Why must neonatal pku testing be performed by heel stick rather than venipuncture?

Phlebotomy is the specialized practice of withdrawing blood from a patient’s vein for medical purposes. Using a needle or dermal puncture lancet, blood is collected into a series of collection tubes, each treated with specific chemical additives to process the blood for a variety of diagnostic medical tests. Phlebotomy is an important service; up to two thirds of all patient diagnoses depend partially or wholly upon laboratory results. What’s more, many patients require continual monitoring of blood tests, and so may require serial blood draws, or venipunctures.

For newborn babies (neonates), blood is typically collected in very small amounts via the neonatal heel prick method. When performed properly, this method is very effective at yielding the appropriate amount of blood for testing purposes. Special skills are needed when performing phlebotomy procedures on small infants, as you must take into consideration not only their size, but also their vulnerability to pain and the anxiety that the procedure induces in their parents.

What types of blood tests do newborns need?

Although newborns can potentially require any type of testing, there are a handful of routine tests that are common in neonatal medicine:

Phenylketonuria (PKU) – This free, state-performed screening test is done as a public service for all newborns. Although its named after just one particular metabolic disorder, the test actually screens for about a dozen critical disorders, including thyroid dysfunction, sickle-cell anemia, and cystic fibrosis.

Bilirubin – Immediately after birth, many newborns can develop jaundice, a yellowing of the skin and eyes caused by the buildup of bilirubin. Although a temporary condition, bilirubin levels need to be monitored with blood tests until the condition resolves.

Complete blood count – Usually called a CBC, blood counts are an important monitoring tool for a number of issues that can arise with neonates. CBCs are used to screen for anemia, infection, or disorders of red cell shape and function. These are usually performed on newborns that are staying in the hospital.

Genetic studies – A number of chromosomal disorders can be detected using the DNA contained within white blood cells.

Blood collection methods

The neonatal heel prick, also known as a dermal puncture, is by far the most popular way to collect blood from newborns and infants up to about six months of age. Dermal punctures are preferable because, when done correctly, they are guaranteed to produce blood, removing the uncertainty of needle sticks. The heel prick is used to fill small collection tubes called pediatric “bullet” tubes, named after their shape. The steps for a proper neonatal heel prick are as follows:

1 – Be sure to place the infant in a safe, comfortable position, face-up, either on an infant draw table or a parent’s lap. Leave the baby as swaddled as possible for comfort, only exposing one leg for the draw.

2 – Using an approved heel-warmer pad, heat the baby’s entire foot for approximately one minute until the skin is very warm to the touch. This technique dilates capillaries in the area, maximizing blood flow.

3 – Gently flexing the foot upward, encase the baby’s entire foot in one hand. Rather than pinching the heel directly, you will be using a full-foot massage technique to collect the blood. This ensures both the baby’s comfort and a much better blood flow.

4 – Wipe the heel with an alcohol pad and allow it to air dry. Do not blow on the foot as it dries. Softly squeeze the baby’s foot so that the skin of the heel is bunched up. Press the lancet flat against the inner, bottom edge of the heel, along the same side as the big toe, and depress the trigger. The lancet will make a quick swipe of the heel with a clicking sound.

5 – Wipe the first drop of blood away with a clean cotton pad. Then, using a massaging pattern, gently squeeze and release the foot several times, allowing blood drops to form a drip from the end of the heel.

6 – As the blood drops collect, let them flow openly into the bullet tube or PKU card without scraping the blood along the skin. This helps avoid damaging, or “hemolysing”, the red blood cells.

7 – When a sufficient amount of blood has been collected, place a clean cotton pad across the puncture site and apply moderate pressure for approximately one minute.
With your free hand, close the cap of the tube and gently invert it several times to mix the tube additive with the blood. Don’t shake too vigorously.

Heel sticks are not appropriate for every type of lab test. In the event that specialty testing is ordered for blood banking, blood culture, or tests that require larger amounts of product, you will need to perform a regular venipuncture with a needle. For babies, use a tourniquet that has been cut smaller, and spend extra time locating the antecubital vein on one or both arms. Use a 23g or 25g sized butterfly needle, and be sure that the arm is completely restrained for the duration of the draw to avoid an injury.

Special considerations for neonatal phlebotomy

Neonatal phlebotomy can be understandably intimidating. There are certain things to consider when performing a phlebotomy procedure on a young infant. Be cognizant of the fact that their small body and thin veins are more vulnerable to damage, and use the utmost caution at all times. Also, infants have a miniscule blood volume compared to adults, so only draw the amount needed for testing.

Their tininess does also have some benefits; unlike older people, babies cannot anticipate or remember pain, so there is no anxiety on their part. Parents, on the other hand, can create a bigger challenge. Always take the time to explain the procedure well, and offer compassion and reassurance for nervous parents. If you require their assistance in holding or restraining an infant, be certain of their competence to do so. Distressed babies are notorious for erasing their parents’ ability to remain objective! If you do not feel the phlebotomy can be comfortably assisted by a parent, ask a coworker to help instead.

Becoming a phlebotomist

When performing a capillary puncture on newborns and infants What part of the heel pad is used?

Which of the following is a complication that can result from multiple deep heel sticks on an infant?

How long should the heel of a neonate be warmed prior to a heel stick?

Which is the preferred site for a heel stick?