Capillary Collections, Point-of-care Testing, and Pediatrics

WEEK 8 Capillary collections, Point-of-care testing, and pediatrics 01 CAPILLARY COLLECTION Capillary collection, also called dermal puncture or skin puncture, is the usual collection procedure for infants. In adults, it is an alternative collection procedure when minute amounts of blood are needed for testing, or for patients for whom venipuncture is inadvisable or impossible. In addition, it is typically used when collecting blood from infants and point-of-care testing. The depth of puncture must be carefully controlled to produce adequate flow while avoiding contact with underlying bone. Skin puncture devices deliver a precise incision, and microsample containers, sized to fit the desired sample, collect the blood from the puncture site. Reasons for Performing Capillary Collection Although venipuncture is the most common way to obtain a blood sample, at times it is impossible or inadvisable to do so. In these situations, capillary collection (also called skin puncture or dermal puncture) offers a valuable alternative. Capillary collection is the preferred method for obtaining blood from newborns and infants for neonatal bilirubin, newborn screening, and point-of-care testing. Capillary collection is also used for ancillary blood glucose testing. A requisition form generally does not state that a capillary collection should be performed, and it is up to the phlebotomist to choose the best collection method for the tests ordered. For this reason, you must be familiar with the advantages, limitations, and appropriate uses of capillary collection. Knowing how and when to perform a capillary collection is a vital skill for a phlebotomist. Capillary collection is preferred in several situations and for several types of patients. Adult patients undergoing frequent glucose monitoring are excellent candidates for capillary collection, because the test requires only a small amount of blood, which must be taken frequently. Access to venipuncture sites may be difficult with obese patients, whose veins are often hard to find, and with geriatric patients, who often have small or fragile veins that can make obtaining venous blood difficult. Venipuncture may be contraindicated for patients with burns or scars over venipuncture sites or for those at risk for venous thrombosis (caused when clots form within the veins), because venipuncture increases the risk of venous thrombosis. Other patients may be at risk for serious complications associated with deep venipuncture, including iatrogenic anemia, hemorrhage, infection, organ or tissue damage, arteriospasm, or cardiac arrest. Iatrogenic anemia is anemia caused by excessive blood draws. Capillary collection is usually the preferred method of collection for newborns, infants, and children younger than 2 years. Young children’s smaller veins and lower blood volume make venipuncture both difficult and potentially dangerous. Reducing blood volume through venipuncture is a concern for newborns and infants. It may lead to anemia and even cardiac arrest and death. In addition to serving as a substitute for venipuncture, capillary blood gas (CBG) determination can be used as an alternative to arterial puncture for arterial blood gas (ABG) determination in infants. However, some tests cannot be performed on blood from a capillary collection. These include blood cultures, erythrocyte sedimentation rate (ESR), coagulation tests, and tests requiring larger volumes of plasma or serum. Capillary collection may not be appropriate for severely dehydrated patients, because test results may not be accurate. (This may also be a concern with venipuncture for such patients.) Capillary collection should not be used at sites that are swollen or where circulation or lymphatic drainage is compromised (such as a limb on the same side as a mastectomy). Differences Between Venous and Capillary Blood A capillary puncture collects blood from capillaries. Because capillaries are the bridges between arteries and veins, blood collected by capillary puncture is a mixture of venous blood and arterial blood. The arterial proportion in the sample is increased when the collection site is warmed, as may be done to help increase blood flow before collection. Small amounts of tissue fluid from the puncture site may also be in the sample, especially in the first drop. The levels of many substances are the same in both capillary and venous blood, but this is not the case for all substances, as indicated in the table below. For instance, the normal potassium reading obtained by capillary collection is lower than that in a plasma sample obtained by venipuncture, but higher than that in a serum sample obtained by venipuncture. Because of these differences, results obtained by the two techniques cannot be compared. For this reason as well, it is important to record that the sample was obtained by capillary collection. Equipment for Capillary Collection Capillary collection equipment allows the phlebotomist to puncture the skin safely and collect the sample quickly and efficiently, with a minimum of discomfort for the patient. Skin Puncture Devices Skin puncture devices in the modern healthcare 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 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. There are heel incision devices specifically for newborns and premature babies. The method of puncture by the blade is different from a typical puncture device. The puncture is a sweeping arc instead of a perpendicular puncture. This is a smoother cut that minimizes bruising and reduces contact with nerve endings, therefore minimizing pain to the infant. Various sizes are available based on the age, birth status, and size of the infant. The Lasette laser lancing device is unique in that it uses a laser, rather than a sharp instrument, to pierce the skin, which causes less pain and bruising at the sample site. Cross-contamination between patients is avoided by disposing of the single-use lens cover between uses. The device is approved for patients 5 years and older. Microsample Containers Microsample containers (also called bullets) come in a range of sizes that accommodate various volumes of blood. The fill volumes of the microsample containers range from 250 μL up to 750 μL. Microcollection tubes are made of plastic and contain a variety of anticoagulants and additives. 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 are available in plastic and come in a range of sizes. These tubes are used primarily for the collection of samples for CBG (capillary blood gases) determinations. Caraway or Natelson pipets are narrow plastic tubes with capacities up to about 470 μL. 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 plastic tubes, 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. Additional Supplies As in venipuncture, alcohol pads are used to prepare the site, and gauze pads are used to help stop the bleeding. A sharps disposal container is needed for the lancet. Warming devices increase arterial circulation by sevenfold. 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.