Blood Studies: Hematology and Coagulation



Blood Studies: Hematology and Coagulation






OVERVIEW OF BASIC BLOOD HEMATOLOGY AND COAGULATION TESTS


Composition of Blood

The average person circulates about 5 L of blood (1/13 of body weight), of which 3 L is plasma and 2 L is cells. Plasma fluid derives from the intestines and lymphatic systems and provides a vehicle for cell movement. The cells are produced primarily by bone marrow and account for blood “solids.” Blood cells are classified as white cells (leukocytes), red cells (erythrocytes), and platelets (thrombocytes). White cells are further categorized as granulocytes, lymphocytes, monocytes, eosinophils, and basophils.

Before birth, hematopoiesis occurs in the liver. In midfetal life, the spleen and lymph nodes play a minor role in cell production. Shortly after birth, hematopoiesis in the liver ceases, and the bone marrow is the only site of production of erythrocytes, granulocytes, and platelets. B lymphocytes are produced in the marrow and in the secondary lymphoid organs; T lymphocytes are produced in the thymus.


Blood Tests

Tests in this chapter are basic screening tests that address disorders of hemoglobin (Hb) and cell production (hematopoiesis), synthesis, and function. Blood and bone marrow examinations constitute the major means of determining certain blood disorders (anemias, leukemia and porphyrias disorders, abnormal bleeding and clotting), inflammation, infection, and inherited disorders of red blood cells,
white blood cells, and platelets. Specimens are obtained through capillary skin punctures (finger, toe, heel), dried blood samples, arterial or venous sampling, or bone marrow aspiration. Specimens may be tested by automated or manual hematology instrumentation and evaluation.


BLOOD SPECIMEN COLLECTION PROCEDURES

Proper specimen collection presumes correct technique and accurate timing when necessary. Most hematology tests use liquid ethylenediaminetetraacetic acid (EDTA) as an anticoagulant. Tubes with anticoagulants should be gently but completely inverted end over end 7 to 10 times after collection. This action ensures complete mixing of anticoagulants with blood to prevent clot formation. Even slightly clotted blood invalidates the test, and the sample must be redrawn.

For plasma coagulator studies, such as prothrombin time (PT) and partial thromboplastin time (PTT), the tube must be allowed to fill to its capacity or an improper blood-to-anticoagulant ratio will invalidate coagulator results. Invert 7 to 10 times to prevent clotting.


Capillary Puncture (Skin Puncture)

Capillary blood is preferred for a peripheral blood smear and can also be used for other hematology studies. Adult capillary blood samples require a skin puncture, usually of the fingertip. For children, the tip of the finger is also often the choice. Infants younger than 1 year of age and neonates yield the best samples from the great toe or side of the heel.




Venipuncture

Venipuncture allows procurement of larger quantities of blood for testing. Care must be taken to avoid sample hemolysis or hemoconcentration and to prevent hematoma, vein damage, infection, and discomfort. Usually, the antecubital veins are the veins of choice because of ease of access. Blood values remain constant no matter which venipuncture site is selected, so long as it is venous and not arterial blood. Sometimes, the wrist area, forearm, or dorsum of the hand or foot must be used. Blood values remain consistent for all of these venipuncture sites.



  • Observe standard precautions (see Appendix A). If latex allergy is suspected, use latex-free supplies and equipment.


  • Position and tighten a tourniquet on the upper arm to produce venous distention (congestion). For elderly persons, a tourniquet is not always recommended because of possible rupture of capillaries. Large, distended, and highly visible veins increase the risk for hematoma.


  • Ask the patient to close the fist in the designated arm. Do not ask patient to pump the fist because this may increase plasma potassium levels by as much as 1 to 2 mEq/L (mmol/L). Select an accessible vein.


  • Cleanse the puncture site, working in a circular motion from the center outward, and dry it properly with sterile gauze. Chlorhexidine must dry thoroughly.


  • To anchor the vein, draw the skin taut over the vein and press the thumb below the puncture site. Hold the distal end of the vein during the puncture to decrease the possibility of rolling veins.


  • Puncture the vein according to accepted technique. Usually, for an adult, anything smaller than a 21-gauge needle might make blood withdrawal more difficult. A Vacutainer system syringe or butterfly system may be used.



  • Once the vein has been entered by the collecting needle, blood will fill the attached vacuum tubes automatically because of negative pressure within the collection tube.


  • Remove the tourniquet before removing the needle from the puncture site or bruising will occur.


  • Remove needle. Apply pressure and sterile dressing strip to site.


  • The preservative or anticoagulant added to the collection tube depends on the test ordered. In general, most hematology tests use EDTA anticoagulant. Even slightly clotted blood invalidates the test, and the sample must be redrawn.


  • Take action to prevent these venipuncture errors:



    • Pretest errors



      • Improper patient identification


      • Failure to check patient compliance with dietary restrictions


      • Failure to calm patient before blood collection


      • Use of wrong equipment and supplies


      • Inappropriate method of blood collection


    • Procedure errors



      • Failure to dry site completely after cleansing with alcohol


      • Inserting needle with bevel side down


      • Using too small a needle, causing hemolysis of specimen


      • Venipuncture in unacceptable area (e.g., above an intravenous [IV] line)


      • Prolonged tourniquet application


      • Wrong order of tube draw


      • Failure to mix blood immediately that is collected in additive-containing tubes


      • Pulling back on syringe plunger too forcefully


      • Failure to release tourniquet before needle withdrawal


    • Posttest errors



      • Failure to apply pressure immediately to venipuncture site


      • Vigorous shaking of anticoagulated blood specimens


      • Forcing blood through a syringe needle into tube


      • Mislabeling of tubes


      • Failure to label specimens with infectious disease precautions as required


      • Failure to put date, time, and initials on requisition


      • Slow transport of specimens to laboratory





Arterial Puncture

Arterial blood samples are necessary for arterial blood gas (ABG) determinations or when it is not possible to obtain a venous blood sample. “Arterial sticks” are usually performed by a physician or a specially trained nurse or technician because of the potential risks inherent in this procedure. Samples
are normally collected directly from the radial, brachial, or femoral arteries. If the patient has an arterial line in place (most frequently in the radial artery), samples can be drawn from the line. Be sure to record the amounts of blood withdrawn because significant amounts can be removed if frequent samples are required.

ABG determinations are used to assess the status of oxygenation and ventilation, to evaluate the acid-base status by measuring the respiratory and nonrespiratory components, and to monitor effectiveness of therapy. The ABGs are also used to monitor critically ill patients, to establish baseline laboratory values, to detect and treat electrolyte imbalances, to titrate appropriate oxygen therapy, to qualify a patient for home oxygen use, and to assess the patient’s status in conjunction with pulmonary function testing.

Arterial puncture sites must satisfy the following requirements:



  • Sites must have available collateral blood flow.


  • Sites must be easily accessible.


  • Sites must be relatively nonsensitive as periarterial tissues.



Bone Marrow Aspiration

Bone marrow is located within cancellous bone and long bone cavities. It consists of a pattern of vessels and nerves, differentiated and undifferentiated hematopoietic cells, reticuloendothelial cells, and fatty tissue. All of these are encased by endosteum, the membrane lining the bone marrow cavity. After proliferation and maturation have occurred in the marrow, blood cells gain entrance to the blood through or between the endothelial cells of the sinus wall.

A bone marrow specimen is obtained through aspiration or biopsy or needle biopsy aspiration. A bone marrow examination is important in the evaluation of a number of hematologic disorders and infectious diseases. The presence or suspicion of a blood disorder is not always an indication for bone marrow studies. A decision to employ this procedure is made on an individual basis.

Sometimes, the aspirate does not contain hematopoietic cells. This “dry tap” occurs when hematopoietic activity is so sparse that there are no cells to be withdrawn or when the marrow contains so many tightly packed cells that they cannot be suctioned out of the marrow. In such cases, a bone marrow biopsy would be advantageous. Before the bone marrow procedure is started, a peripheral blood smear should be obtained from the patient and a differential leukocyte count done.


Reference Values


Normal







BASIC BLOOD TESTS


Hemogram

A hemogram consists of a white blood cell count (WBC), red blood cell count (RBC), hemoglobin (Hb), hematocrit (Hct), red blood cell indices, and a platelet count. A complete blood count (CBC) consists of a hemogram plus a differential WBC. Table 2.2 contains the normal values for a hemogram.








TABLE 2.2 Normal Values for Hemogram








































































































































Age


WBC (× 103/mm3)


RBC (× 106/mm3)


Hb (g/dL)


Hct (%)


MCV (fL)


Birth-2 wk


9.0-30.0


4.1-6.1


14.5-24.5


44-64


98-112


2-8 wk


5.0-21.0


4.0-6.0


12.5-20.5


39-59


98-112


2-6 mo


5.0-19.0


3.8-5.6


10.7-17.3


35-49


83-97


6 mo-1 y


5.0-19.0


3.8-5.2


9.9-14.5


29-43


73-87


1-6 y


5.0-19.0


3.9-5.3


9.5-14.1


30-40


70-84


6-16 y


4.8-10.8


4.0-5.2


10.3-14.9


32-42


73-87


16-18 y


4.8-10.8


4.2-5.4


11.1-15.7


34-44


75-89


>18 y (males)


5.0-10.0


4.5-5.5


14.0-17.4


42-52


84-96


>18 y (females)


5.0-10.0


4.0-5.0


12.0-16.0


36-48


84-96


Age


MCH (pg/cell)


MCHC (g/dL)


Platelets (× 103/mm3)


RDW (%)


MPV (fL)


Birth-2 wk


34-40


33-37


150-450




2-8 wk


30-36


32-36





2-6 mo


27-33


31-35





6 mo-1 y


24-30


32-36





1-6 y


23-29


31-35





6-16 y


24-30


32-36





16-18 y


25-31


32-36





>18 y


28-34


32-36


140-400


11.5-14.5


7.4-10.4




Complete Blood Count (CBC)

The CBC is a basic screening test and is one of the most frequently ordered laboratory procedures. The findings in the CBC give valuable diagnostic information about the hematologic and other body systems, prognosis, response to treatment, and recovery. The CBC consists of a series of tests that determine number, variety, percentage, concentrations, and quality of blood cells:



  • White blood cell count (WBC): leukocytes fight infection


  • Differential white blood cell count (Diff): specific patterns of WBC


  • Red blood cell count (RBC): red blood cells carry O2 from lungs to blood tissues and CO2 from tissue to lungs


  • Hematocrit (Hct): measures RBC mass


  • Hemoglobin (Hb): main component of RBCs and transports O2 and CO2


  • Red blood cell indices: calculated values of size and Hb content of RBCs; important in anemia evaluations


  • Mean corpuscular volume (MCV)


  • Mean corpuscular hemoglobin concentration (MCHC)


  • Mean corpuscular hemoglobin (MCH)


  • Stained red cell examination (film or peripheral blood smear)


  • Platelet count (often included in CBC): thrombocytes are necessary for clotting and control of bleeding


  • Red blood cell distribution width (RDW): indicates degree variability and abnormal cell size


  • Mean platelet volume (MPV): index of platelet production

These tests are described in detail in the following pages.



Reference Values


Normal



Interfering Factors


RBCs



  • Many physiologic variants affect outcomes: posture, exercise, age, altitude, pregnancy, and many drugs.


Hematocrit



  • Physiologic variants affect Hct outcomes: age, sex, and physiologic hydremia of pregnancy.


Hemoglobin



  • Physiologic variations affect test outcomes: high altitude, excessive fluid intake, age, pregnancy, and many drugs.



MCHC



  • High values may occur in newborns and infants.


  • Presence of leukemia or cold agglutinins may increase levels. MCHC is falsely elevated with a high blood concentration of heparin.


MCH



  • Hyperlipidemia and high heparin concentrations falsely elevate MCH values.


  • WBC counts greater than 50,000/mm3 falsely elevate Hb values and falsely elevate the MCH.


WBC Count



  • Hourly variation, age, exercise, pain, temperature, and anesthesia affect test results.


Neutrophils and Eosinophils



  • Physiologic conditions such as stress, excitement, exercise, and obstetric labor increase neutrophil levels. Steroid administration affects levels for up to 24 hours.


  • The eosinophil count is lowest in the morning and then rises from noon until after midnight. Do repeat tests at the same time every day. Stressful states such as burns, postoperative states, and obstetric labor decrease the count. Drugs such as steroids, epinephrine, and thyroxine affect eosinophil levels.


Platelets



  • Physiologic factors include high altitudes, strenuous exercise, excitement, and premenstrual and postpartum effects.


  • A partially clotted blood specimen affects the test outcome.



TESTS OF WHITE BLOOD CELLS


White Blood Cell Count (WBC; Leukocyte Count)

White blood cells (or leukocytes) are divided into two main groups: granulocytes and agranulocytes. The granulocytes receive their name from the distinctive granules that are present in the cytoplasm of neutrophils, basophils, and eosinophils. However, each of these cells also contains a multilobed nucleus, which accounts for their also being called polymorphonuclear leukocytes. In laboratory terminology, they are often called “polys” or PMNs. The nongranulocytes, which consist of the lymphocytes and monocytes, do not contain distinctive granules and have nonlobular nuclei that are not necessarily spherical. The term mononuclear leukocytes is applied to these cells.

The endocrine system is an important regulator of the number of leukocytes in the blood. Hormones affect the production of leukocytes in the blood-forming organs, their storage and release from the tissue, and their disintegration. A local inflammatory process exerts a definite chemical effect on the mobilization of leukocytes. The life span of leukocytes varies from 13 to 20 days, after which the cells are destroyed in the lymphatic system; many are excreted from the body in fecal matter.

Leukocytes fight infection and defend the body by a process called phagocytosis, in which the leukocytes actually encapsulate foreign organisms and destroy them. Leukocytes also produce, transport, and distribute antibodies as part of the immune response to a foreign substance (antigen).

The WBC serves as a useful guide to the severity of the disease process. Specific patterns of leukocyte response can be expected in various types of diseases as determined by the differential count (percentages of the different types of leukocytes). Leukocyte and differential counts, by themselves, are of little value as aids to diagnosis unless the results are related to the clinical condition of the patient—only then is a correct and useful interpretation possible. Signs and symptoms of increased WBCs include fever, bruising, petechiae, fatigue, anemia, bleeding of mucous membranes, weight loss, and history of infections.


Reference Values


Normal

Black adults: 3.2-10.0 × 103 cells/mm3 or × 109/L or 3200-10,000 cells/mm3

Adults: 4.5-10.5 × 103 cells/mm3 or × 109/L or 4500-10,500 cells/mm3

Children:

0-2 weeks: 9.0-30.0 × 103 cells/mm3 or × 109/L or 9000-30,000 cells/mm3

2-8 weeks: 5.0-21.0 × 103 cells/mm3 or × 109/L or 5000-21,000 cells/mm3

2 months-6 years: 5.0-19.0 × 103 cells/mm3 or × 109/L or 5000-19,000 cells/mm3

6-18 years: 4.8-10.8 × 103 cells/mm3 or × 109/L or 4800-10,800 cells/mm3





Jun 11, 2016 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Blood Studies: Hematology and Coagulation
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