Site Puncture

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The page below is a sample from the LabCE course Pre-analytical Challenges Encountered with Capillary Blood Collection and Testing. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

Learn more about Pre-analytical Challenges Encountered with Capillary Blood Collection and Testing (online CE course)
Site Puncture

Before capillary puncture is performed, the site may need to be warmed in order to increase arterial blood flow to the puncture area. There are several commercially available thermal packs also known as "heel warmers", but a towel soaked in warm water can also do the trick. Heat should be applied for 3-5 minutes. Care needs to be taken in controlling the temperature in order to prevent burning the patient. It is recommended to warm the site to 42°C.
After the site is properly cleansed, a spring loaded safety lancet is recommended for use. Use of a syringe needle or an old fashioned lancet is not recommended due to safety concerns. Also, spring-loaded lancets used for glucose monitoring are not recommended as they are designed to make a smaller cut enough to produce only 2-3 drops. A previous puncture site should be avoided because of increased possibility of infection.
Finger stick
For a finger stick, the cut should be made perpendicular to the fingerprint line. This will provide the best possible drop and blood flow. If the cut is made along the fingerprint lines, the blood will follow the lines of the fingerprint and will not form the best drop for collection, therefore it should be avoided.
Heel stick
For a heel stick, care needs to be taken so the incision is not too deep. The capillary bed on an infant's heel is about 0.35-1.6 mm beneath the skin. The recommended depth of incision is about 2 mm, anything deeper will present the risk of nerve or bone damage and should be avoided.
For both finger and heel sticks, the first drop of blood produced should not be used and should be wiped away since it contains interstitial fluid which aids in the clotting process.
If a good puncture is made, about 0.5 mL of blood should be collected. Applying gentle pressure to the area and releasing will produce a better blood flow. The pressure should be applied to the surrounding area, about 1/2 inch away from the puncture site. Tightly squeezing the area will cut the blood flow and should be avoided. Milking or scooping the site should also be avoided because they could cause hemolysis and contamination with tissue fluid.