33: Heelstick (Capillary Blood Sampling)

1. Collect blood samples when only a small amount of blood is needed or when there is difficulty obtaining samples by venipuncture or other source. Common studies include complete blood count (CBC), chemistries, liver function tests, sickle cell anemia, thyroid, bilirubin levels, toxicology/drug levels, bedside glucose monitoring, and newborn metabolic screening. Coagulation studies, chromosomal analyses, immunoglobulin titers, and some other, more sophisticated tests cannot be done on capillary sampling.

2. Capillary blood gas determination gives satisfactory pH and Pco₂, but not Po₂.

3. Blood cultures when venous access or other access is not possible. Sterile technique is required but heelstick is not the preferred method.

4. Newborn metabolic screen is ideally performed in the first 48–72 hours of life (however, can be done after 24 hours). It should be done before a transfusion and antibiotic therapy and ideally after receiving breast milk or formula to ensure accurate testing. See Chapter 15.

Automated self-shielding lancets are preferred in neonates (Table 33–1); sterile manual lancets are not recommended but may be used in some units if automated lancets are not available (sizes: 2 mm for <1500 g and 4 mm for >1500 g). Capillary tube (for rapid hematocrit and bilirubin tests) or larger BD Microtainer™ collection tubes (if more blood is needed [eg, for chemistry determinations]), filter paper card for newborn screening (if appropriate), clay to seal the capillary tube, a warm washcloth or heel warming device (eg, a chemical activated packet), antiseptic solution, gloves, and a diaper.

1. Automated self-shielding lancets are preferred in neonates because they are associated with fewer complications and decreased pain. Automated devices cause less hemolysis and less lab value error, and provide an exact width and depth of incision. Manual unshielded lancets are no longer recommended (unless automated lancets are not available) because they cause more pain, may penetrate too deeply, and can injure health care providers. Studies have shown that using the Tenderfoot Preemietm (ITC, Edison, NJ) automated lancet versus a manual lancet resulted in fewer heel punctures, less collection time, and a lower recollection rate. There are two types of devices: puncture and incision (see Table 33–1).

   1. Puncture devices. (eg, BD Microtainer Contact-Activated Lancet.) Activate only when positioned and pressed against the skin. These puncture the skin by inserting a blade or needle vertically into the tissue. Puncture-style devices typically deliver a single drop of blood and are better for sites that are repeatedly punctured (eg, for glucose testing).

   2. Incision devices. (eg, BD Microtainer Quickheel Lancets, Tenderfoot, BD, babyLance.) These devices slice through the capillary beds. These are less painful and require fewer repeat incisions and shorter collection times, and are recommended for infant heelsticks. Incision devices deliver a small flow of blood as opposed to a drop and are better to fill Microtainer tubes.

2. Capillary blood sampling is considered the most common painful procedure but the least invasive and safest of all the blood drawing methods done in the neonatal intensive care unit (NICU). Sampling is done by puncturing the dermis layer of the skin to access capillaries running through the subcutaneous layer of the skin. The sample is a mixture of arterial and venous blood (from arterioles, venules, and capillaries) plus interstitial and intracellular fluids. The proportion of arterial blood is greater than that of venous blood due to increased pressure in the arterioles leading into the capillaries. Warming of the puncture site further arterializes the blood. The areas on the bottom surface of the heel contain the best capillary bed. Note: Cochrane review states that venipuncture, not capillary blood sampling, by a skilled operator is the method of choice for blood sampling in term neonates. Lower pain scores are seen with venipuncture.

3. Heelsticks are contraindicated. If there is local infection, poor perfusion, significant edema, any injury of the foot, or any congenital anomaly of the foot.

4. Infant should be supine. Some advocate for infant to be on stomach with the limb lower than the level of the heart to increase blood flow.

5. Wrap the foot in a warm washcloth and then in a diaper for 3–5 minutes (moist heat helps to increase blood flow). Commercial packs are available to heat the heel and should be applied for 5 minutes. A warming pad may be used, but its temperature should not exceed 40°C (104°F). This prewarming of the blood (arterialization of capillary blood) increases the local blood flow and reduces the difference between the arterial and venous gas pressures. Although not mandatory, it will produce hyperemia, which increases vascularity, making blood collection easier. It is mandatory when collecting a sample for a blood gas or pH determination. One study suggests that heel warming is an unnecessary step. Topical nitroglycerine did not facilitate blood collection in a heelstick study.

6. Pain management. Factors that contribute to pain responses from a heelstick are size of needle, gestational age, repeated exposure, squeezing of the heel, severity of illness, and behavioral state of the infant.

   1. The American Academy of Pediatrics (AAP) recommends nonpharmacologic pain prevention such as oral sucrose/glucose, breast milk, kangaroo care, swaddling, pacifier, or other methods.

   2. EMLA (eutectic mixture of lidocaine and prilocaine). Not found to be effective in heelsticks.

   3. Automated devices. Cause less pain than manual devices.

   4. Other methods found to help. Sugar-coated pacifier, pacifier-activated lullaby, Yakson therapy (Korean touching method of laying a hand on the back and caressing the abdomen for 5 minutes), and mechanical vibration.

7. Choose the area of puncture. (Figure 33–1) Blue area on the picture is preferred. An alternative site is the area between the sides of the heel (plantar area), but it should only be used if the other areas are used extensively. Avoid the end (crown) of the heel (the posterior curvature of the heel where the calcaneus bone is close to the skin), as this area is associated with an increased incidence of osteomyelitis. Fingertips and toes are not recommended in infants and are only recommended in children >1 year. Vary the sites to prevent bruising. One recommendation to prevent osteomyelitis is to use the most medial or lateral portions of the plantar surface of the heel, at a depth no more than 2.4 mm; never on the posterior curvature and not through a previous heelstick site.

8. Wipe the area with povidone-iodine, followed by a saline wipe. Some advocate only a 70% alcohol prep pad and let dry. Do not use cotton balls. (Note: Povidone-iodine can interfere with potassium, bilirubin, phosphorus, and uric acid. If the area is wet with alcohol, hemolysis may occur, altering the results.)

9. Two general devices are available: automated and manual

   1. Using an automated lancet (preferred method). Common devices are noted in Table 33–1. Prepare the unit and hold the device 90 degrees to the surface. The device can be oriented perpendicularly or at 90 degrees to the long axis of the foot (see Figure 33–1A). Depress the trigger with your index finger to activate the device and automatically make the puncture. Immediately discard the device.

   2. Using a standard manual heel lancet. Encircle the heel with the palm of your hand and index finger (Figure 33–1B). Make a quick, deep (<2.0 mm) puncture. Never puncture more than 2 mm to avoid complications.

10. Wipe off the first drop of blood with gauze as the first drop of blood is often contaminated with tissue fluid and may have a high potassium level, causing specimen dilution, hemolysis, and clotting. Wiping off the first drop also permits the sample to flow better as platelets aggregate at the site and may stop bleeding. Gently apply pressure to the heel (“tennis racket grip”), and place the collection tube at the site of the puncture. The capillary tubes will automatically fill by capillary action; gently “pump” the heel to continue the blood flow to collect drops of blood in a larger tube. Allow enough time for capillary refill of the heel, and apply pressure so the incision is opened with each pumping maneuver. Do not squeeze, milk, scoop, scrape, or massage the area as these may affect the test results.

11. Seal the end of the capillary tube with clay. Collect the larger samples in the BD Microtainer or similar tubes.

12. Collect the blood gas sample first. The blood becomes more venous if the collection is delayed. Send it to the lab promptly, making sure there are no air bubbles. Hematology studies should be done next, then chemistry. Also, if the CBC is delayed there is an increased chance of erroneous cell counts due to platelet clumping. Use the following “Order of Draw”: blood gases, ethylenediaminetetraacetic acid (EDTA) tubes, other additive tubes, serum tubes.

13. For filter paper newborn screening. (See Chapter 15.) The paper can be directly applied to the heel or the blood can be transferred to a capillary tube (without anticoagulants) and then applied to the filter paper. This testing is usually done at least 24 to 48 hours after birth.

14. Maintain pressure on the puncture site with a dry sterile gauze pad until the bleeding stops and elevate the foot. A 4 × 4 gauze pad can be wrapped around the heel and left on to provide hemostasis; adhesive bandages are not recommended.

15. Inaccurate laboratory results. Falsely elevated glucose/dextrostix, potassium, hematocrit, and inaccurate blood gas values (slightly lower pH, slightly higher Pco₂, and markedly lower Pco₂) can occur with heelstick sampling. Milking or squeezing causes hemolysis and inaccurate values.

1. Infection

   1. Cellulitis. Risk can be minimized with the proper use of sterile technique. A culture from the affected area should be obtained and the use of broad-spectrum antibiotics considered.

   2. Osteomyelitis. Usually occurs in the calcaneus bone. Avoid the center area of the heel, and do not make the puncture opening too deep. If osteomyelitis occurs, tissue should be obtained for culture, and broad-spectrum antibiotics should be started until a specific organism is identified. Infectious disease and orthopedic consultation is usually obtained.

   3. Other infections. Abscess and perichondritis have been reported.

2. Scarring of the heel. Occurs when there have been multiple punctures in the same area. If extensive scarring is present, consider another technique of blood collection, such as central venous sampling.

3. Pain. Caused by heelsticks in premature infants can cause declines in hemoglobin oxygen saturation as measured by pulse oximetry.

4. Calcified nodules. These can occur but usually disappear by 30 months of age.

5. Other complications. Include nerve damage, tibial artery laceration (medial aspect of heel), burns, bleeding, bruising, hematoma, and bone calcification.