Methods of
Hematology

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3-3. CAPILLARY PUNCTURE

a. Site. Several different sites are suitable for capillary puncture. Because it is the most accessible, the palmer or lateral surface of the tip of the finger (preferably ring finger) is the most common site in adults. However, certain problems can be encountered such as heavy calloused areas or excessive tissue fluids (edema) that tend to result in non-representative samples. The lobe of the ear can be used for capillary puncture. However, differences in cell concentration do occur when blood is obtained from this site, primarily because of higher lymphocyte concentrations in the ear lobe. Because of the small amount of tissue on the fingers of infants, preferred site is the heel or big toe. A modification of the normal technique that has proven quite satisfactory when working with the heel of infants is to make two incisions in a crisscross fashion or “T”.

NOTE: To be a valid report, work done on capillary blood must be from a free-flowing puncture wound.

b. Equipment.

(1) Gauze pads 2 x 2 inches.
(2) Blood lancet.
(3) Glass slides, heparinized capillary tubes, and other devices to receive the specimen.
(4) Isopropyl alcohol, 70 percent, prep pads.

c. Procedure.

(1) The puncture site should be warm to assure good circulation of blood. If it is cold, apply warm water (38-40C) for a few minutes. If blood is to be drawn from the ear, the edge of the lobe, not the flat side, should be punctured.
(2) The site to be punctured is first rubbed with alcohol prep pads to remove dirt and epithelial debris, increase circulation, and render the area reasonably disinfected (see figure 3-4 capillary puncture procedure, a through d).

Figure 3-4a. Capillary puncture procedure: Clean the puncture site.

(3) Allow sufficient time for the circulation to equalize.
(4) While making a finger puncture, apply gentle pressure to the finger to hold the skin taut. Hold the finger in one hand and the lancet in the other. The puncture is made perpendicular to the lines of the fingerprints, which results in a more free-flowing wound (see figure 3-4b).

Figure 3-4b. Capillary puncture procedure: Puncture the finger.

(5) The first drop of blood that appears is wiped away before specimens are taken (see figure 3-4c).

Figure 3-4c. Capillary puncture procedure: Wipe away first drop of blood.

(6) The blood must not be squeezed out since this dilutes it with fluid from the tissues, thus altering the ratio of cellular elements to fluid, as well as the ratio of cellular elements to each other.
(7) After the desired specimens have been collected, have the patient hold a sterile dry gauze pad over the wound until bleeding stops (see figure 3-4d).

Figure 3-4d. Capillary puncture procedure: Apply pressure to the site.

d. Discussion.

(1) A disposable lancet is a very satisfactory instrument for puncture of the skin. However, if this is not available a sharp pointed surgical blade is quite suitable.

(2) Do not use the finger on a hand, which has been hanging over the side of the bed as it is likely to be congested. Edematous or cyanotic areas should not be used.

(3) The finger should be thoroughly dry prior to puncture; blood will not well up on a finger that is moist. Furthermore, the alcohol or other antiseptic used can coagulate the blood proteins causing cell clumping and erroneous values as well as dilute cell volumes. This will result in incorrect counts and differentials.

(4) Finger punctures should be made along the lateral aspect of the fingertip. More nerve endings are located on the fingerprint area of the fingers; therefore, more pain results from punctures in this area. Scars can also form in these sensitive areas, and difficulty may be encountered in puncturing a callous. All of these difficulties are eliminated by drawing the blood from the lateral rather than the ventral aspect of the finger.


Curriculum design: David L. Heiserman
Publisher: SweetHaven Publishing Services

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