Capillary Puncture Sites: 5 Common Methods for Microsampling

Capillary blood sampling provides a less invasive alternative to venipuncture, requiring only small volumes, typically tens of microliters, to test for conditions such as anemia, blood sugar levels, thyroid hormones, drug concentrations, and more.

However, the location of the puncture site is crucial for obtaining an adequate sample. Selecting the right site helps to maximize blood flow, minimize pain, and prevent complications. Here are five commonly used sites for capillary puncture:

Finger (Fingertip)

For both adults and children, the third or fourth finger is typically the preferred choice for puncture. Avoid using the thumb (which may cause excessive bleeding), the index finger (often calloused or sensitive), and the little finger (which has insufficient tissue). Puncture the palmar surface either to the left or right of the midline, staying clear of the fingernails.

Heel

This puncture site is used for newborns and infants, less than one year old, because their fingers are too tiny for finger-stick blood collection. The puncture is done on the farthest lateral or medial aspect of the plantar surface of the heel, not on the bottom. Punctures done on the plantar surface can potentially damage cartilage or bone.

Ear lobe

The ear lobe has been used for capillary blood sampling but is no longer recommended. Studies have shown that the blood flow in the ear lobe is less than that in either the fingertip or the heel. The ear lobe can still be used if no other site is available.

Great toe (Big toe)

The big toe can be used, if necessary, in infants who are not yet walking. Callous formation after a child starts walking can interfere with blood collection and is a contraindication. The site should be lateral to the midline of the plantar surface of the toe. Only the great toe has the amount of tissue necessary to protect the bone from injury.

Palm

There are two areas on the palm that can be used, if necessary. Especially in diabetics who test their own blood sugars frequently, alternate sites give some relief from reusing the same sites over and over. The thenar (fleshy mound at base of thumb) and hypothenar (fleshy mound at base of fourth finger) eminences have capillary circulation equivalent to the fingertip. Some people sample from the "heel" of the hand, though this may not produce enough sample volume.

Best Practices & Things to Keep in Mind

• Always clean and sterilize the puncture site properly.
• Avoid the fingernail and calloused areas.
• Ensure a gentle poke, using the correct lancet depth.
• Discard the first drop of blood (can be diluted with interstitial fluid).
• Rotate sites when possible to avoid soreness and callous formation.
• For infants, always handle heel and toe sites carefully to avoid damage and discomfort.

Learn more about what industries have adopted capillary blood microsampling.