HEMATOLOGY INTRODUCTION

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.

Composition

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.

Composition

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.

Collecting Sample

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.

Capilary puncture

Observe standard precautions. Check for latex allergy. If allergy is present, do not use latex-containing products.

Obtain capillary blood from fingertips or earlobes (adults) or from the great toe or heel (infants). Avoid using the lateral aspect of the heel where the plantar artery is located.

Disinfect puncture site, dry the site, and puncture skin with sterile disposable lancet, perpendicular to the lines of the patient's fingers, no deeper than 2 mm. If povidone-iodine is used, allow to dry thoroughly.

Wipe away the initial drop of blood. Collect subsequent drops in a microtube or prepare a smear directly from a drop of blood.

After collection, apply a small amount of pressure briefly to the puncture site to prevent painful extravasation of blood into the subcutaneous tissues.

Capilary puncture - Procedure

In this method, a lancet is used, and the resulting droplets of blood are collected by blotting them with filter paper directly.

Check the stability of equipment and integrity of supplies when doing a finger stick. If provided, check the humidity indicator patch on the filter paper card. If the humidity circle is pink, do not use this filter paper card. The humidity indicator must be blue to ensure specimen integrity.

After wiping the first drop of blood on the gauze pad, fill and saturate each of the circles in numerical order by blotting the blood droplet with the filter paper. Do not touch the patient's skin to the filter paper; only the blood droplet should come in contact with the filter paper.

If an adult has a cold hand, run warm water over it for approximately 3 minutes. The best flow occurs when the arm is held downward, with the hand below heart level, making effective use of gravity. If there is a problem with proper blood flow, milk the finger with gentle pressure to stimulate blood flow or attempt a second finger stick; do not attempt more than two.

When the blood circles penetrate through to the other side of the filter paper, the circles are fully saturated.

Dried Blood Spot

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.

Venipuncture

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.

Venipuncture - procedure

Cleanse the puncture site, working in a circular motion from the center outward, and dry it properly with sterile gauze. Povidone-iodine 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.

Venipuncture - procedure

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.

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

Pretest Error

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 (eg, 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

Procedure Error

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

Posttest Error

Instruct patient regarding sampling procedure. Assess for circulation or bleeding problems and allergy to latex. Verify with the patient any fasting requirements. Diagnostic blood tests may require certain dietary restrictions of fasting for 8 to 12 hours before test. Drugs taken by the patient should be documented because they may affect results.

Reassure patient that mild discomfort may be felt when the needle is inserted.

Place the arm in a fully extended position with palmar surface facing upward (for antecubital access).

Pretest Patient Care

If withdrawal of the sample is difficult, warm the extremity with warm towels or blankets. Allow the extremity to remain in a dependent position for several minutes before venipuncture. For young children, warming the draw site should be routine to distend small veins.

Be alert to provide assistance should the patient become lightheaded or faint.

Prescribed local anesthetic creams may be applied to the area before venipuncture; allow 60 seconds for light-skinned persons and 120 seconds for dark-skinned persons before performing the procedure.

Pretest Patient Care

Be aware that the patient occasionally becomes dizzy, faint, or nauseated during the venipuncture. The phlebotomist must be constantly aware of the patient's condition. If a patient feels faint, immediately remove the tourniquet and terminate the procedure. Place the patient in a supine position if possible. If the patient is sitting, lower the head between the legs and instruct the patient to breathe deeply. A cool, wet towel may be applied to the forehead and back of the neck, and, if necessary, ammonia inhalant may be applied briefly. Watch for signs of shock, such as increased heart rate and decreased blood pressure. If the patient remains unconscious, notify a physician immediately.

Posttest Patient Care

Prevent hematomas by using proper technique (not sticking the needle through the vein), releasing the tourniquet before the needle is withdrawn, applying sufficient pressure over the puncture site, and maintaining an extended extremity until bleeding stops. If a hematoma develops, apply a warm compress.

Assess the puncture site for signs and symptoms of infection, subcutaneous redness, pain, swelling, and tenderness.

Posttest Patient Care

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.

Arterial Puncture

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

Absence of a palpable radial artery pulse

Positive Allen's test result, which shows only one artery supplying blood to the hand

Negative modified Allen's test result, which indicates obstruction in the ulnar artery (ie, compromised collateral circulation)

Cellulitis or infection at the potential site

Presence of arteriovenous fistula or shunt

Severe thrombocytopenia (platelet count 20,000/mm3)

Prolonged prothrombin time or partial thromboplastin time (>1.5 times the control is a relative contraindication)

Contra Indication

Perform a modified Allen's test by encircling the wrist area and using pressure to obliterate the radial and ulnar pulses. Watch for the hand to blanch, and then release pressure only over the ulnar artery. If the result is positive, flushing of the hand is immediately noticed, indicating circulation to the hand is adequate. The radial artery can then be used for arterial puncture. If collateral circulation from the ulnar artery is inadequate (ie, negative test result) and flushing of the hand is absent or slow, then another site must be chosen. An abnormal Allen's test result may be caused by a thrombus, an arterial spasm, or a systemic problem such as shock or poor cardiac output.

Intratest patient care

Elevate the wrist area by placing a small pillow or rolled towel under the dorsal wrist area. With the patient's palm facing upward, ask the patient to extend the fingers downward, which flexes the wrist and positions the radial artery closer to the surface.

Palpate for the artery, and maneuver the patient's hand back and forth until a satisfactory pulse is felt.

Swab the area liberally with an antiseptic agent such as Chloraprep.

Intratest patient care

Prepare a 20- or 21-gauge needle on a preheparinized, self-filling syringe; puncture the artery; and collect a 3- to 5-mL sample. The arterial pressure pushes the plunger out as the syringe fills with blood. (Venous blood does not have enough pressure to fill the syringe without drawing back on the plunger.) Air bubbles in the blood sample must be expelled as quickly as possible because residual air alters ABG values. The syringe should then be capped and gently rotated to mix heparin with the blood.

Intratest patient care

When the draw is completed, withdraw the needle, and place a 4- — ร 4-inch absorbent bandage over the puncture site. Do not recap needles; if necessary, use the one-handed mechanical, recapping, or scoop technique or commercially available needles (eg, B-D Safety-Glide [Franklin Lakes, NJ] or Sims Portex Pro-Vent [Keene, NH]). Maintain firm finger pressure over the site for a minimum of 5 minutes or until there is no active bleeding evident. After the bleeding stops, apply a firm pressure dressing but do not encircle the entire limb, which can restrict circulation. Leave this dressing in place for at least 24 hours. Instruct the patient to report any signs of bleeding from the site promptly and apply finger pressure if necessary.

Intratest patient care

Frequently monitor the puncture site and dressing for arterial bleeding for several hours. The patient should not use the extremity for any vigorous activity for at least 24 hours.

Monitor the patient's vital signs and mental function to determine adequacy of tissue oxygenation and perfusion.

The arterial puncture site must have a pressure dressing applied and should be frequently assessed for bleeding for several hours. Instruct the patient to report any bleeding from the site and to apply direct pressure to the site if necessary.

Posttest patient care

For patients requiring frequent arterial monitoring, an indwelling arterial catheter (line) may be inserted. Follow agency protocols for obtaining arterial line blood samples. The procedure varies for neonate, pediatric, and adult patients.

Label all specimens appropriately, and document pertinent information in the health care record.

Posttest patient care