Capillary collection

Skin puncture devices in the modern health care environment are designed with a retractable blade per Occupational Safety and Health Administration (OSHA) safety regulations. Some devices are contact activated, whereas others require the phlebotomist to depress a trigger to release the blade. Automatic puncture devices (Figure 10-1) deliver a swift puncture to a predetermined depth, which can be a significant advantage at sites where the bone is close to the skin (see “Site Selection”). The dimensions of the puncture are controlled by the width and depth of the point. Some automatic puncture devices have a platform that is positioned over the puncture site and color coded for different depths. (The automatic devices designed for home glucose testing make a cut that is too small for multiple tests or for filling several microsample containers and are not used in clinical practice.) Safety features include retractable blades and locks that keep a blade from being used a second time and prevent accidental sticks to the phlebotomist.

Microcollection tubes (also called “bullets”) hold up to 750 μL of blood. They are made of plastic and are available with a variety of anticoagulants and additives (Figure 10-2). The tubes are color coded by additive to match the coding of evacuated containers. Microcollection tubes are used for all types of dermal puncture collections and are the most common type of collection containers used for dermal puncture samples. They are also available with a plastic capillary tube that is fitted inside the container to aid in the collection of the sample.

Capillary tubes come in either plastic or glass, and in a range of sizes. These tubes are used primarily for the collection of samples for CBG determinations. Caraway or Natelson pipets are narrow glass tubes with capacities up to about 470 μL. The use of these tubes has declined in recent years because of heightened attention to sharps injury prevention, and OSHA recommends against using glass tubes of this sort. Plastic capillary tubes are increasingly being used instead.

Capillary tubes are available either plain, with a blue band, or heparin coated, with a red, yellow, or green band (colors may vary with the manufacturer). Once the sample is in the tube, one or both ends are sealed. Sealing used to be done by pushing the ends into soft clay. New, safer sealing methods are recommended that instead fit the ends with small plastic caps. Both ends must be sealed for a CBG determination. In the laboratory, the sample may be centrifuged to separate cells from plasma or serum. The sample can be removed with a syringe.

Microhematocrit tubes are small tubes, either plastic or glass, with a volume up to 75 μL. These tubes are used infrequently and have been largely replaced by microcollection containers. However when a spun hematocrit is requested, a plastic capillary tube can be used.

Finally, capillary collection is also used in neonatal screening for a variety of inherited diseases. In this situation, no collection container is used. Instead, drops of blood are applied directly to specially prepared filter paper.

Warming devices increase circulation. Simple towels or washcloths may be soaked in warm water and applied to the site. Be sure that the site is completely dry before puncture, however, as residual water will cause hemolysis and dilution of the specimen. Commercial warming packs are also available. The pack is first wrapped in a dry towel and then activated by squeezing. Such “heel warmers” are often used when blood must be collected from infants. The temperature of the device should not exceed 42° C, and it should be applied for 3 to 5 minutes. Glass slides are used to prepare blood smears for microscopic examination of blood cells.

The depth of the puncture depends on the site and the patient (Table 10-2). To minimize the risk of inflammation and infection, the lancet should never penetrate more than 3.0 mm. For a heel puncture, the maximum depth is 2.0 mm, because the calcaneus, or heel bone, can lie very close to the surface. For premature babies, the recommended depth is 0.65 to 0.85 mm.

Puncture width should not exceed 2.4 mm (Figure 10-3). At the right site, this achieves adequate blood flow but remains well above the bone. Puncture width is actually more important than depth in determining blood flow, because capillary beds may lie close to the skin, especially for newborns. A wider cut severs more capillaries and produces greater flow.

For adults and for children older than 1 year, dermal punctures are almost always performed on the fingertips of the nondominant hand. The best sites are the palmar surface of the distal segments of the third (middle) and fourth (ring) fingers (Figure 10-4). The thumb is likely to be callused, and the index finger’s extra nerve endings make punctures more painful. The little finger (pinky) has too little tissue for safe puncture. If the fingers cannot be used, the big toe may be an option—check the policy at your workplace. Earlobes are never used for dermal puncture.

The puncture should be made near the fleshy center of the chosen finger. Avoid the edge of the finger, as the underlying bone is too close to the surface. As indicated in Figure 10-4, the puncture should be made perpendicular to (across) the ridges of the fingerprint, which lessens the flow of blood into the grooves.

For children younger than 1 year, there is too little tissue available in any of the fingers. For this reason, dermal puncture is performed in the heel (Figure 10-5). As shown in Figure 10-6, only the medial and lateral borders of the plantar (bottom) surface can be used. The center of the plantar surface is too close to the calcaneus, as is the posterior (back) surface. The arch is too close to nerves and tendons. For older infants, the big toe may be used if the heel is unacceptable. Be aware that the heel may be callused on young children who have begun to walk.

A bleeding time (BT) test measures the length of time required for bleeding to stop after an incision is made. In the past, this test was used before surgery to help assess the overall integrity of primary hemostasis. Inconsistencies in the test have led to its replacement by the platelet function assay (PFA) or other tests. These are performed not on dermal puncture samples but on venous blood. The procedure is included here because you may be called upon to perform it nonetheless. To perform a BT test, an automated incision device is used, with the depth set at 1 mm, and length at 5 mm. Some devices make one incision; others make two incisions at the same time. The steps of the BT test are shown in Procedure 10-2.

PROCEDURE 10-2

Bleeding Time Test

1. Assemble your equipment. You will need the following: • Antiseptic materials (alcohol pads) • Blood pressure cuff • Automated BT device • Stopwatch or timer with a second hand • Filter paper • Bandages
2. Prepare the patient. Explain the procedure to the patient. Be sure to explain that scarring may occur, especially in dark-skinned patients. Ask the patient about any medications he or she is taking or has taken recently, especially aspirin or other drugs that interfere with clotting. Salicylates, including aspirin, inhibit platelet function for 7 to 10 days after the last dose. Ibuprofen inhibits function for 24 hours. If the patient has taken such medications recently, follow the policy of your institution or consult with your supervisor about proceeding with the test.
3. Position the arm, select the site, and clean the site. Place the arm on a flat, steady surface, with the palm facing up. Select a site 5 cm below the antecubital crease. The area must be free of veins, scars, hair, and bruises. For patients with very hairy arms, you may need to shave the site before cleaning it. Clean the site and allow it to dry, as you would for other dermal procedures.
4. Apply the blood pressure cuff. Place the cuff on the upper arm. Inflate it to 40 mm Hg. This pressure must be maintained throughout the procedure. Wait 30 to 60 seconds to ensure that the pressure is stable before making the incision.
5. Position the device. The blade is placed perpendicular to the antecubital crease on the volar surface of the forearm.
6. Make the incision, and start timing. Press the device on the arm without making an indentation. Press the trigger. Start timing as soon as the cut is made. Remove the device only after the blade has retracted.
7. Wick the blood away every 30 seconds. Use filter paper to absorb blood from the cut without touching the incision. Touch the edge of the filter paper to the surface of the blood drop without touching the skin or the incision. Wick until the drop disappears. Repeat every 30 seconds. Be careful not to touch the site between times, as this may disrupt the clot and alter the test results. If two incisions are made, follow this procedure for each incision independently.
8. Complete the test. When blood is no longer absorbed by the filter paper, bleeding has stopped. Record the time. Remove the pressure cuff.
9. Attend to the patient. Clean the arm around the incision site with antiseptic wipe and apply a butterfly bandage. Be sure to pull the edges of the incision together when applying the butterfly bandage. Instruct the patient to keep the bandage in place for 24 hours to minimize scarring.