematocrit; that opposite problem is not on the AMT term list, but it explains why a hematocrit can be high as well as low.

Leukocytes: five types you must name

A leukocyte is a white blood cell (WBC). Leukocytes defend against infection and participate in inflammation and immune response. AMT lists the five circulating types you must recognize.

Memory aid: Never Let Monkeys Eat Bananas (neutrophil, lymphocyte, monocyte, eosinophil, basophil) is the usual descending order of abundance in a healthy adult. Typical textbook adult examples are neutrophils about 50–70%, lymphocytes 20–40%, monocytes 2–8%, eosinophils 1–4%, and basophils 0–1%. Those figures are teaching examples, not a universal AMT cutoff and not pediatric truth. Automated differentials and smear reviews belong to medical laboratory technicians (MLT) and medical laboratory scientists (MLS) (or equivalent qualified testing personnel). A CMLA does not independently perform a high-complexity manual differential or sign out cell morphology.

Mononucleosis is a clinical syndrome classically associated with Epstein–Barr virus, often with atypical lymphocytes on a smear that qualified personnel review. The heterophile agglutination (mono) test is an immunology/serology procedure (Chapter 12). In this hematology section, mononucleosis is a term you recognize as a reason a CBC or a mono test may be ordered—not a cassette you interpret here.

Thrombocytes (platelets)

A thrombocyte, also called a platelet, is a small cytoplasmic fragment of a megakaryocyte. Platelets adhere at endothelial injury, aggregate, and provide phospholipid surface for coagulation. They are not oxygen carriers and they are not leukocytes. A low platelet count (thrombocytopenia) increases bleeding risk; a very high count can increase clotting risk. Platelet estimates on a smear and instrument flags for clumping are MLT/MLS work. The assistant's job is an adequately mixed EDTA specimen without clots (Section 11.2).

Complete blood count and erythrocyte sedimentation rate

A complete blood count (CBC) is the panel that conceptually reports:

• Red-cell count, hemoglobin, hematocrit, and red-cell indices (mean corpuscular volume and related calculated values)
• White-cell count, often with an automated differential of the five leukocyte types
• Platelet count, sometimes with mean platelet volume

The CBC is a whole-blood EDTA assay. It is not a serum chemistry panel. Many CBCs are moderate- or high-complexity tests on automated counters. The CMLA collects and mixes the tube, recognizes clots as rejection, and may perform waived hemoglobin, hematocrit, or related point-of-care methods (Section 11.4). You do not independently operate a core-lab hematology analyzer or report a morphologic differential.

Erythrocyte sedimentation rate (ESR, sed rate) measures how far red cells fall in a vertical column of anticoagulated blood in a timed interval (commonly one hour in classic Westergren methods). Inflammation increases plasma proteins that make red cells stack (rouleaux) and fall faster. ESR is a nonspecific inflammation marker, not a cell count and not a CBC component. Some ESR kits are waived; many Westergren methods are not. Perform only the method your SOP and Clinical Laboratory Improvement Amendments (CLIA) certificate assign.

Functions of blood and functional differences among the three formed elements

Functions of blood:

• Transport: oxygen and carbon dioxide (erythrocytes and hemoglobin), nutrients, wastes, hormones, heat
• Regulation: pH, fluid distribution, body temperature
• Protection: leukocytes against microbes; platelets and coagulation proteins against blood loss; antibodies and complement in plasma

Functional differences CMLA will test:

Platelets start the plug; plasma coagulation factors (measured with prothrombin time and partial thromboplastin time in Section 11.3) stabilize it with fibrin. Do not call a platelet a coagulation factor, and do not call hemoglobin a leukocyte enzyme.

Exam traps

• Serum versus plasma: both can look yellow; neither is the CBC specimen.
• Hemoglobin (protein concentration) versus hematocrit (packed-cell fraction) versus erythrocyte count (number of cells).
• A reticulocyte is a young red cell, not a white cell and not a platelet.
• Mononucleosis as a word belongs here; the mono cassette belongs in immunology.
• A CBC conceptually includes a differential, but a CMLA does not independently perform a high-complexity differential.

When a stem names a hematology word, give its job in one sentence, name the specimen it needs, and name the look-alike it is not.